Vol.:(0123456789)
1 3
Biochar (2022) 4:16
https://doi.org/10.1007/s42773-022-00135-4
ORIGINAL RESEARCH
Amending atropical Arenosol: increasing shares ofbiochar andclay
improve thenutrient sorption capacity
ChristineBeusch1 · DennisMelzer1,2· ArneCierjacks3,4 · MartinKaupenjohann1
Received: 15 August 2021 / Accepted: 30 December 2021
© The Author(s) 2022
Abstract
Tropical Arenosols may be challenging for agricultural use, particularly in semi-arid regions. The aim of this study was to
evaluate the impact of the addition of increasing shares of biochar and clay on the nutrient sorption capacity of a tropical
Arenosol. In batch equilibrium experiments, the sorption of ammonium-N (
NH+
4-N
), nitrate-N (
NO−
3-N
), potassium (
K+
),
and phosphate-P (
PO3
−
4-P
) was quantified for mixtures of an Arenosol with increasing shares of biochar and clay (1%, 2.5%,
5%, 10%, 100%) and the unmixed Arenosol, biochar, and clay. The mid-temperature biochar was produced from Prosopis
juliflora feedstock; the clayey material was taken from the sedimentary parent material of a temporarily dry lake. Only the
Arenosol–biochar mixture with 10% biochar addition and the biochar increased the
NH+
4-N
maximum sorption capacity
(
qmax
) of the Arenosol, by 34% and 130%, respectively. The
qmax
of
PO3
−
4-P
slightly increased with ascending biochar shares
(1–10%) by 14%, 30%, 26%, and 42%, whereas the undiluted biochar released
PO3−
4-P
. Biochar addition slightly reduced
NO−
3-N
release from the Arenosol but strongly induced
K+
release. On the other hand, clay addition of 10% and clay itself
augmented
qmax
of
NH+
4-N
by 30% and 162%; ascending clay rates (1–100%) increased
qmax
for
PO3−
4-P
by 78%, 130%,
180%, 268%, and 712%. Clay rates above 5% improved
K+
sorption; however, no
qmax
values could be derived. Sorption of
NO−
3-N
remained unaffected by clay amendment. Overall, clay addition proved to enhance the nutrient sorption capacity of
the Arenosol more effectively than biochar; nonetheless, both materials may be promising amendments to meliorate sandy
soils for agricultural use in the semi-arid tropics.
* Christine Beusch
christine.beusc[email protected]
1 Institute ofEcology, Chair ofSoil Science, Technische
Universität Berlin, Ernst-Reuter-Platz 1, 10587Berlin,
Germany
2 Institute ofAgricultural andUrban Ecological Projects
(IASP), Affiliated toBerlin Humboldt University,
Philippstraße 13, Haus 16, 10115Berlin, Germany
3 Institute ofEcology, Chair ofEcosystem Science/Plant
Ecology, Technische Universität Berlin, Rothenburgstr. 12,
12165Berlin, Germany
4 Department ofLandscape Development/Vegetation
Technology, University ofApplied Sciences Dresden,
Pillnitzer Platz 2, 01326Dresden, Germany
Highlights
• Biochar addition slightly increased NH4
+-N and PO4
3−-P
sorption, reduced NO3
−-N release but triggered K+
release.
• Clay addition enhanced sorption of NH4
+-N, K+, and
PO4
3−-P better than biochar but had no effect on NO3
−-N.
• Both substrates can contribute to fertility of Arenosols by
increasing their nutrient content and sorption capacity.
Keywords Coarse-textured soils· Biochar· Clay· Batch equilibrium experiment· Sorption isotherms
Biochar (2022) 4:16
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16 Page 2 of 23
1 Introduction
Arenosols are among the most abundant soil types world-
wide. They cover an area of about 1300 million ha, cor-
responding to roughly 10% of the land surface. Even
though Arenosols can be found globally, they predominate
in vast areas of arid and semi-arid regions (IUSS Work-
ing Group WRB 2014; Yost etal. 2019). This also includes
the Itaparica region, located in the state of Pernambuco in
the north-east of Brazil (AraújoFilho etal. 2013). These
coarse-textured soils have low contents of organic carbon
(C) and clay, resulting in limited capacity to retain water and
nutrients (Huang and Hartemink 2020). One possible land
management strategy to meliorate soil fertility could be the
application of soil amendments such as biochar and clay.
By increasing soil C and clay content, the nutrient retention
of the Arenosol may be enhanced, leading to more efficient
fertiliser use and hence contributing to food security in the
region.
Many studies in recent years have revealed the posi-
tive effects of biochar amendment on physical and chemi-
cal soil properties (Beusch 2021), such as the increase of
cation exchange capacity (CEC) (Das etal. 2021; Liang
etal. 2006) and soil water-holding capacity (e.g., Abel etal.
2013; Bruun etal. 2014; Hien etal. 2021). Furthermore,
an increase in crop yield has been documented for different
biochars, soil types, and regions (e.g., Gopal etal. 2020;
Jeffery etal. 2011; Steiner etal. 2007; Xu etal. 2015; Zhang
etal. 2015). Various studies have proved the potential of
biochars to retain nutrients in the soil. Most of them focused
on the inorganic forms of nitrogen (N); ammonium (
NH+
4
)
and nitrate (
NO−
3
) sorption were, for instance, reported by
Aghoghovwia etal. (2020), Fatima etal. (2021), Kameyama
etal. (2012),and Pratiwi etal. (2016). Moreover, pyrolysis
conditions determine the potential of the biochar to retain
NH+
4
and
NO−
3
. However, sorption of
NH+
4
decreases with
increasing production temperatures of biochar (Gai etal.
2014; Li etal. 2018; Takaya etal. 2016), whereas
NO−
3
sorp-
tion increases with higher temperatures (Fatima etal. 2021;
Yao etal. 2012; Zheng etal. 2013). The literature, however,
is not consistent since other studies report no sorption for
NH+
4
(Alling etal. 2014) and no sorption or even release of
NO−
3
with the addition of biochar to soils (Hale etal. 2013;
Hollister etal. 2013; Gai etal. 2014; Li etal. 2018). In con-
trast to nitrogen, only a few experiments addressed sorption
or retention of potassium (K) by biochars. Most of them
did not report any sorption but strong K release from bio-
chars (Limwikran etal. 2018; Raave etal. 2014; Rens etal.
2018; Widowati etal. 2014). For phosphate (
PO3−
4
), several
studies revealed sorption to biochar (Rashmi etal. 2020;
Takaya etal. 2016; Wang etal. 2021), but different fac-
tors were assumed to be responsible for
PO3−
4
sorption. For
example, biochar feedstock (Gronwald etal. 2015), soil acid-
ity (Ghodszad etal. 2022), or higher process temperature
(Trazzi etal. 2016; Zhang etal. 2016) were hypothesised as
main factors controlling sorption, while Fatima etal. (2021)
and Morales etal. (2013) reported more
PO3−
4
sorption by
biochars produced with lower temperatures. However, Alling
etal. (2014) and Schneider and Haderlein (2016) found only
weak
PO3−
4
sorption, while other studiesreported none at all
(Morales etal. 2021; Yao etal. 2012; Zheng etal. 2013).
Overall, the knowledge about nutrient sorption to biochar
remains insufficient. Furthermore, the majority of sorption
studies were conducted with pure biochar; only a small num-
ber of experiments addressed different biochar-soil ratios.
Another possibility to meliorate sandy soils is to add
clayey material to increase water and nutrient retention
capacities. However, compared to the number of biochar-
related studies, this practice was only considered by a few
studies. The majority of research regarding clay addition
has been conducted on sandy soils, mostly in semi-arid
and arid climates. Addition of clay to soils is reported to
enhance their water-use efficacy (Al-Omran etal. 2005;
Suzuki etal. 2007), increase the soil water content (Mi etal.
2021), reduce the hydrophobicity of sandy soils (Blackwell
2000; Cann 2000; McKissock etal. 2002; Shanmugam
and Abbott 2015), foster the formation of organo-mineral
complexes (Reuter 2001), reduce soil erosion and restrain
land degradation (Pi etal. 2021), and increase CEC and
soil organic matter content (Hall etal. 2010; Karbout etal.
2021; Schapel etal. 2018). Clay amendment also led to
improved plant nutrition, in particular for K, and enhanced
water infiltration and water distribution (Hall etal. 2010).
In a study about terra preta practice in a tropical rain forest,
Schritt etal. (2020) outlined the potential of biochar and clay
addition to soils and composts to close nutrient cycles. All
these meliorations of soil properties may have contributed
to the increase of yield and the improvement of yield qual-
ity after clay addition that was reported in several studies
(e.g., Al-Omran etal. 2005; Hall etal. 2010; Mi etal. 2020;
Reuter 2001). Other studies focused on adsorption of
NH+
4
,
NO−
3
, or
PO3−
4
from aqueous solutions to clayey material
to investigate sediment–water interactions and to test their
suitability for the construction of wetlands or for removal of
ammonium from waste water (Alshameri etal. 2018; Baker
and Fraij 2010; Durn etal. 2016; Lazaratou etal. 2020; Zhu
etal. 2011). However, even though clayey material is not
considered as a soil amendment, these studies contribute to
a better understanding of the sorption mechanisms of nutri-
ents to clay. In contrast, only some studies have addressed
the effects of clay amendment on nutrient dynamics in soils.
Reuter (1994) reported a decrease in leaching of phospho-
rus (P) and K after clay addition, Dempster etal. (2012)
described a decrease in
NH+
4
leaching, but no effect for
Biochar (2022) 4:16
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Page 3 of 23 16
NO−
3
, Nguyen and Marschner (2013) found a reduction of
N and P leaching, and Ye etal. (2019)reported a reduction
of
NO−
3
leaching. In a field experiment, Beusch etal. (2019)
observed a significant reduction of
NH+
4-N
and
K+
leach-
ing for clay-amended sandy soils, which remained relatively
stable for 1.5 years, but a contrasting increase of
NO−
3-N
leaching. Tahir and Marschner (2017) found an increase of
NH+
4-N
and P sorption of an Arenosol after the addition of
a high-smectite soil collected from a Vertisol. However, the
knowledge about the effects of clay amendments on the sorp-
tion and retention of nutrients remains limited.
The objective of this study was to enhance the nutrient
sorption capacity of an Arenosol from north-eastern Brazil
by using locally available and inexpensive materials. Locally
produced biochar made of Prosopis juliflora (Sw.) DC. and
clayey material, collected from the A-horizon of a Vertisol
that developed at the bottom of a temporarily dry lake, were
mixed with the Arenosol in different ratios. To illuminate the
sorption behaviour of different nutrients, sorption isotherms
according to Langmuir and Freundlich were derived for
NH+
4-N
,
NO−
3-N
,
K+
, and
PO3−
4-P
for different mixtures of
the Arenosol with biochar and clay, respectively. We hypoth-
esised that the addition of biochar and clay would have sig-
nificant effect on the sorption of (i)
NH+
4-N
, (ii)
NO−
3-N
, (iii)
K+
, and (iv)
PO3−
4-P
. In particular, we expected that biochar
would increase the sorption of
NH+
4-N
and
NO−
3-N
, and that
clay would increase the sorption of
NH+
4-N
,
K+
, and
PO3−
4-P
compared to unamended Arenosol.
