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How and why does willow biocha inc ease a clay soil wa e e en ion capaci y?
© 2018 The Au ho s. Published by Else ie L d.
Published e sion
Rasa, Kimmo; Heikkinen, Jaakko; Hannula, Ma kus; A s ila, Kai; Kulju, Sampo;
Hy äluoma, Ja i
Rasa, K., Heikkinen, J., Hannula, M., A s ila, K., Kulju, S., & Hy äluoma, J. (2018). How and why
does willow biocha inc ease a clay soil wa e e en ion capaci y?. Biomass and Bioene gy, 119,
346-353. h ps://doi.o g/10.1016/j.biombioe.2018.10.004
2018
Con en s lis s a ailable a ScienceDi ec
Biomass and Bioene gy
jou nal homepage: www.else ie .com/loca e/biombioe
Resea ch pape
How and why does willow biocha inc ease a clay soil wa e e en ion
capaci y?
Kimmo Rasa
a,∗
, Jaakko Heikkinen
a
, Ma kus Hannula
b
, Kai A s ila
c
, Sampo Kulju
a
,
Ja i Hy äluoma
a
a
Na u al Resou ces Ins i u e Finland, Luke, FI-31600, Jokioinen, Finland
b
BioMediTech Ins i u e and Facul y o Biomedical Sciences and Enginee ing, Tampe e Uni e si y o Technology, FI-33101, Tampe e, Finland
c
Uni e si y o Jy askyla, Depa men o Physics, P.O. Box 35, FI-40014, Finland
ARTICLE INFO
Keywo ds:
Biocha
X- ay omog aphy
3D image analysis
Helium ion mic oscopy
Soil wa e e en ion
Plan a ailable wa e
ABSTRACT
Addi ion o biocha in o a soil changes i s wa e e en ion p ope ies by modi ying soil ex u al and s uc u al
p ope ies. In addi ion, in e nal mic ome e -scale po osi y ha is able o di ec ly s o e eadily plan a ailable
wa e affec s soil wa e e en ion p ope ies. This s udy shows how p ecise knowledge o he in e nal mic o-
me e -scale po e size dis ibu ion o biocha can deepen he unde s anding o he biocha -wa e in e ac ions in
soils. The mic ome e -scale po osi y o willow biocha was quan i a i ely and quali a i ely cha ac e ized using
X- ay omog aphy, 3D image analysis and Helium ion mic oscopy. The effec o biocha applica ion on clay soil
wa e e en ion was s udied by con en ional wa e e en ion cu e app oach. The esul s indica e ha he in-
e nal po es o biocha , wi h sizes o a 50 and 10 μm (equi alen po e diame e ), inc eased soil po osi y and he
amoun o eadily plan a ailable wa e . A e biocha addi ion, changes in soil po osi y we e de ec ed a po e
size egimes 5–10 and 25 μm, i.e. biocha po e sizes mul iplied by ac o 0.5. The de ec ed po e size dis ibu ion
o biocha does no p edic di ec ly (1:1 compa ibili y) he changes obse ed in he soil mois u e cha ac e is ics.
I is likely ha biocha chemis y and po e mo phology affec biocha -wa e in e ac ions ia e.g. su ace
oughness and con ac angle. In addi ion, biocha induced changes in soil s uc u e and ex u e affec ed soil
mois u e cha ac e is ics. Howe e , he app oach p esen ed is an a ac i e pa hway o mo e gene alized un-
de s anding on how and why biocha in e nal po osi y affec s soil mois u e cha ac e is ics.
1. In oduc ion
While biocha is conside ed as a po en ial measu e o seques e
ca bon in o soil [1], i s seconda y effec s on soil p ope ies a e o en
con o e sial [2]. Biocha has been shown o inc ease soil wa e
holding capaci y (WHC), bu no solid unde s anding exis s on he e -
ec s o , e.g., soil ype, biocha quali y o clima e. Addi ion o biocha
in o a gi en soil changes soil ex u al and s uc u al p ope ies. These
changes modi y soil mois u e cha ac e is ics in a specific manne de-
pending on biocha ype and soil p ope ies (indi ec mechanism).