2 Material andmethods
2.1 Characterisation ofsubstrates
All materials used in this study have their origin at the
site of a field experiment (8° 57′ 24.1″ S and 38° 15′ 00.4″
W), located in the Itaparica area, Pernambuco state, north-
eastern Brazil. Details about the field site can be found in
Beusch etal. (2019) and Mertens etal. (2017a, b). For the
batch experiment in this study, material from the same site
was used. The soil studied is a Protic Arenosol (according
to IUSS Working Group WRB 2014). It has a sandy texture
with low contents of silt and clay and a low pH (Table1).
For this experiment, a mixed soil sample was taken from 0
to 30 cm depth, air-dried, and sieved to
2 mm
.
The biochar was made from trunks and branches of the
Fabaceae tree Prosopis juliflora (Sw.) DC., which is an
invasive species in the area (Sena etal. 2021) and has,
other than endemic Caatinga species, no restrictions in
cutting. It was produced through slow pyrolysis by a local
charcoal burner in a traditional way, using a clay kiln with
a burrow sealed with corrugated sheet. No data on pyroly-
sis temperature and residence time are available. However,
an estimation of the production temperature based on
temperature-controlled parameters like hydrogen (H), oxy-
gen (O), C, ash, and volatile matter (VM) content, along
with molar O:C and H:Cratios (e.g., Chen etal. 2008;
Spokas 2010; Zhao etal. 2013) was possible. Our data
Table 1 Standard chemical and physical substrate properties for
chemical parameters
n=3
, for physical parameters (specific surface
area (SSA), grain size distribution)
n=2
; SSA was determined in
N2
(
∗
) or
CO2
(
†
); n.d. not determined
For analytical methods, see Sect.2.2
Properties Unit Arenosol Biochar Clay
0B/0C 100B 100C
pH (
H2O
) 5.1 9.1 8.3
pH (
KCl
) 4.2 8.2 7.3
Total C
g kg−1
1.78 852.71 17.07
Total N
g kg−1
0.21 4.76 0.67
Total Al
g kg−1
4.91 0.24 64.14
Total Fe
g kg−1
2.10 0.20 61.00
Total Mn
g kg−1
0.00 0.01 0.87
Total Ca
g kg−1
n.d. 6.50 38.00
Total Mg
g kg−1
0.12 1.17 28.77
Total Na
g kg−1
n.d. 1.14 0.23
Total K
mg kg−1
n.d. 7697.75 17533.69
Total P
mg kg−1
42.93 580.79 488.31
Plant-available K
mg kg−1
27.45 1060.40 288.67
Plant-available P
mg kg−1
1.24 71.20 9.80
Cation exchange capac-
ity
cmolckg−1
0.87 n.d. 72.57
SSA
m2g−1
1.80* 249.15†108.25*
Clay % 4.3 n.d. 69.8
Silt % 0.4 n.d. 30.2
Sand % 95.3 n.d. 0.0
Table 2 Biochar specific properties (
n=3
)
Molar H:C and O:C ratios and O content were calculated; pyrolysis
temperature was estimated based on biochar properties. For details,
see Sect.2.1
Properties Unit Biochar
100B
Total H
g kg−1
31.6
Total O
g kg−1
120.7
Total S
g kg−1
1.0
Molar H:C 0.46
Molar O:C 0.11
Ash content % 2.2
Volatile matter % 19.1
Estimated pyrolysis tempera-
ture
°C Approx. 450
Biochar (2022) 4:16
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16 Page 4 of 23
(Table2) correspond well to those given by Trompowsky
etal. (2005) for two woody biochars made of Eucalyp-
tus saligna and Eucalyptus grandis and to a pine biochar
by Ronsse etal. (2012). These biochars were produced
under slow pyrolysis conditions at a temperature of 450
°C. Consequently, we assume an approximate pyrolysis
temperature of 450 °C, indicating a medium-temperature
biochar, according to Chen etal. (2015). Before use, the
air-dried biochar was pestled and sieved to
<0.5 mm
. We
refrained from washing the biochar before the conduction
of the experiment to simulate real-life field practice in
tropical agriculture, where washing of large amounts of
biochar would (i) not be economically feasible and (ii) lead
to a loss of desirable nutrients.
The clayey material applied in this experiment is an
A-horizon of a Vertisol, originating from the sediment of
a periodically dried-up lake close to the experimental site
described in Beusch etal. (2019) and dominated by a smec-
titic mineralogy. It has a high clay content of almost 70%,
leading to a large specific surface area (SSA) and large CEC
(Table1). The clay contains the following clay minerals:
40% to 60% smectite, 10% to 30% illite, and 20% to 30%
kaolinite, plus less than 10% calcium carbonate, organic
material, and iron oxides (Mertens etal. 2017a). For this
experiment, mixed samples were taken from 0 to 30 cm
depth, air-dried, carefully crushed and sieved to
<0.5 mm
.
2.2 Laboratory analyses
Before all analyses, substrates were air-dried and sieved:
Arenosol to
<2 mm
, clay and biochar to
<0.5 mm
. Sub-
strates were ground in tungsten carbide vessels (MM200,
Retsch, Haan, Germany) for analysis of C, H, N, and sul-
fur (S). The pH was analysed in millipore water and in 1 M
KCl
, in a soil/solution ratio (w/v) of 1:2.5 using an inoLab
pH/Cond Level 1, SenTix 41 electrode (WTW, Weilheim,
Germany). Electric conductivity (EC) was analysed with
the same device, using a WTW TetraCon 325 conductivity
cell. Soil texture was determined by sieving and sedimen-
tation according to DINISO11277. Total C, H, N, and S
concentrations were analysed in triplicate after dry combus-
tion, using an Elementar Analyzer (Vario EL III, Elementar,
Hanau, Germany), while H was only derived for biochar.
Effective CEC was analysed for clay and the Arenosol using
1 M
NH4Cl
extraction with a soil/solution ratio (w/v) of
1:20. The concentrations of extracted aluminium (Al), cal-
cium (Ca), iron (Fe), magnesium (Mg), manganese (Mn),
sodium (Na), and K were analysed with an ICP-OES iCAP
6000 ICP Spectrometer (Thermo Fisher Scientific, Dreieich,
Germany). Plant-available
P
and
K
were determined after
extraction with 0.05 M calcium-acetate-lactate in a soil/
solution ration (w/v) of 1:20. Analysis was conducted with
the ICP-OES (iCAP 6000 ICP Spectrometer, Thermo Fisher
Scientific, Dreieich, Germany). SSA for biochar, Areno-
sol, and clay was measured by gas adsorption, applying the
Brunauer–Emmett–Teller (BET) equation (ISO 9277:1995)
in an Autosorb-1 (Quantachrome, Odelzhausen, Germany),
using
N2
as an adsorbate for clay and Arenosol, and carbon
dioxide (
CO2
) for biochar. For further methodological details
of the conducted gas adsorption measurements, see Wagner
and Kaupenjohann (2014). Total contents of Al, Fe, Mn, Ca,
Mg, Na, K, and P were determined after digestion with
HNO3
(biochar) and aqua regia (Arenosol and clay), respectively.
All substrates were ground and dried at 105°C prior to diges-
tion. For
HNO3
digestion,
5 mL
of 69%
HNO3
(suprapur) was
added to
250 mg
biochar; for aqua regia digestion,
10 mL
aqua regia (1 part 69%
HNO3
suprapur, three parts 37%
HCl
suprapur) was added to
500 mg
Arenosol and clay, respec-
tively. All extracts were heated in pressure-tight vessels for
6h
at 185 °C and were analysed with ICP-OES. An overview
of the chemical and physical parameters of the materials is
given in Table1. Some parameters were derived for biochar
only: Volatile matter content was determined in triplicate
according to DIN 51720 in crucibles covered with lids to
avoid oxygen contact for
7 min
under 900 °C in muffle fur-
nace LV 5/11, controller P330, controller S27” (Nabertherm,
Lilienthal, Germany); ash content was determined in tripli-
cate according to DIN 51719 for
60 min
under 815 °C in the
same furnace; oxygen content was calculated according to
DIN 51733 by subtracting ash content, C, H, N, and S (%
w/w each) from 100; and molar H:C and O:C ratios were
calculated for the biochar used (Table2).
The aqueous solutions obtained from the batch experi-
ments were stored at −18 °C until analysis. Directly after
melting the aqueous solutions,
NH+
4-N
and
NO−
3-N
were
analysed by CFA Auto Analyzer 3 MT7 (SEAL Analytical,
Norderstedt, Germany). In the concentration range from 0
to 5 mg
L−1
,
PO3−
4-P
was measured with the same CFA Auto
Analyzer, when above
5 mg L−1
, P was detected by ICP-OES
iCAP 6000 ICP Spectrometer (Thermo Fisher Scientific,
Dreieich, Germany). In the range from 0 to 2 mg
L−1
,
K+
was analysed by flame AAS (1100B Perkin Elmer, Rodgau,
Germany) and caesium was added to avoid ionisation;
K+
concentrations above
2 mg L−1
were analysed by ICP-OES
iCAP 6000 ICP Spectrometer (Thermo Fisher Scientific,
Dreieich, Germany).
2.3 Batch experiment
Several mixtures of Arenosol–biochar and Arenosol–clay, as
well as the plain Arenosol, biochar, and clay were tested for
their sorption capacity for
NH+
4-N
,
NO−
3-N
,
K+
, and
PO3−
4-P
according to OECD guideline 106 for testing of chemicals
(Adsorption/Desorption using a Batch EquilibriumMethod
OECD 2000). Where necessary, the method was adjusted to
the experimental needs of this study.
Biochar (2022) 4:16
1 3
Page 5 of 23 16
Labelling of the samples is based on the percentage of
soil conditioner mixed with the Arenosol, “B” refers to
biochar, “C” to clay. The label 0B/0C corresponds to the
unamended Arenosol, with no biochar or clay addition; 1B
refers to a biochar content of 1% (w/w), whereas 100C refers
to the clay itself, 100B to the biochar itself, etc. All sorption
experiments have been conducted with unamended Arenosol
(0B/0C), biochar (100B), clay (100C), and different mixtures
of the Arenosol with biochar and clay, respectively. The mix-
tures cover a realistic range of applicable biochar and clay
amounts (1%, 2.5%, 5%, 10%; w/w).
The nutrient solution was composed to reflect a realis-
tic fertilisation scheme. Batch experiments were conducted
in a quarternary system containing N, P, and K in a mass
ratio of 1:0.4:1, where N consisted of 50% each of
NH+
4-N
and
NO−
3-N
. The stock solution was produced by dissolving
ammonium nitrate (
NH4NO3
, Chemical Abstracts Service
(CAS) number 6484-52-2, Merck) and water-free dipotas-
sium hydrogen phosphate (
K2HPO4
, CAS number 7758-11-
4, Merck) in millipore water with an EC of
<0.055 μS cm−1
.
Nomenclature of the samples referred to the assumed annual
average (AVG) fertiliser input of
200 kg N ha−1
, set as
1AVG. The 1AVG NPK solution had a pH of 7.7 and an EC
of
100.1 μS cm−1
. The six applied batch equilibrium concen-
trations were adapted to this annual average; consequently,
2AVGreferred to
400 kg N ha−1
, 0.5AVG to
100 kg N ha−1
.
All concentrations are displayed in Table3. Hence, the
concentrations of the calculated target nutrient solutions
applied in the batch experiment ranged from
3.1 mg L−1
(0.25AVG) to
50 mg L−1
(4AVG) for N and K, correspond-
ing to
1.6 mg L−1
and
25 mg L−1
NH+
4-N
and
NO−
3-N
, and
1.2 mg L−1
to
19.8 mg L−1
PO3−
4-P
.
Prior to conduction of the batch experiments, all vessels and
devices used were rinsed in a 0.1 M
HCl
acid bath. Exactly
10 g
of substrate was weighed into
100 mL
Duran GL45 boro-
silicate glass vessels in five replicates for the variants with only
millipore water (0AVG) and in triplicate for the treatments
with addition of NPK solution (0.25AVG, 0.5AVG, 1AVG,
2AVG, 4AVG). Also, controls with no soil but only the NPK
solution (
C0
) were set up in five replicates. These initial NPK
concentrations were used as a reference value to calculate the
sorbed amount of nutrients. According to OECD guideline
106, the samples were pre-equilibrated at room temperature
overnight in
45 mL
millipore water (13 to 21 h) on an orbital
shaker (KS 501 D, IKA, Staufen, Germany) at
130 rpm
. After
pre-equilibration,
5 mL
of tenfold-concentrated NPKsolu-
tion was added to reach the target concentration and soil/
solution ratio of 1:5 (w/v) (
10 g
soil,
50 mL
solution). This
suspension was equilibrated for 24 h on an end-over-end
shaker at
10 rpm
. After shaking, the suspension was allowed
to sediment for 5 to 10 min. Then, it was filtrated in a slow-
draining NPK-free folded filter, grade 131,
125 mm
diameter
of Munktell, Bärenstein, Germany, which was rinsed with
100 mL
of millipore water before use. This pre-filtrate was
discarded. All filtrates were ultracentrifugated (Optima L-90K,
rotor 45Ti, Beckmann Coulter, Krefeld, Germany) for
20 min
at
39, 000 rpm
and 10 °C to match the preconditions in clean-
liness of particles for instrumental analysis. After this step,
all samples were frozen at −18 °C until further analysis. As
the preconditions for particle-free samples were not fulfilled
by ultrazentrifugation, all thawed samples were filtered in a
0.45 μm
glass fibre syringe pre-filter (Sartorius, Göttingen,
Germany) before being analysed. Due to its low stability in
aqueous solutions,
NH+
4-N
was analysed promptly after thaw-
ing the samples to minimise the risk of volatilisation losses.
2.4 Derivation ofsorption isotherms according
toFreundlich andtoLangmuir
The nutrient concentrations in the aqueous phase and the initial
concentrations of the NPK solution without substrate were
analysed, and the nutrient concentrations of the solid phase
were calculated using the following equation:
where
Cs
represents the calculated amount of nutrients in
the solid phase at equilibrium (
mg kg−1
), v refers to the vol-
ume of the solution (
L
), m to the mass of the substrate (
kg
),
C0
is the initial nutrient concentration of the NPK solution
without substrate (
mg L−1
), and
Cw
is the analysed nutrient
concentration in the aqueous phase after
24 h
of equilibra-
tion (
mg L−1
).
(1)
C
s=
v
m
×(C0−Cw)
,
Table 3 Applied batch
equilibrium concentrations
(0AVG−4AVG) and
corresponding annual
fertilisation schemes for
Ntotal
,
NH+
4-N
,
NO−
3-N
,
PO3−
4-P
, and
K+
Concentrations
Ntotal
(
kg ha−1
)
NH+
4-N
(
kg ha−1
)
NO−
3-N
(
kg ha−1
)
PO3−
4-P
(
kg ha−1
)
K+
(
kg ha−1
)
0AVG 0 0 0 0 0
0.25AVG 50 25 25 19.8 50
0.5AVG 100 50 50 39.6 100
1AVG 200 100 100 79.2 200
2AVG 400 200 200 158.4 400
4AVG 800 400 400 316.8 800
Biochar (2022) 4:16
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16 Page 6 of 23
As most substrates contained water-soluble nutrients,
release of nutrients into the equilibrium solution occurred,
so the initial nutrient concentrations without addition of
NPK were not zero. In several cases, this led to a greater
Cw
than
C0
, which resulted in negative values for the cal-
culated
Cs
. Hence, the dimensionless term a was added
to the Freundlich and Langmuir equations to display the
intersection of the isotherm with the y-axis, indicating
the initial concentration of the water-soluble fraction of a
nutrient in the substrate. For describing nutrient sorption
on the different materials, two adsorption isotherm equa-
tions were applied:
Freundlich:
Langmuir:
where
Cs
represents the amount of nutrient removed at equi-
librium (
mg kg−1
), a displays the additional term to display
the initial concentration of the water-soluble fraction of a
nutrient in the substrate,
Kf
is the Freundlich affinity coef-
ficient (
mg(1−n)
L
−n
g
−1
),
Cw
is the equilibrium concentra-
tion of the sorbate in solution (
mg L−1
), n is the Freundlich
linearity constant,
Kl
represents the Langmuir bonding
term related to interaction energies (
L mg−1
), and
qmax
is
the Langmuir maximum adsorption capacity (
mg kg−1
). All
sorption isotherms were calculated by using the open source
Linux-based software QtiPlot (Version 0.9.9-rc10).
All isotherms were derived from six different NPK con-
centrations (0AVG, 0.25AVG, 0.5AVG, 1AVG, 2AVG,
4AVG), with the exception of the isotherms of
NH+
4-N
to
clay. Due to inconsistencies with the sample preparation,
(2)
Cs=a+Kf×Cn
w,
(3)
C
s=a+
K
l
×q
max
×C
w
1+K
l
×C
w
,
the sorption data of 1AVG and 2AVG for
NH+
4-N
had to
be discarded. Therefore, the sorption isotherms of
NH+
4-N
were only derived from four different NPK concentrations
(0AVG, 0.25AVG, 0.5AVG, 4AVG).
2.5 Calculation ofrelative difference ofinitial
andfinal NPK concentrations insolution
In addition to the sorption isotherms, the relative differ-
ences (RD) were calculated for all substrate mixtures and
NPK solution concentrations using the following equation:
where RD displays the relative difference between
Cw
and
C0
[%],
Cw
is the final nutrient concentration in solution
(
mg L−1
), and
C0
the initial concentration of the NPK solu-
tion without substrate (
mg L−1
). RD values above 0 indicate
sorption, an RD of 100% means that all available nutrients
were sorbed by the substrate, whereas RD values below zero
indicate release of water-soluble nutrients from the substrate
into the solution.
3 Results
3.1 Sorption of
NH+
4-N
3.1.1
NH+
4-N
sorption onbiochar
Release of
NH+
4-N
from biochar (100B) equilibrated in mil-
lipore water for
24 h
was low, with only
0.5 mg kg−1
. In con-
trast, release from the Arenosol was almost eightfold higher
(Table4). An increasing share of biochar in the substrate
(4)
RD
=
(C
0
−C
w
)×100
C0
,
Table 4 Nutrients released
from the solid phase (
Cs
) of the
Arenosol, biochar (B), clay (C),
and their mixtures into aqueous
solution after
24 h
of shaking in
millipore water without addition
of NPK solution (0AVG;
n=5
)
The second column indicates the percentage (w/w) of biochar or clay in the substrate mixture. Concentra-
tions above 0 indicate release of water-soluble nutrients from the substrate
Substrate sample Description
NH+
4-N
(
mg kg−1
)
NO−
3-N
(
mg kg−1
)
K+
(
mg kg−1
)
PO3−
4-P
(
mg kg−1
)
0B/0C Arenosol 3.9 27.1 13.8 0.6
1B 1% biochar 3.3 12.8 22.5 0.3
2.5B 2.5% biochar 3.4 7.2 39.0 0.7
5B 5% biochar 2.4 5.8 81.8 0.6
10B 10% biochar 0.7 4.1 177.8 0.8
100B 100% biochar 0.5 3.1 1734.4 24.5
1C 1% clay 4.1 2.6 17.2 0.1
2.5C 2.5% clay 4.0 2.7 22.2 0.0
5C 5% clay 3.4 3.1 20.2 0.0
10C 10% clay 2.6 4.3 23.1 0.0
100C 100% clay 1.7 19.0 27.9 0.0
Biochar (2022) 4:16
1 3
Page 7 of 23 16
decreased the amount of
NH+
4-N
being dissolved. No
NH+
4-N
release occurred for any substrate mixture that received the
NPK solution (Fig.3a). The Freundlich model described
the
NH+
4-N
sorption to biochar better for the unamended
Arenosol and all biochar-Arenosol mixtures (
R2
0.96 to
0.99; Table5), whereas the sorption to biochar itself was
better described by the Langmuir model (
R2=0.68
). As
displayed in Figs.1a and 3a, the addition of lower doses
of biochar to sandy soil had no effects on sorption or even
decreased
NH+
4-N
sorption. When compared with the una-
mended Arenosol (0B/0C), the addition of 1%, 2.5%, 5%
did not show any effect. Only the addition of 10% and 100%
showed an increase of
NH+
4-N
sorption to 27.6% and 86.2%,
respectively, when compared to
C0
at the highest added NPK
concentration (4AVG).
3.1.2
NH+
4-N
sorption onclay
Clay released only the very low amount of
1.7 mg NH+
4
-N kg
−1
for 100C (Table4); a larger amount
of clay also decreased release of
NH+
4-N
. Only marginal
release occurred for the substrate mixtures with a low
share of clay (1C, 2.5C) that received the NPK solution
(Fig.3b). Half of the isotherms were better described by
the Freundlich model (0C, 2.5C, 5C) than by Langmuir
(1C, 10C, 100C), whereas the differences were marginal.
It is noticeable that the maximal sorption capacity (
qmax
=
51.8 mg kg−1
) of the Arenosol decreased when 1% clay was
added (
qmax
=
27.9 mg kg−1
). Addition of 2.5% and 5% clay
also evoked only marginal effects. Starting from 10%, clay
addition showed a visible effect of increased
NH+
4-N
sorp-
tion (
qmax
=
97.6 mg kg−1
). However, due to loss of experi-
mental data (as described in Sect.2.4), the results of this
treatment are not based on the five test substance concen-
trations recommended by OECD guideline 106 (OECD
2000), hence their validity may be limited compared to
the other treatments in this study.
3.2 Sorption of
NO−
3-N
3.2.1
NO−
3-N
sorption onbiochar
Unamended Arenosol showed the greatest
NO−
3-N
release
of
27.1 mg kg−1
. Addition of biochar subsequently reduced
NO−
3-N
release to only
3.1 mg kg−1
for the biochar (Table4).
The Langmuir model could not be applied to any of the
sandy mixtures for
NO−
3-N
, only to 100B (
R2=0.87
).
For the unamended Arenosol, no sorption model could
be applied at all. In contrast, the Freundlich model could
be applied to all substrate mixtures. The
R2
for Freun-
dlich ranged from only 0.64 (1B) to 0.86 (100B), with the
Freundlich linearity constant (n) being above 1, resulting
in convex isotherms (see Fig.1c). Only the addition of
the highest NPK concentration (4AVG) led to sorption of
NO−
3-N
to the unamended Arenosol and all Arenosol mix-
tures (Figs.1c, 3c). Only the biochar itself did not release
NO−
3-N
and had a
qmax
of
183.9 mg kg−1
. All other concen-
trations provoked release of
NO−
3-N
. However, as displayed
in Fig.3c,
NO−
3-N
release was highest in the unamended
Arenosol; the larger the amount of biochar present in the
substrate, the less
NO−
3-N
was released. With the addition of
higher NPK concentrations,
NO−
3-N
release diminished, as
indicated by less negative RD values. On the other hand, the
RD for biochar decreased from 98.5% (0.25AVG) to 47.7%
(4AVG), indicating that the maximum sorption capacity of
the biochar mixture was already reached.