Howe e , biocha as a highly po ous ma e ial also di ec ly affec s soil
wa e holding capaci y ia i s in e nal po osi y. The aim o his pape is
o s udy how his in e nal po osi y and po e size dis ibu ion a e ela ed
o he amoun o plan a ailable wa e in a clay soil.
A la ge numbe o pape s desc ibe biocha effec on soil hyd aulic
p ope ies and wa e holding capaci y [3–6] wi h he e ogeneous de-
sc ip ions o biocha physical quali y. Physical cha ac e is ics o
biocha is mos commonly s udied by gas adso p ion echniques ac-
companied by he B unaue -Emme -Telle (BET) modelling o de-
e mine he specific su ace a ea [7] o Ba e -Joyne -Halenda (BJH)
modelling o de e mine po e size dis ibu ion [8]. Howe e , po e space
analysis based on gas adso p ion measu emen s is limi ed o po es
smalle han 300 nm, whe eby i does no ell much abou he po osi y
in he size ange ha is impo an o plan wa e up ake (i.e. mic o-
me e -scale po es).
The inadequacy o gas adso p ion s udies and d awbacks ela ed o
po osi y measu emen s using me cu y po osime y ha e been ad-
d essed in ea lie s udies [9,10]. NMR C yopo ome y ha e also been
used o s udy po ous ma e ials a nanome e leng h-scale, howe e ,
esolu ion in he uppe end is limi ed o ew mic ome e s [11]. Kinney
e al. [4] add essed he need o quan i a i e echniques o cha ac e ize
mic ome e -scale po es wi hin he biocha s due o hei expec ed im-
po ance on soil WHC. All hese s udies suppo heo y ha la ge in-
e nal mic ome e -scale po es o biocha may ha e ema kable di ec
h ps://doi.o g/10.1016/j.biombioe.2018.10.004
Recei ed 21 Decembe 2017; Recei ed in e ised o m 28 Sep embe 2018; Accep ed 2 Oc obe 2018
∗
Co esponding au ho . Na u al Resou ces Ins i u e Finland, Luke, P oduc ion sys ems, Bio efine y and Biop oduc s, Finland.
E-mail add ess: kimmo. asa@luke.fi(K. Rasa).
Biomass and Bioene gy 119 (2018) 346–353
0961-9534/ © 2018 The Au ho s. Published by Else ie L d. This is an open access a icle unde he CC BY license
(h p://c ea i ecommons.o g/licenses/BY/4.0/).
T
effec on soil mois u e cha ac e is ics (indi ec effec e e s o changes
in soil ex u e and o ma ion o soil agg ega es h ough bindings be-
ween biocha and soil pa icles, see Re . [12]). Howe e , he effec o
biocha mic ome e -scale po osi y on soil mois u e cha ac e is ics has
no been quan i a i ely analysed.
We an icipa e ha he way o wa d in inc easing unde s anding o
biocha -wa e in e ac ions goes h ough undamen al esea ch on in-
e nal po e s uc u e o biocha s. La ely, X- ay omog aphy and 3D
image analysis me hods ha e been adap ed o biocha esea ch. This
app oach is able o isualize he in e nal po e s uc u e o biocha , and
o di ec ly de e mine po e cha ac e is ics such as po e size dis ibu ion
and po e con inui y. In ecen s udies X- ay omog aphy has been used
o cha ac e ize po e s uc u e o biocha s de i ed om a ious aw
ma e ials (e.g. Pinus syl es is,Miscan hus, Populus spp. L., co onseed
hull, hay) wi h esolu ion anging om 0.74 o 21 μm[13–17]. Hy ä-
luoma e al. imaged a ious ypes o biocha s and hyd ocha s wi h ca.
1μm esolu ion and ound ha conside able pa o he biocha olume
consis o po es in size ange ele an o hyd ological p ocesses and
s o age o plan a ailable wa e [18]. In addi ion, hey ound high
a ia ion in po osi y, po e size dis ibu ion and s uc u al aniso opy
be ween diffe en biocha s. Also agg ega es o biocha -amended soil
ha e been imaged wi h X- ay omog aphy, al hough a his leng h-scale
X- ay omog aphy esolu ion canno cap u e he in e nal po osi y o
biocha [19].
The lack o p ecise da a on mic ome e -scale po osi y migh be one
eason ha he impo ance o biocha in e nal po osi y di ec ly a -
ec ing he soil wa e holding capaci y has been o e looked. In his
s udy we used X- ay mic o omog aphy, 3D image analysis and Helium
ion mic oscopy o s udy s uc u al p ope ies o willow biocha wi h
emphasis on mic ome e -scale po es con ibu ing o s o age o plan
a ailable wa e . Fu he , we conduc ed an incuba ion expe imen o
s udy he effec o his specific biocha on soil mois u e cha ac e is ic
and o iden i y po e egimes al e ed by biocha amendmen . The e-
la ionship be ween biocha po e sys em and i s specificeffec on soil
mois u e cha ac e is ics is an a ac i e pa hway owa ds de elopmen
o p ecisely ailo ed biocha s aimed o enhance wa e use efficiency.
2. Ma e ials and me hods
2.1. Expe imen al soil
Soil o incuba ion expe imen was aken om Ko kanoja long- e m
d ainage expe imen si e [20] loca ed in Jokioinen in sou he n Finland
(N 60.82°, E 23.51°). The s udy ma e ial was collec ed om he opmos
10 cm laye o an annually ploughed plo in sp ing 2016. The hea y
clay soil had 64.8%, 30.5% and 4.7% o clay, sil and sand, espec i ely
(mass ac ions de e mined by pipe e me hod [21]). Soil o ganic
ma e con en was 9.2% (loss o igni ion a 550 °C), ca bon con en
2.9% (Leco analysa o ), pH 6.3 and elec ical conduc i i y
63.1 μScm
−1
(soil o wa e a io 1:5).
2.2. Biocha
The biocha used in he expe imen was py olysed in an indi ec ly
hea ed pilo -scale ba ch- ype py olysis acili y using Willow s em wood
(Salix sp, wi h ba k) as aw ma e ial. Tempe a u e p ofile o py olysis
p ocess consis ed o wo s eps. Fi s , he empe a u e was aised o
280 °C, and hen u he o 320 °C wi h a es o 2.2 and 0.2 K min
−1
,
espec i ely.
The elemen al analysis (CHNSO) o biocha was ca ied ou using
FLASH 2000 se ies analyse and he esul s we e used o calcula e O:C
and H:C a omic a ios. The biocha was analysed o pH (SF EN 13037,
1:5 cha o wa e a io), elec ical conduc i i y (EC, SF EN 13038, 1:5
cha o wa e a io), nu ien s and hea y me als (SFS-EN 13650 Aqua
Regia ex ac ion and ICP-measu emen ), BET su ace a ea (ISO
9277:2010(E) B unaue –Emme –Telle (BET) Su ace A ea) and Ash
con en (SFS 3008, loss o igni ion a 550 °C).
The BET su ace a ea o biocha p oduced was 6.8 ± 0.43 m
2
g
−1
.
The elemen al mass ac ions o C, H, N, S and O we e 74.0 ± 0.0%,
4.1 ± 0.1%, 0.4 ± 0.0%, 0.0 ± 0.0% and 15.8 ± 0.3%, espec-
i ely. The ash mass ac ion o he biocha was 3.13 ± 0.15%. The
co esponding mole a ios o H:C and O:C we e 0.66 and 0.16, e-
spec i ely. Biocha had low con en o main nu ien s (1.5 g kg
−1
,
3.6 g kg
−1
, 9.1 g kg
−1
o P, K, Ca espec i ely) and hea y me als Cd, Cu
and Pb (0.79 mg kg
−1
, 6.8 mg kg
−1
,<3mgkg
−1
, co esponding limi s
a e 1.5, 600 and 100 mg kg
−1
, espec i ely) con en s we e below limi s
se o soil amendmen s in Finland.