3.2.2
NO−
3-N
sorption onclay
In contrast to biochar, addition of clay led to greater
NO−
3-N
release rates compared to the unamended Arenosol, up to
19 mg kg−1
for the clay itself (Table4). A Freundlich iso-
therm could only be derived for the unamended Areno-
sol, whereas the creation of Langmuir isotherms was not
possible. However, the very low
R2
of 0.002 indicated no
relation between sorbate and sorbent. Release of
NO−
3-N
was observed up to
30.1 mg kg−1
for clay itself at the high-
est NPK concentration (4AVG). Even though this was the
greatest amount released, the RD compared to the initial
NO−
3-N
content decreased to −20.5% (Fig.3d). For the low-
est NPK dose (0.25AVG; data not shown), the RD was as
large as −320%, indicating more than three times as much
NO−
3-N
release as initially added to the 100C substrate.
3.3 Sorption of
K+
3.3.1
K+
sorption onbiochar
All mixtures, including the ones that received no NPK
solution, showed release of
K+
. The unamended Areno-
sol released
13.8 mg K+kg−1
. With greater biochar share,
K+
release significantly increased up to
1734.4 mg kg−1
(Table4). The
R2
was greater for Freundlich models of 0B,
1B, and 2.5B (up to 0.99); for 5B and 10B, the Langmuir
model fitted better, with
R2
decreasing to only 0.12 for 10B.
The
K+
release from the biochar itself could not be described
by any of the models. Biochar showed very strong
K+
release
of up to almost
400 mg kg−1
for the highest NPK concentra-
tion added (4AVG; data not shown). Figure2a shows the
clear relation of biochar share and the increase of
K+
release:
for the unamended Arenosol, the two highest NPK concen-
trations led to sorption of
K+
, whereas the addition of lower
doses only released
K+
. The addition of 1% biochar to the
Biochar (2022) 4:16
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16 Page 8 of 23
Table 5 Sorption parameters of Freundlich and Langmuir adsorption isotherms for
NH+
4-N
,
NO−
3-N
,
K+
, and
PO3−
4-P
to the unamended Areno-
sol (0B/0C), biochar (100B), clay (100C), and their mixtures
The numbers for the substrate samples refer to the percentage (w/w) of the biochar (B) or clay (C) addition to the Arenosol. Values are shown for
the intersection of the isotherm with the y-axis (a), Freundlich affinity coefficient (
Kf
), Freundlich linearity constant (n), the Langmuir bonding
term (
Kl
), Langmuir maximum adsorption capacity (
qmax
), coefficients of determination (
R2
), and standard deviations of the respective isotherms
(STDV). In several cases, no sorption isotherms could be determined (n.d.)
Substrate sample Freundlich Langmuir
a
Kf
(
mg(1−n) L−n g−1
)n
R2
STDV a
Kl
(
L mg−1
)
qmax
(
mg kg−1
)
R2
STDV
NH+
4-N
0B/0C −6.19 4.595 0.625 0.961 1.748 −3.50 0.029 75.033 0.954 1.897
1B −7.25 5.790 0.560 0.985 1.185 −3.94 0.046 57.469 0.979 1.395
2.5B −6.34 5.081 0.587 0.977 1.423 −3.54 0.041 59.191 0.972 1.569
5B −4.16 3.743 0.682 0.987 1.084 −2.62 0.031 71.317 0.984 1.205
10B −1.55 3.874 0.768 0.981 1.709 −0.87 0.029 100.411 0.982 1.657
100B 10.67 32.565 0.762 0.636 23.200 9.66 0.299 172.690 0.681 21.700
1C −7.47 4.409 0.486 0.785 3.433 −5.01 0.073 27.902 0.792 3.381
2.5C −6.36 3.207 0.713 0.981 1.441 −5.28 0.034 63.372 0.981 1.441
5C −9.98 8.274 0.545 0.993 1.167 −6.37 0.080 60.948 0.993 1.156
10C −9.27 10.121 0.632 0.998 0.975 −6.35 0.077 97.580 0.998 0.885
100C −122.13 160.690 0.263 0.998 2.117 −38.48 0.691 196.223 0.998 1.991
NO−
3-N
0B/0C −13.02 0.216 0.728 0.002 12.798 n.d. n.d. n.d. n.d. n.d.
1B −8.69 0.105 1.603 0.631 3.706 n.d. n.d. n.d. n.d. n.d.
2.5B −5.46 0.030 1.982 0.838 2.236 n.d. n.d. n.d. n.d. n.d.
5B −4.15 0.053 1.726 0.802 2.031 n.d. n.d. n.d. n.d. n.d.
10B −1.63 0.003 2.738 0.842 2.071 n.d. n.d. n.d. n.d. n.d.
100B 4.21 4.980 0.940 0.859 7.829 3.29 0.033 183.891 0.867 7.602
1C n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
2.5C n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
5C n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
10C n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
100C n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
K+
0B/0C −20.18 3.404 0.754 0.987 1.962 −17.46 0.009 190.048 0.987 1.988
1B −36.06 6.180 0.595 0.981 2.102 −28.52 0.014 134.467 0.979 2.206
2.5B −40.82 0.223 1.342 0.933 3.256 n.d. n.d. n.d. n.d. n.d.
5B −104.34 7.735 0.486 0.575 7.800 −93.04 0.015 93.785 0.579 7.768
10B −452.29 208.991 0.094 0.119 21.668 −204.94 0.018 116.322 0.120 21.645
100B n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
1C −26.51 4.882 0.475 0.864 3.036 −21.32 0.025 46.095 0.869 2.975
2.5C −20.04 0.071 1.771 0.863 7.022 n.d. n.d. n.d. n.d. n.d.
5C −26.00 1.048 1.195 0.993 2.140 n.d. n.d. n.d. n.d. n.d.
10C −36.08 2.186 1.152 0.996 2.257 n.d. n.d. n.d. n.d. n.d.
100C −96.74 6.137 1.412 0.996 4.473 n.d. n.d. n.d. n.d. n.d.
PO3−
4-P
0B/0C −26.51 33.172 0.115 0.994 0.561 −1.28 0.516 21.718 0.975 1.165
1B −4.95 10.796 0.315 0.994 0.521 −0.25 0.275 24.674 0.984 0.970
2.5B −4.68 9.732 0.367 0.981 1.118 −0.48 0.205 28.277 0.970 1.415
5B −9.55 15.699 0.262 0.996 0.544 −1.10 0.311 27.354 0.987 0.954
10B −13.41 20.132 0.242 0.985 1.184 −1.58 0.324 30.923 0.973 1.564
100B −21.51 0.016 2.427 0.563 6.491 n.d. n.d. n.d. n.d. n.d.
1C −2.87 16.705 0.347 0.986 1.568 0.65 0.543 38.709 0.992 1.236
2.5C −1.27 16.046 0.429 0.987 1.795 0.72 0.388 49.846 0.995 1.136
5C −0.54 18.010 0.461 0.991 1.743 1.37 0.349 60.766 0.991 1.724
10C −0.27 20.530 0.511 0.995 1.491 2.10 0.276 79.860 0.988 2.255
100C 4.55 91.896 0.690 0.986 3.858 4.82 1.016 176.397 0.985 3.942
Biochar (2022) 4:16
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Page 9 of 23 16
Arenosol resulted in greater
K+
release, but also showed
some sorption for the highest NPK dose. The Arenosol–bio-
char mixtures with 2.5%, 5%, and 10% biochar exclusively
showed
K+
release. The increase of
K+
release responded to
the share of biochar in the mixture. However, when set in
relation to
C0
, all mixtures showed a release of
K+
(Fig.3e).
The addition of biochar clearly led to a strong increase of
K+
release, with the biochar itself evoking
K+
release up
to 4000 times the amount of
K+
added at the lowest addi-
tion of NPK solution (0.25AVG). The RD decreased with
increasing NPK concentration to eight times the amount of
K+
released for 4AVG.
3.3.2
K+
sorption onclay
All clay mixtures showed medium
K+
release of
17.2 mg kg−1
(1C) to
27.9 mg kg−1
(100C; Table4). The Freundlich model
described all mixtures better than Langmuir;
R2
values were
high, between 0.86 and 0.99. The isotherms of 0C and 1C
had an n of below 1, whereas the others showed a convex
form with n above 1. When low concentrations of NPK were
added,
K+
was only released for the two highest concentra-
tions applied (2AVG and 4AVG); all isotherms showed
sorption (Figs.2a and 3e). These figures also showed
that for 1C and 2.5C, sorption decreased compared to the
Fig. 1 Measured nutrient concentrations in the aqueous solutions
(
Cw
) and calculated nutrient concentrations in the substrates (
Cs
) for
NH+
4-N
(top row: a, b) and
NO−
3-N
(bottom row: c, d), and for sub-
strates with biochar (left side: a, c) and clay share (right side: b, d).
0B/0C refers to the unamended Arenosol, 100B to the biochar itself,
and 100C to the clay. Symbols, lines, and colours are explained in
the legend box below the figures. The numbers in the legend indi-
cate the percentage (w/w) of biochar (B) or clay (C) addition to the
Arenosol. Data points below the x-axis indicate nutrient release; data
points above indicate nutrient sorption to the substrate. Adsorption
isotherms according to Freundlich were applied if possible. Due to
loss of data, the
NH+
4-N
sorption isotherms for clay were only derived
from four different NPK concentrations instead of six. For details, see
Sect.2.4
Biochar (2022) 4:16
1 3
16 Page 10 of 23
unamended Arenosol. In contrast, clay addition to the
Arenosol above 5% increased
K+
sorption of the Arenosol.
A steep isotherm with n of 6.1 indicated strong sorption of
K+
to the clay itself.
3.4 Sorption of
PO3
−
4-P
3.4.1
PO3
−
4-P
sorption onbiochar
The unamended Arenosol and its mixtures with biochar only
released very low amounts of
PO3−
4-P
, up to
0.8 mg kg−1
,
whereas the plain biochar showed around 30 times more
PO3−
4-P
release (Table4). The biochar only provoked
release of
PO3−
4-P
; no sorption model could be applied.
For all other substrates, Freundlich described the sorption
best (
R2
of 0.98 and 0.99, respectively). When compared
to initial
PO3−
4-P
concentrations added, the
PO3−
4-P
release
decreased from more than 300% (0.25AVG) to slight sorp-
tion of 0.9% for 4AVG (Fig.3g). Figs.2c and 3g show
that more biochar addition led to gradually greater
PO3−
4-P
sorption. However, the differences were marginal; when
displayed with the Langmuir model, the initial
qmax
of the
Arenosol (
21.7 mg kg−1
) slightly increased to
24.7 mg kg−1
(1B), and finally to
30.9 mg kg−1
for 10B, not indicating a
great increase.
3.4.2
PO3
−
4-P
sorption onclay
All mixtures containing clay without NPK addition
showed no
PO3−
4-P
release at all (Table4). With the excep-
tion of 2.5C, the Freundlich model explained the sorption
of
PO3−
4-P
to clay best. All
R2
values were high (0.99).
All substrates showed only low or no
PO3−
4-P
release, but
Fig. 2 Measured nutrient concentrations in the aqueous solutions
(
Cw
) and calculated nutrient concentrations in the substrates (
Cs
)
for
K+
(top row: a, b) and
PO3
−
4-P
(bottom row: c, d), and for sub-
strates with biochar (left side: a, c) and clay share (right side: b, d).
0B/0C refers to the unamended Arenosol, 100B to the biochar itself,
and 100C to the clay. Symbols, lines, and colours are explained in
the legend box below the figures.The numbers in the legend indi-
cate the percentage (w/w) of biochar (B) or clay (C) addition to the
Arenosol. Data points below the x-axis indicate nutrient release; data
points above indicate nutrient sorption to the substrate. Adsorption
isotherms according to Freundlich were applied if possible. Due to
strong
K+
release, axis breaks of x- and y-axis were applied for (a)
and (b)
Biochar (2022) 4:16
1 3
Page 11 of 23 16
sorption. When clay was added,
PO3−
4-P
sorption increased
gradually (Fig.2d). For Langmuir, there was an increase
of
qmax
from 21.7 (0C) to 38.7 (1C) and 49.8 (2.5C), up
to a
qmax
of
176.4 mg kg−1
for 100C. The sorption isotherm
of 100C was the steepest. In relation to the initially added
NPK concentration, the relative difference of the equilib-
rium concentration for the 100C remained very high at
above 95% (Fig.3h). However, the sorption capacity for
the clay–Arenosol mixtures was already decreasing, indi-
cating that they had already reached their sorption capac-
ity. As the majority of isothermswere better described
by Freundlich than by the Langmuir model, indicated by
greater coefficient of determination (
R2
; see Table5), all
Fig. 3 Relative difference (RD) of the nutrient concentration in
the aqueous phase (
Cw
) compared to the initial concentration of the
NPK solution (
C0
) for samples with addition of NPK solution. Col-
ours are explained in the legend box below the figures.The diagrams
show RD values for the different concentrations of the NPK solution
(0.25AVG, 0.5AVG, 1AVG, 2AVG, 4AVG), nutrients (from top
to bottom: a, b:
NH+
4-N
; c, d:
NO−
3-N
; e, f:
K+
; g, h:
PO3
−
4-P
), and
different shares of biochar (left side) and clay (right side) in the sub-
strates (0B/0C = unamended Arenosol; 1B/1C = 1% (w/w) biochar
or clay addition, 2.5B/2.5C = 2.5% biochar or clay; 5B/5C = 5% bio-
char or clay; 10B/10C = 10% biochar or clay; 100B = 100% biochar;
100C = 100% clay). Data points below the x-axis (
x=0
) indicate
nutrient release; data points above indicate nutrient sorption to the
substrate. Error bars display the standard error. Due to loss of data,
the RD of 1AVG and 2AVG cannot be displayed in diagram (b); for
details, see Sect.2.4. For
NO−
3-N
,
K+
, and
PO3
−
4-P
, axis breaks of the
y-axis were applied
Biochar (2022) 4:16
1 3
16 Page 12 of 23
sorption isotherms displayed in this study were derived by
the Freundlich equation (Figs.1 and 2).
4 Discussion
The nutrient adsorption of most substrates and substrate
mixtures in our study was better described by Freundlich
than by the Langmuir model, in particular for the una-
mended Arenosol, the clay itself, most Arenosol–clay
mixtures, and the Arenosol–biochar mixtures up to 5%
biochar addition. This implies that in these cases most
multisite adsorption occurred on heterogeneous surfaces
(Freundlich) rather than monolayer adsorption with no
interactions between the adsorbed molecules (Langmuir)
(e.g., Limousin etal. 2007; Oliveira etal. 2017; Rens etal.
2018; Yao etal. 2013). The latter may apply for mixtures
with 10% biochar addition and for the biochar itself, where
R2
of the Langmuir isotherm is greater thanthat of the
Freundlich model (see Table5). However, the differences
in
R2
between both adsorption models remain very low.
4.1 Sorption of
NH+
4-N
4.1.1
NH+
4-N
sorption onbiochar
The presence of biochar in the Arenosol showed no effects
or even decreased the maximum sorption capacity
qmax
for the lower doses
<10
% but increased
qmax
by 34% for
10B and 130% for 100B. Only a small number of studies
conducted batch experiments with mixtures of soil and
biochar, most of them reporting only marginal or no effects
on
NH+
4-N
sorption. Gronwald etal. (2015), for example,
found a slight increase of
NH+
4-N
sorption of a sandy loam
after addition of biochars made of woodchips (up to 8%)
and digestate (up to 17%). The addition of the same bio-
chars to a silty loam showed no effects or even resulted in
NH+
4-N
release. Rens etal. (2018)reported similar find-
ings: a slight but significant increase of
NH+
4-N
sorption
after application of a woody mid-temperature biochar to
a sandy soil at lower
NH+
4-N
concentration of the nutrient
solution added. However, the same biochar significantly
decreased
NH+
4-N
sorption with greater biochar addition
when added to a silty clay loam soil. Alling etal. (2014)
found no significant changes in
NH+
4-N
concentration after
addition of a woody low-temperature biochar to seven
acidic tropical peat and mineral soils. These results are
in line with our findings for biochar-soil mixtures. Most
batch equilibrium experiments were conducted with pure
biochar and showed significant sorption of
NH+
4-N
(e.g.,
Aghoghovwia etal. 2020; Cui etal. 2016; Fidel etal.
2018; Li etal. 2018; Liu etal. 2016; Zheng etal. 2018).
Takaya etal. (2016), for example, revealed 10% to 13%
sorption of
NH+
4-N
in the initial nutrient solution for bio-
chars from different feedstocks with pyrolysis tempera-
tures between 400 and 650 °C. Several authorsreported
increasing
NH+
4-N
sorption capacity with decreasing
pyrolysis temperature, with maximum sorption capacities
for low-temperature biochars at pyrolysis temperatures
below 400 °C (e.g., Li etal. 2018; Zheng etal. 2013).
The
qmax
of our biochar, produced at approx. 450 °C, is
172.7 mg kg−1
and rather low.
Surface complexation of the nutrient with negatively
charged oxygen-containing functional groups like car-
boxyl and carbonyl is an important mechanism for
NH+
4
sorption to biochar surfaces (e.g., Cui etal. 2016; Kizito
etal. 2015; Zheng etal. 2013, 2018). With higher pyroly-
sis temperatures, the total content of
O
and total amount
of acidic oxygen-containing functional groups on the sur-
face of biochars decrease (Zheng etal. 2013), leading to a
reduction in negative charge on the biochar surface (Chen
etal. 2008). Consequently, less surface complexation and
less electrostatic attraction occur, resulting in a decrease
of
NH+
4
sorption with higher pyrolysis temperatures. This
explains the higher sorption capacity of low-temperature
biochars for
NH+
4
compared to high-temperature biochars.
The negative charge of biochar surfaces leads to electro-
static attraction of the positively charged
NH+
4
, resulting in
cation exchange;
Al3+
,
Ca2+
, and
Mg2+
may be exchanged
by
NH+
4
(e.g., Cui etal. 2016; Ding etal. 2010; Gai etal.
2014; Li etal. 2018; Zheng etal. 2013, 2018). In con-
trast, sequestration in pores plays no decisive role for
NH+
4
adsorption to biochars (e.g., Cui etal. 2016; Zheng etal.
2013). In our study, biochar addition of less than 10%
showed no or converse effects, whereas
NH+
4-N
sorption to
biochar itself sharply increased. This may result from the
larger surface area offering more binding sites for
NH+
4-N
compared to the Arenosol.
Another effect that may or may not have contributed
to the results in this studywas
NH3
volatilisation. As the
pH (
H2O
) of the biochar was alkaline (9.1) and the pH of
the NPK solution addedwas relatively neutral (7.7),
NH3
volatilisation that can occur at pH values above 7 (Som-
mer etal. 2004) may have taken place. In batch experi-
ments, nutrient sorption to the solid phase is calculated
by the difference between initial and final nutrient solution
in the aqueous phase; however, the mechanisms behind
the nutrient decrease remain unknown. Other processes
like decomposition, precipitation, or volatilisation may
occur and lead to an overestimation of the actual amount
of nutrient sorbed. Hence, we may misinterpret
NH3
volatilisation as
NH+
4-N
“sorption”, in particular for the
biochar itself (Fig.1a). This is in line with Wang etal.
(2015a), who assumed
NH3
volatilisation to be the reason
for incomplete recovery of only about 60% of
NH+
4-N
after
Biochar (2022) 4:16
1 3
Page 13 of 23 16
a batch adsorption experiment with biochar of high pH
values. However, as no measures were applied to detect
NH3
volatilisation, possible
NH3
volatilisation cannot be
excluded, confirmed, or quantified.
4.1.2
NH+
4-N
sorption onclay
Clay addition also enhanced
NH+
4-N
sorption of the
Arenosol, whereas the maximum sorption capacity
(
qmax
) of the clay itself was
196.22 mg kg−1
, 13.6% greater
than that for biochar. These results are in line with sev-
eral studies that reported sorption of
NH+
4-N
onto clayey
material. In a batch experiment using a clayey sediment
with 55% calciummontmorillonite content, Baker and
Fraij (2010) found
NH+
4-N
removal rates from aque-
ous solution up to 45%. Alshameri etal. (2018) evalu-
ated six natural clay minerals for their suitability to
remove
NH+
4-N
from waste water, where vermiculite and
montmorillonite showed the highest
NH+
4-N
maximum
adsorption capacities of
50.06 mg g−1
and
40.84 mg g−1
,
respectively. Kaolinite, in contrast, had a maximum
adsorption capacity of
15.58 mg g−1
; the Vertisol used
in our study contained 20% to 30% Kaolinite. Zhu etal.
(2011), who tested a wide range of materials for wet-
land construction,reported a maximum
NH+
4-N
adsorp-
tion capacity of
3.33 mg g−1
for the 2:1 clay mineral ver-
miculite that had a medium shrink-swell capacity. Only
two studies were found that conducted batch experiments
with
NH+
4-N
on clayey material as soil amendment. Both
studies did not derive sorption isotherms. Dempster etal.
(2012) conducted a sorption experiment with clayey sub-
soil that had a clay content of 60.4% and mainly consisted
of kaolinite and quartz. Maximal
NH+
4-N
sorption from
aqueous solution was 32%. Tahir and Marschner (2017)
mixed a high-smectite subsoil (73% clay content) and a
low-smectite subsoil (42% clay content) with an Arenosol
(3% clay content). The addition of 20% (w/w) clayey sub-
soil led to a 20-fold increase of
NH+
4-N
sorption for the
high-smectite subsoil (
139 mg g−1
) and a 14-fold increase
for the low-smectite (
99 mg g−1
) subsoil.
Several studies claim cation exchange to be the main
sorption mechanism for
NH+
4
to clay (e.g., Alshameri etal.
2018; Zhu etal. 2011). This is a result of the large chemi-
cally active surface area, the variable net negative charge,
and variable interlayer spacing of the predominant 2:1
smectite clay minerals that account for 40% to 60% of the
clay applied in our study. Smectites exhibit a large cation
exchange capacity; in our study, the addition of 1% clay
increased the initial CEC of the Arenosol of
0.9 cmolckg
−
1
3.8 fold (data not shown). Therefore, we assume cation
exchange to be the main mechanism for
NH+
4-N
sorption
on clay in our study. As the pH of the NPK solution in
this batch experiment was 7.7 and the pH (
H2O
) of the
clay was 8.0, volatilisation of
NH3
might have occurred.