2.3. X- ay omog aphy
The X- ay compu ed mic o omog aphy imaging was conduc ed wi h
Zeiss X adia Mic oXCT-400 (Zeiss, Pleasan on, CA, USA) de ice. Sou ce
ol age was 40 kV and sou ce cu en was 250 μA. The pixel size was
1.14 μm. A 20× objec i e was used wi h 2 binning. 1600 p ojec ions
we e aken in ull 360°. Each p ojec ion was exposed wi h X- ays o 3 s.
No fil e s we e used in he imaging p ocess. Zeiss XMRecons uc o
so wa e wi h he fil e ed back p ojec ion algo i hm was used in he
econs uc ion o he image s acks.
Fo quan i a i e s uc u al analysis he o iginal g ey-scale image
was de-noised and segmen ed in o solid and oid phases. De ails o
hese image p ocessing s eps ha e been desc ibed by Hy äluoma e al.
[18] and e iewed in Appendix A. The sub olume used in he u he
image analysis consis ed o 1.9·10
8
oxels (sample size ca. 0.5 mm).
The po osi y o he sample was calcula ed by di iding he numbe o
oid oxels by he o al numbe o oxel in he analysed olume. The
po e-size dis ibu ion was de e mined using an app oach based on
ma hema ical mo phology [22] by successi ely applying mo phological
opening ope a ions (sphe e wi h adius was used as he s uc u ing
elemen ) on he po e space [23]. The po e size based on mo phological
opening closely ela es o he s a iona y dis ibu ion o we ing and
nonwe ing fluids in po e space and he ela ed capilla y p essu e ia
he Young-Laplace equa ion
=
p
γcosθ
2
c(1)
whe e γis he su ace ension, θ he con ac angle, and he po e adius
defined as he adius o he s uc u ing elemen used in he mo pho-
logical opening [24]. This me hod is pa icula ly sui able o ou pu -
poses whe e he po e-size dis ibu ion de e mined om imaged po e
space is linked o measu ed soil mois u e cha ac e is ic cu e. The
s uc u al aniso opy o he sample was quan ified by calcula ing he
deg ee o aniso opy using he me hod based on g ey-scale g adien
enso [25].
2.4. Helium ion mic oscopy
Helium ion mic oscopy (HIM) is a no el de elopmen wi hin he
amily o scanning beam mic oscopes. Ins ead o using elec ons o
beam pa icles, as in con en ional scanning elec on mic oscopes
(SEM), helium ions a e used. Helium ions ha e a ocal dep h ha is
5–10 imes la ge han elec ons in SEM, a beam spo size below 0.5 nm,
and e y small in e ac ion olume p oducing seconda y elec ons, e-
sul ing in images wi h e y high esolu ion. Ano he impo an ad-
an age o HIM is i s capabili y o image insula ing ma e ials wi hou
cha ging effec s (no me al coa ing is needed) as he posi i e su ace
cha ging om helium ions and seconda y elec ons can be neu alized
wi h an elec on flood gun. A Zeiss O ion NanoFab Helium ion mic o-
scope was used in his wo k.
2.5. Soil sample p epa a ion
Expe imen al soil was ai -d ied in oom empe a u e and sie ed
K. Rasa e al. Biomass and Bioene gy 119 (2018) 346–353
347
h ough 6 mm mesh. Fou diffe en kinds o samples we e p epa ed o
incuba ion expe imen . Hal o he samples had na u al (NAT) ag-
g ega e s uc u e (< 6 mm). The soil o o he hal was homogenized
(HOM) using a olle -mill and hen sie ed (2 mm) in o de o des oy
soil mac o agg ega es. Samples wi h NAT and HOM s uc u e ecei ed
0% o 5% mass ac ion biocha amendmen (on d y ma e basis).
Biocha used in he expe imen was c ushed in o fine powde using
blende . Al oge he 14 samples we e p epa ed.
The incuba ion expe imen was conduc ed in 190 cm
3
(7.1 cm in
diame e ) me al cylinde s. One end o each cylinde was co e ed wi h a
hin ab ic o p e en sample o loss om he cylinde . The cylinde s
we e filled wi h soil o mix u e o soil and biocha . The samples we e
gen ly shaken and p essed using 10 kg weigh . Finally, he excess ma-
e ial was emo ed by spa ula.