However, as in the case of biochar mentioned above, this
possible mechanism cannot be excluded.
4.2 Sorption of
NO−
3-N
4.2.1
NO−
3-N
sorption onbiochar
The Arenosol showed distinct
NO−
3-N
release that was
diminished by increasing biochar share. In contrast, undi-
luted biochar sorbed
NO−
3-N
up to a
qmax
of
183.89 mg kg−1
.
Slightly rising
Kf
values with increasing biochar share indi-
cate a marginal increase of
NO−
3-N
sorption with increasing
biochar share. Several studies have tested biochars for their
ability to sorb
NO−
3
. Most studiesrevealed a close correla-
tion between high pyrolysis temperatures and
NO−
3
sorption.
With production temperatures up to 550 °C, no significant
NO−
3
sorption on low-temperature biochars could be detected
(e.g., Alling etal. 2014; Hale etal. 2013; Hollister etal.
2013; Li etal. 2018; Paramashivam etal. 2016). In contrast,
regardless of feedstock, many studiesreported
NO−
3
sorption
on high-temperature biochars above 500 °C (e.g., Chintala
etal. 2013; Fatima etal. 2021; Fidel etal. 2018; Pratiwi
etal. 2016; Yang etal. 2017; Zheng etal. 2013). Maximum
qmax
values in these studies were
533.5 mg kg−1
for biochar
produced from giant reed at 600 °C (Zheng etal. 2013) and
785 mg kg−1
for biochar produced from lignin biomass com-
ponent at 700 °C (Yang etal. 2017). For a Eucalyptus bio-
char produced at 600 °C, Dempster etal. (2012) reported
NO−
3-N
removal rates from aqueous solution between 38%
at the highest addition rate (
50 mg NO−
3
-N L
−1
) and 80% at
the lowest addition rate (
2.5 mg NO−
3
-N L
−1
). In our study,
removal rates for biochar ranged between 98.5% for the
lowest added concentration (
1.6 mg NO
−
3
-N L−
1
) and 47.7%
for the highest added concentration (
25 mg NO−
3
-N L
−1
).
However, all mixtures with less than 10% biochar addition
predominantly showed
NO−
3-N
release. As with most batch
equilibrium experiments, our experiments were conducted
with fresh and untreated biochars. In consequence, no state-
ments regarding the sorption characteristics of biochar over
time can be made.
In general,
NO−
3
is very weakly retained by most soils
due to its great solubility in water and negative charge; it
is therefore easily leached (Addiscott 2005). Several fac-
tors and mechanisms influence
NO−
3
sorption on biochars.
One factor is the feedstock used for biochar production.
Strong variations in adsorption capacities in regard to bio-
char feedstock were mentioned by Gronwald etal. (2015)
and Yang etal. (2017), the latter reporting relatively large
NO−
3
sorption for biochars made from lignin while biochars
from cellulose exhibited only low capacity for
NO−
3
sorption.
Biochar (2022) 4:16
1 3
16 Page 14 of 23
Another important factor that controls most biochar proper-
ties and therefore
NO−
3
sorption is the temperature at which
biochar was produced. SSA, pore volume, anion exchange
capacity (AEC), and concentration of base functional groups
increase with higher pyrolysis temperatures (Al-Wabel etal.
2013). Due to their large SSA and large pore volume, bio-
chars exhibit a high number of potential sorption sites with
different properties. Yang etal. (2017)proposed the large
SSA of biochars to be the controlling parameter for
NO−
3
sorption. In accordance, Gronwald etal. (2015) found a
pronounced correlation between SSA and
NO−
3
sorption. A
further mechanism that is proposed to contribute to sorption
of
NO−
3
to biochar is capture of dissolved
NO−
3
in micro-
and nano-pores (e.g., Kammann etal. 2015; Knowles etal.
2011; Prendergast-Miller etal. 2014; Zheng etal. 2013).
However, this sorption effect may attenuate over time, when
dissolved organic matter and mineral particles cover biochar
surfaces and clog pores, thus reducing the inner reactive
surface (e.g., Eykelbosh etal. 2015; Kanthle etal. 2016;
Pignatello etal. 2006; Yang etal. 2017). Physical sorption of
NO−
3
, based on electrostatic attraction, depends on the pres-
ence of positively charged sorption sites, expressed by AEC.
Some biochars exhibit significant levels of AEC, which may
enhance sorption of anionic nutrients like
NO−
3
and
PO3−
4
(Chintala etal. 2014; Lawrinenko and Laird 2015). Biochar
AEC increases with higher pyrolysis temperatures and with
decreasing pH. Lawrinenko and Laird (2015) assumed that
oxonium functional groups contribute to pH-independent
AEC. Only a few studies have investigated the AEC con-
tents of biochars. Reported AEC levels differ between 0.60
and
27.76 cmolc
kg
−1
for a wide range of biochar feedstocks
(e.g., Cheng etal. 2008; Inyang etal. 2011; Lawrinenko and
Laird 2015; Silber etal. 2010). At neutral soil pH, AEC is
predominantly controlled by pyrolysis temperature. Ageing
is changing AEC over time, especially in low-temperature
biochars. Several authors have discussed the potential role
of base functional groups for
NO−
3
sorption. Higher pyroly-
sis temperatures promote the formation of base functional
groups to which
NO−
3
is chemically sorbed (Al-Wabel etal.
2013; Fatima etal. 2021; Kameyama etal. 2012). However,
in Fourier-transform infrared spectra of various biochar sam-
ples, Yang etal. (2017) could not observe obvious base func-
tional groups. The pyrolysis temperature is responsible for
biochar properties like SSA, presence of positively charged
sorption sites, and the number of base functional groups.
We assume that the medium pyrolysis temperature of our
woody, lignin-rich biomass feedstock is the reason for the
comparably medium sorption capacity for
NO−
3-N
.
4.2.2
NO−
3-N
sorption onclay
Other than biochar, clay addition triggered
NO−
3-N
release and
none of the substrate mixtures that contained clay showed any
NO−
3-N
sorption. This is in line with the few other studies that
report no effects of clay amendment to
NO−
3
retention or even
an increase of
NO−
3
release. In the field experiment related to
this study, the addition of clay to an Arenosol had no effect
on the leaching of
NO−
3-N
over the 1.5 years in the unferti-
lised treatment. In contrast, in the second period (months 8 to
16) the
NO−
3-N
leaching of the fertilised treatment increased
by 57.4% compared to the unamended control (Beusch etal.
2019). In a 20-day lysimeter experiment by Dempster etal.
(2012), cumulative
NO−
3-N
leaching significantly decreased
by 16% after the addition of clayey subsoil, mainly contain-
ing kaolinite and quartz, to a Sodosol. This may be related to
the significant increase of water-holding capacity after the
addition of the clay because in a batch equilibrium experi-
ment of the same study, clay exhibited no capacity to sorb
NO−
3-N
. Abdelwaheb etal. (2019)stated an increase of
NO−
3
sorption with increasing clay content from
qmax
of
0.25 mg g−1
for unamended sand to
1.60 mg g−1
for a mixture of sand and
clay with a clay content of 30%. In the area of water purifi-
cation, some studies tested the ability of unmodified natural
clay minerals and clayey materials to remove anions like
NO−
3
or
PO3−
4
from water. Most clays exhibit only low to medium
AEC, which results in
NO−
3
removal between 1 and
16 mg g−1
for clay minerals and
<2 mg g−1
for clayey materials at pH 6
and 7 (Lazaratou etal. 2020).
As reported by Dempster etal. (2012), the addition of
clay may enhance the water-holding capacity of a soil and,
as a result, retain
NO−
3
solved in the soil solution and there-
fore prevent
NO−
3
leaching. Several other studies also stated
an increase of water-holding capacity due to clay amend-
ment (e.g., Al-Omran etal. 2010; Costa etal. 2009; deLima
etal. 1998; Mojid etal. 2010; Sabrah etal. 1993; Suzuki
etal. 2007). However, due to electrostatic repulsion of the
negatively charged anion
NO−
3
from the net negative surface
charge of clays, no physical adsorption of
NO−
3
to the clay
surface itself or interlayers is expected under neutral pH con-
ditions (Addiscott 2005). When conditions change to acidic,
the number of positively charged hydroxyl groups on the sur-
faces of metal oxides and the edges of silicate clays increase
(Lazaratou etal. 2020). Consequently, the AEC increases
and allows sorption of
NO−
3
(Mohanty etal. 2015). Özcan
etal. (2005)reported
NO−
3
removal from an aqueous solution
of
23 mg g−1
by a natural sepiolite. In a study by Mohanty
etal. (2015), 27.5% of initial
NO−
3
was adsorbed on clay
kaolin; both studies were conducted at pH 2, so they do not
reflect the conditions of soils used for agriculture.
4.3 Sorption of
K+
4.3.1
K+
sorption onbiochar
Increasing biochar application sharply enhanced the release
of
K+
. Only a few batch equilibrium experiments exist for
Biochar (2022) 4:16
1 3
Page 15 of 23 16
the sorption of
K+
on biochars. Rens etal. (2018) report
a decrease of
K+
adsorption capacity after biochar addi-
tion to a silty clay loam soil, but no effect on a sandy soil.
Most studies that examined biochar effects on K reported
no sorption or retention but an increase of
K
release (e.g.,
Limwikran etal. 2018; Raave etal. 2014; Widowati etal.
2014). In theory, due to its large surface area and net nega-
tive charge, biochar exposes many potential binding sites for
positively charged nutrients like
K+
and
NH+
4
. Usually, K
contents of biochars are relatively large because the original
feedstock contains a certain quantity, on average 1% K in
plant shoot dry matter (Kirkby and Marschner 2012), which
is enriched in the pyrolysis process to a multiple of the initial
K content (Al-Wabel etal. 2013). As biochar K is predomi-
nantly only weakly bound, large amounts of
K+
are dissolved
in the soil solution. In an incubation study by Limwikran
etal. (2018), up to 64% of biochar
K+
was soluble in water
and up to 75% of this
K+
diffused rapidly into the soil.
Rinsing the biochars beforehand would have removed
water-soluble
K+
and decreased the amount of
K+
and other
nutrients leached (e.g., Hale etal. 2013); it woud also have
allowed a better estimation of the sorption capacity of
K+
to biochars. However, the intention of this study was to
approach real-life biochar production and field conditions
in the tropical north-east of Brazil to the extent possible for
the conduction of a batch equilibrium experiment. Washing
would not be economically feasible for large amounts of
biochar. Also, the fertilising effect of plant-available biochar
K+
in soils is desirable.
4.3.2
K+
sorption onclay
In contrast to biochar, clay addition led to a release of
K+
from the unamended Arenosol, clay, and all mixtures for
the lower NPK concentrations added, but increased sorp-
tion relative to the initial NPK concentration (
Cini
) for the
higher concentrations. Few existing studies deal with K
sorption or retention of clayey soil amendments. One batch
equilibrium study mixed peat for horticultural use with sev-
eral clayey materials. In that study by Binner etal. (2017),
K sorption was found to be positively correlated with CEC
(
R2=0.84
) but not with clay content. Also, the SSA, con-
tent of Fe oxides, and smectite content correlated with K
sorption. The greatest K adsorption was measured in a smec-
titic bentonite (
qmax
=5760 mg kg−1
),which also showed
great ability to desorb almost all initially sorbed K. This
indicated that sorbed K was not fixed in clay interlayers but
remained exchangeable at a higher rate than for illitic or
kaolinitic clay. Only two other studies regarding
K+
sorp-
tion or retention that used clayey materials to amend soils
are known to us. In a 1.5 year leaching study that Beusch
etal. (2019) conducted at the field site where the material of
our study has its origin, the addition of the same smectitic
clay resulted in a significant decrease of
K+
leaching over
two consecutive periods of 8 months, by 51.0% and 45.2%,
respectively. Reuter (1994) reported approx. 60% reduction
of K leaching in a pot experiment with a sandy soil after
2.4% (w/w) brick clay, mainly containing illite, was added.
With the addition of only 0.3% (w/w) bentonite, the same
effect could be achieved. The bentonite predominantly con-
tained montmorillonite. This study shows that the sorption
of
K+
and other nutrients is largely dependent on the mineral
composition of clays.
The clay applied in our study contained 40% to 60% smec-
tites and expansible layer phyllosilicates. The interlayer sites
of smectites provide a high number of cation adsorption sites
(Sparks and Carski 1985). Besides vermiculites, smectites
are the clay minerals with the largest SSA of
600 m2g−1
to
800 m2g−1
and the largest surface charge, resulting in a high
CEC of
80 cmolckg−1
to
150 cmolckg−1
(Sparks 2003b). We
assume these characteristics to be the reason for the increase
of
K+
adsorption after the addition of clay to an Arenosol. The
release of
K+
to the solutions with no or only small amounts
of NPK can be explained by the content of
288.7 mg kg−1
plant-available K in the clay applied in our study. Soil K is
divided into soluble, exchangeable, fixed, and structural K
(Sparks and Carski 1985). One portion of plant-available K
is water-soluble K in the soil solution and is directly released
to the mixing solution; the other portion is exchangeable K,
which is only weakly bound to the surface of the clay minerals
and can be rapidly exchanged by other cations in the solution
(Sparks 1987). K that is released from clays can contribute to
the K supply of plants in the soil (Binner etal. 2017).
4.4 Sorption of
PO3
−
4-P
4.4.1
PO3
−
4-P
sorption onbiochar
In our study, biochar addition to the Arenosol slightly
enhanced maximum
PO3−
4-P
sorption capacity, whereas
undiluted biochar induced release of
PO3−
4-P
. Only a few
studies have conducted batch equilibrium experiments and
derived sorption isotherms to examine P adsorption on bio-
chars, and a very limited number of studies have worked on
P adsorption of biochar-soil mixtures at varying pyrolysis
temperatures (e.g., Eduah etal. 2019; Ghodszad etal. 2021).
As for other nutrients, most batch experiments were con-
ducted with pure biochars; we only found a few studies that
have tested soil/biochar mixtures for their maximum P sorp-
tion capacity. Addition of 1% rice husk biochar produced at
650 °C to an Arenosol at neutral pH doubled maximum P
sorption capacity
qmax
to
203 mg kg−1
(Eduah etal. 2019).
P sorption capacity increased with increasing pyrolysistem-
perature for corn cob and rice husk biochars. In contrast,
Biochar (2022) 4:16
1 3
16 Page 16 of 23
biochar addition to two acidic soils, an Acrisol and a Fer-
ralsol with pH (
H2O
) 5.03 and 4.73, decreased P sorption
with increasing pyrolysis temperature. These resultswere
confirmed in a study by Ghodszad etal. (2021), who alsore-
ported an increase of P sorption with increasing pyrolysis
temperature in an alkaline soil (pH 7.4) after addition of 6%
wheat straw-derived biochar, but a decrease in two acidic
soils (pH 4.6 and 6.0). The biochars in this study were pro-
duced at 300 °C and 600 °C. Xu etal. (2014) also reported
highest P sorption capacity for an acidic Inceptisol (pH
(
H2O
) 3.83) with a
qmax
of
769 mg kg−1
after addition of 10%
wheat straw biochar produced at 350 °C to 550 °C. Chintala
etal. (2014) found the greatest
qmax
values (
34 mmol L−1
) for
an acidic soil with 4% (w/w) switchgrass biochar pyrolysed
at 650 °C. In a study by Rens etal. (2018), the addition of
a white wood biochar pyrolysed at 450 °C to 500 °C to a
sandy soil led to a 75% increase of
qmax
to
28 mg kg−1
. In
contrast, addition of the same biochar to a silt loam soil
increased
qmax
less than 2% to
185 mg kg−1
. This effect is in
line with several studies that have found more pronounced
effects of biochar addition to sandy soils compared to loamy
soils (e.g., Gronwald etal. 2015).
Greater
qmax
values were derived in batch equilib-
rium experiments with pure biochars: Trazzi etal. (2016)
presented a maximum
PO3−
4-P
sorption capacity
qmax
of
16.1 mg g−1
for a biochar produced from miscanthus at
700◦C
; Li etal. (2019) found maximum P sorption capac-
ity of
4.8 mg g−1
for a woody biochar produced at
500◦C
.
Takaya etal. (2016) reported maximum
PO3−
4-P
sorption
capacities
qmax
between 0 and
30 mg g−1
for several biochars
produced from different feedstocks. The greatest
qmax
in that
study was
30 mg g−1
, reported for biochar made of presscake
from anaerobic digestion at
600◦C
to
650◦C
. One of the fac-
tors that favoured this relatively very high sorption capacity
is that Takaya etal. (2016) used high P concentrations of the
initial batch equilibrium solutions (
125 mg P L−1
) with only
very small amounts of biochar (
0.1 g
biochar per
100 mL
test
solution). In contrast, initial P concentrations added to the
soil mixtures in our study ranged from 1.2 to
19.8 mg P L−1
(Sect.2.3), with a substrate/solution ratio of 1:5 (
10 g
bio-
char per
50 mL
test solution). This observation is in line
with other studies reporting improved P adsorption with
increasing initial P concentrations (e.g., Chintala etal. 2014;
Takaya etal. 2016; Wang etal. 2009). However, in the study
by Takaya etal. (2016), the maximum amount of initially
added
PO3−
4-P
that sorbed to biochar accounted for only
7%. Other batch equilibrium studies describedno P sorp-
tion (e.g., Alling etal. 2014; Hollister etal. 2013; Morales
etal. 2021; Palanivell etal. 2020; Soinne etal. 2014), or
even P release (e.g., Yao etal. 2012; Zhang etal. 2016).
Addition of 4% (w/w) rice husk char produced at
600◦C
to loamy soils triggered
PO3−
4-P
release up to 72% com-
pared to the unamended soil (Pratiwi etal. 2016). Hale etal.
(2013)reportedpronounced
PO3−
4-P
release from unwashed
biochars; however, after washing with millipore water, corn
cob and cacao shell biochars produced in traditional kilns at
400 °C and 350 °C showed
PO3−
4-P
sorption.
In general, due to its low mobility and the ability of most
soils to sorb P in the form of
PO3−
4
, P loss due to leach-
ing is not of great concern in most soils (Sparks 2003a).
Sorption of P in soils is influenced by various soil param-
eters, like pH, contents of iron and aluminium oxides, car-
bonates, organic matter, clay content, and clay mineralogy
(Coulombe etal. 1996). The reddish colour and significant
amounts of total
Fe
(
2.10 g kg−1
) and
Al
(
4.91 g kg−1
; see
Table1) indicate the presence of
Fe
- and
Al
-oxides in the
Arenosol that might have contributed to the
PO3−
4-P
sorp-
tion of
21.7 mg kg−1
(
qmax
) of the unamended Arenosol.
According to Sparks (2003b),
Fe
-,
Al
- and
Mn
-oxides play
extremely important roles in sorption processes in soils due
to their large SSA and reactivity, even if they are only pre-
sent in small quantities. In our study, no further characterisa-
tion of metal oxides was conducted. We suppose that the
Fe
-
and
Al
-oxides were responsible for
PO3−
4-P
sorption in the
unamended Arenosol. The stepwise addition of biochar to
the Arenosol led to a slight increase of
qmax
, presumably due
to the addition of a certain amount of sorption sites for
PO3−
4
on the biochar. However, the effect was not linear and was
more pronounced for the lowest biochar addition rates. Other
than for
NO−
3
and
NH+
4
, SSA seems not to be an important
factor in
PO3−
4
sorption (e.g., Takaya etal. 2016; Wang etal.
2015b); however, elemental composition of biochars seems
to play a major role (e.g., Gronwald etal. 2015; Takaya etal.
2016; Xue etal. 2009). Several authors reported a positive
correlation between
PO3−
4
adsorption and the presence of
mineral salts, in particular
Ca2+
or
Mg2+
; precipitation and
surface deposition on
Ca2+
or
Mg2+
present on the biochar
surface are assumed to be the main mechanisms controlling
PO3−
4
sorption (e.g., Chintala etal. 2014; Pratiwi etal. 2016;
Xue etal. 2009; Yao etal. 2013; Zeng etal. 2013).
Moreover, several authors assume that the increase in P
sorption or precipitation after biochar addition is, due to an
increase of pH, more pronounced for
Ca
-induced sorption
and, to a lesser extent, for
Fe
- and
Al
-oxides (e.g., Eduah
etal. 2019; Takaya etal. 2016; Xu etal. 2014). This assump-
tion is in line with Schneider and Haderlein (2016), who
analysed P fractions of several biochars andreportedthat
more than two-thirds of the total P of all analysed pyro-
charswas inorganic
Ca
-bound P, whereas
Fe
- and
Al
-bound
organic and inorganic P added up to less than 10%. The bio-
char used in this study had substantial amounts of
6.5 g kg−1
Ca
and
1.17 g kg−1
Mg
, which may be an important factor
for
PO3−
4-P
sorption. Moreover, Wang etal. (2015b) found a
positive linear correlation (
R2=0.73
) between
PO3−
4-P
sorp-
tion and the amount of base functional groups on biochar
Biochar (2022) 4:16
1 3
Page 17 of 23 16
surfaces. The contents of base functional groups and mineral
salts in biochars increase with higher pyrolysis temperature
(e.g., Al-Wabel etal. 2013; Cheng etal. 2018; Jung etal.
2016); this may be one possible reason for the tendency for
greater
PO3−
4
sorption rates in high-temperature biochars, as
reported by several authors (e.g., Jung etal. 2016; Takaya
etal. 2016; Trazzi etal. 2016; Wang etal. 2015b). In con-
trast, the biochar itself used in our study did not show any
sorption at all, only release of
24.5 mg PO3
−
4
-P g−
1
(Table4)
when equilibrated for
24 h
in millipore water. With a total P
content of
580.8 mg kg−1
, the biochar used in our study had
a large P reservoir; the plant-available P fraction, determined
by CAL-extraction, amounted to
71.2 mg kg−1
(Table1). As
the water-soluble P fraction of the biochar was not removed
by washing before the experiment, the equilibration released
large amounts of water-soluble P that presumably exceeded
the
PO3−
4-P
sorption capacity of the biochar and evoked
release of P.
4.4.2
PO3−
4-P
sorption onclay
Addition of clay stepwise increased the maximum
PO3−
4-P
sorption capacity of the Arenosol and
qmax
of the Vertisol
itself to
176.4 mg kg−1
. Compared with biochar, the addition
of 10% clay more than doubled
qmax
. All batch equilibrium
experiments relating to
PO3−
4
sorption reviewed for our study
report
PO3−
4
sorption for numerous types of clays, e.g., Verti-
sols (Nunes etal. 2012; Nychas and Kosmas 1984; Solis and
Torrent 1989), terra rossa (Durn etal. 2016), clayey sedi-
ments (Wang etal. 2009), clayey Cerrado soil (Barros etal.
2005), clay from crushed recycled building bricks (White
etal. 2011), and clays originating from primary and sec-
ondary types of deposits (Binner etal. 2015). All the main
clay minerals, such as kaolinite, illite, smectite, montmo-
rillonite, bentonite, and saprolite, are represented in these
studies. Maximum
qmax
values in the studies are
402 mg kg−1
for a clayey river sediment (Wang etal. 2009), around
450 mg kg−1
for a Vertisol (Solis and Torrent 1989) and a
fine calcined clay (White etal. 2011),
1430 mg kg−1
for terra
rossa (Durn etal. 2016), and
2566 mg kg−1
for another Ver-
tisol (Nunes etal. 2012). Maximum
PO3−
4
sorption (
qmax
) up
to approx.
3500 mg kg−1
is reported by Binner etal. (2015)
for a smectitic clay with large content of Fe oxides.
Conduction of batch experiments is a common practice
to reveal sorption capacities of soils and soil components
for dissolved substances such as nutrients, heavy metals,
or organic compounds. However, all the above-mentioned
studies derived sorption isotherms from pure clays. We only
found one study, Abdelwaheb etal. (2019), which involved
a batch experiment with mixtures of a sandy soil with a
clayey soil. They reported a more than ninefold increase of
maximum sorption capacity
qmax
of the (
0.14 mg g−1
) sand
after 30% clay addition up to
1.28 mg g−1
. Several other stud-
ies examined the effects of clay addition to a soil in regard
to
PO3−
4
sorption or retention, but did not derive sorption
isotherms. In an incubation study, Nguyen and Marschner
(2013), for example, reported a significant reduction of P
release after the addition of 20% (w/w) sandy clay loam
to sandy soil amended with compost. Tahir and Marschner
(2017) demonstrated an increase of P sorption after addi-
tion of 20% (w/w) of a clayey subsoil; while Arenosol had
no P sorption capacity at all, the addition of a low smectite
clay increased the maximum P sorption to
36 mg kg−1
and
the addition of a high-smectite clay raised it to
46 mg kg−1
.
In pot experiments, Reuter (1994) added increasing shares
of two different clays to a sandy soil. The addition of 2.2%
(w/w) brick clay reduced initial P leaching of around
75 mg
P per pot to 0, whereas only a 0.3% (w/w) addition of ben-
tonite was needed to achieve the same result.
Vertisols, in general, have a large sorption capacity for
PO3−
4
(Nychas and Kosmas 1984). As mentioned in the pre-
vious section, the sorption of
PO3−
4
is influenced by various
soil parameters; in particular, P has a high affinity to sorb
on mineral surfaces like clay minerals, Fe/Al oxides, and
carbonates (Coulombe etal. 1996). However, even though
numerous experiments have tested the
PO3−
4
sorption abil-
ity of these single soil constituents, the relative importance
of each soil parameter still remains unclear (Gérard 2016).
Consequently, several studies come to contradictory expla-
nations regarding the main factors controlling
PO3−
4
sorption
to soils. One factor controlling
PO3−
4
sorption in soils is the
large clay content that accounts for 69.8% of the Vertisol in
our study (see Table1). The dominating clay minerals are
smectites, estimated 40% to 60% for this Vertisol (Mertens
etal. 2017a). Smectites exhibit large SSAs (
600 m2g−1
to
800 m2g−1
), have high layer charge (0.3 to 0.6 electron per
half unit cell), and are rich in iron (up to 10%
Fe2O3
) (Cou-
lombe etal. 1996). Several authorsreported a significant cor-
relation of clay content with
PO3−
4
sorption (e.g., Abdelwa-
heb etal. 2019; Barros etal. 2005; Gérard 2016; Nunes
etal. 2012; Sei etal. 2002). Factors that may explain the
correlation are the large SSA of clay minerals (e.g., Gérard
2016; Nunes etal. 2012; Sei etal. 2002), and their high
content of structural Al and Fe (e.g., Binner etal. 2015; Sei
etal. 2002). Reactive Al sites, present at the edges of clay
minerals, are able to bind P through ligand exchange with
hydroxyl groups (Durn etal. 2016).
Mineral surfaces form another factor of P sorption to
soils. Phosphate has a relatively strong affinity to sorb on
mineral surfaces (Antelo etal. 2005). A strong correlation
between the presence of Al and Fe oxides and
PO3−
4
sorp-
tion is reported by many authors (e.g., Binner etal. 2015;
Gérard 2016; Nunes etal. 2012; Sei etal. 2002; Wang
etal. 2012). Durn etal. (2016), for example, stated the Fe
Biochar (2022) 4:16
1 3
16 Page 18 of 23
content in clayey terra rossa to be the key controlling fac-
tor of P removal from waste water, and Solis and Torrent
(1989) found a high correlation of P sorption with the Fe
content of a Vertisol in short-term sorption. One expla-
nation for the high correlation is the large SSA of Al and
Fe oxides, resulting in a large number of reactive sites on
the surface of the oxides (e.g., Flores-Ramírez etal. 2018;
Gérard 2016; Sei etal. 2002; Wang etal. 2009). Two pos-
sible mechanisms behind
PO3−
4
sorption to Al and Fe oxides
are ligand exchange reactions and electrostatic interaction
of the
PO3−
4
anion with the charged oxide surface, where
the latter is greatly controlled by the solution’s pH (Antelo
etal. 2005). As the Vertisol used in our study contained
substantial amounts of Fe and Al (total Fe content: 6.1%;
total Al content: 6.4% Table1), we assume that Fe and Al
oxides play a major role in the high sorption capacity of the
Vertisol for
PO3−
4
.
Under neutral and alkaline conditions, Vertisols also
contain significant amounts of carbonates (Coulombe etal.
1996). No carbonate analysis was conducted in our study;
however, the rather large total Ca content of 3.8% in combi-
nation with the slightly alkaline pH of 8.3 (
H2O
) indicates
the presence of substantial carbonate supplies in the Verti-
sol. Low but significant correlation between the carbonate
content of clayey material and
PO3−
4
sorption is reported in
several studies (e.g., Nunes etal. 2012; Wang etal. 2009),
especially for long-term sorption up to 144 days after P addi-
tion (Solis and Torrent 1989). However, a significant share
of
PO3−
4
sorption to carbonates may take place as co-precip-
itation of Ca-phosphates with low solubility, contributing
to
PO3−
4
fixation in soils (e.g., Coulombe etal. 1996; Wang
etal. 2009). Even though several authors claimed organic
matter plays a role in P sorption, (e.g., Nychas and Kosmas
1984; Sparks 2003a; Wang etal. 2009), the low total C con-
tent of only 1.7% (Table1) may not play a significant role
for P sorption in this study. Nunes etal. (2012) found only a
low and not significant correlation of organic matter to
PO3−
4
sorption of a Vertisol but acknowledged the relevance of
organic carbon for
PO3−
4
sorption for other soil types. In con-
clusion, we assume that the P sorption to the Vertisol in this
study is mainly controlled by sorption to clay minerals, in
particular to smectites, Fe/Al oxides, and, to a lesser extent,
carbonates that may form low-soluble Ca-phosphates.
5 Conclusions
The biochar and clay tested in this study both indicated
the potential to increase the nutrient sorption capacity of
a tropical Arenosol, even when added in small amounts.
For biochar, batch equilibrium experiments demonstrated a
marginal increase of
PO3−
4-P
and
NH+
4-N
sorption capacity,
a slight reduction of
NO−
3-N
release from the Arenosol, and
massive release of
K+
from the biochar. Biochar itself was
also subject to
PO3−
4-P
release. Clay, in contrast, showed
better efficacy in increasing the sorption capacities of
NH+
4-N
,
K+
, and
PO3−
4-P
but had no effect on
NO−
3-N
sorp-
tion capacity.
Moreover, biochar exhibits the potential to augment soil
fertility by adding nutrients to soils, in particular
K
, and may
act as a slow-release fertiliser over a prolonged time span.
To ensure a greater stability of biochars in soils, we recom-
mend pyrolysis temperatures above 600°C. We also rec-
ommend using locally abundant biomass from sustainable
sources to avoid conflicts with food production for biochar
carbonisation. Before adding large quantities to soils, bio-
chars should be tested for contaminants such as polycyclic
aromatic hydrocarbons, dioxins, furans, and heavy metals
that may be released from the biochar into the soil (Qadeer
etal. 2017).
Clays are more resistant to degradation than biochars and
may perpetuate their soil-amending properties for a long
time. We recommend substrates with large clay content and
preferably smectitic mineralogy, which show high potential
to increase nutrient sorption and water-holding capacity.
Clayey material should come from locally available and sus-
tainable sources, for example, clay-rich subsoils or natural
clay deposits. As in the present study, clayey sediments from
water bodies can also be used as to meliorate soils. However,
a screening for toxic substances should be conducted before
large scale field application to avoid contamination of the
amended soil.
To combine the positive soil-amending effects of both
materials, we suggest mixing biochar and clay. Clay may
help stabilise the organic material and may adsorb the
K
released from the biochar, whereas the biochar may retain
highly mobile
NO−
3-N
. However, further research is needed
to assess the soil-amending potential of biochar–clay
mixtures.
In conclusion, we recommend a careful selection of soil
conditioners according to their properties to complement
the soil. Before field application, the suitability of the soil
conditioner should be tested in preliminary analyses, for
example, batch equilibrium experiments. The high costs
and efforts required to incorporate biochar or clay should
also be considered. To evaluate the long-term efficacy of
the effects of biochar and clay on sorption capacity of soils,
further investigations are needed, particularly under realistic
conditions at the field scale.
Acknowledgements The authors want to thank the technical staff at the
Chairs of Soil Science and Soil Protection at Technische Universität
Berlin for laboratory analyses, especially Claudia Kuntz, Monika Rohr-
beck, and Iris Pieper for analysis of the numerous nutrient samples.
Thanks to Michaela Riese and Susanne Hoffmann at the EVUR insti-
tute at Technische Universität Berlin for the analysis of various biochar
Biochar (2022) 4:16
1 3
Page 19 of 23 16
parameters. Thanks to José Coelho de Araújo Filho for his inspiration
and support, and to Peter Dominik for fruitful discussions and valuable
comments. May you rest in peace! Christine Beusch also wants to thank
her co-workers at The Writing Academic for their empowering com-
munity and support. We also thank the anonymous reviewers for their
valuable suggestions that helped improve our manuscript.
Authors’ contributions CB: writing of manuscript, provisioning of field
samples, design and conduction of experiment, literature research, data
analysis, discussion, revision of text. DM: design and conduction of
experiment, literature research, discussion, revision of text. AC: sup-
port of data analysis, discussion, revision of text. MK: supervision of
experiment, discussion, revision of text. The authors read and approved
the final manuscript.
Funding This study was realised within the German-Brazilian joint
project INNOVATE (Interplay among multiple uses of water reservoirs
via innovative coupling of aquatic and terrestrial ecosystems), which
was funded by the German Federal Ministry of Education and Research
(BMBF), Project number 01 LL 0904 A-E.
Availability of data and materials The data and material used in this
study will be made available by the authors upon request.
Declarations
Conflict of interest The authors declare that they have no conflict of
interest.
Open Access This article is licensed under a Creative Commons Attri-
bution 4.0 International License, which permits use, sharing, adapta-
tion, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article are
included in the article's Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not included in
the article's Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.
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