2.6. Incuba ion
Soil samples we e sa u a ed wi h wa e o six weeks. Sa u a ed
samples we e mo ed o sandbox (Eijkelkamp) and he ma ic po en ial
was g adually adjus ed o −10 kPa. The ea e , he samples we e in-
cuba ed unde cons an ma ic po en ial (−10 kPa) and empe a u e
(5 °C) condi ions o six weeks. Cool incuba ion empe a u e was used
o hinde he mic obial ac i i y in he samples.
2.7. Soil mois u e cha ac e is ic cu e
Volume ac ion o soil wa e in ma ic po en ial ange om −0.3 o
−10 kPa was de e mined using sandbox (Eijkelkamp), whe eas p es-
su e pla e ex ac o (Soilmois u e Equipmen Co p.) equipped wi h
100 kPa, 300 kPa ba and 500 kPa ce amic pla es was used o p oduce
lowe ma ic po en ials. In each s ep he soil samples we e le o
equilib a e be o e de e mina ion o he wa e con en . The lowes ma-
ic po en ials −1500 kPa and −4000 kPa, we e c ea ed osmo ically by
placing 1 g soil sample in o desicca o s equipped wi h con aine s wi h
sa u a ed solu ions o ammonium oxala e ((NH
4
)
2
C
2
O
4
) o sodium
chlo ide (NaCl), espec i ely. The ea e he samples we e emo ed
om desicca o s and he wa e con en o he samples we e de e mined
by weighing he samples be o e and a e d ying hem a 105 °C.
Equilib a ion ime a ied be ween 1 day (a −0.3 kPa) and 6 weeks (a
−1500 and −4000 kPa) depending on he ma ic po en ial.
2.8. Effec o biocha amendmen on soil po osi y
Da a used o de e mine he soil mois u e cha ac e is ic cu e (SMC)
was also used o s udy he effec o added biocha on he po e size
dis ibu ion (See Eq (2). below). Howe e , ins ead o using olume
ac ion o soil wa e , he wa e con en s we e exp essed as a io o
wa e olume o d y mass o sample. This a io was used o ensu e ha
esul s con ain only he po e olume ela ed o biocha o each po e
size in e al. Fi s , he soil wa e po en ials we e con e ed o ap-
p oxima e po e sizes using he Young-Laplace equa ion (Eq. (1)). While
Young-Laplace equa ion is de i ed o s aigh capilla ies wi h ci cula
c oss sec ion and does no di ec ly apply o soils wi h mo e complex
po e geome y, i can be used o de e mine so-called equi alen po e
diame e (EPD) d = 2 . Then, he wa e con en s o he samples we e
in e pola ed on 2.75 μm po e size in e al using linea in e pola ion.
This inc emen co esponds o he inc emen in size o s uc u e ele-
men used in de e mina ion o po e size dis ibu ion om omog aphy
images.
Finally, he effec o biocha on soil po osi y was calcula ed as
ollow:
=− −−
−− −
WWMWWM
Δ
()/()/,
pp p p p p p p p[, Δ] Δ 5% Δ 0% (2)
whe e Δ is he effec o biocha amendmen on specific po e olume o
soil in each po e size in e al (cm
3
/g), W is he olume ac ion o soil
wa e o he sample (=po e olume, cm
3
), M is he d y ma e weigh
o he sample (g), p is he uppe limi and p-Δp lowe limi o po e size
in e al (p = 2.75 μm, 5.5 μm, …;Δp = 2.75 μm) and ˂˃
5%
and ˂˃
0%
ep esen s he a e age o he samples wi h and wi hou biocha .
Calcula ion was pe o med sepa a ely o wo diffe en soil s uc u es
(HOM, NAT).
2.9. S a is ical analysis
The dependence o plan a ailable wa e on he biocha addi ion
and soil s uc u e was s udied using wo-way analysis o a iance.
Dependen a iables we e p opo ions ( alues be ween 0 and 1) and
he e o e logi ans o ma ion was used o make he model assump ion
applicable. Fo he p esen a ion o he esul s, he es ima ed means
we e ans o med back o he o iginal scale. Bon e oni co ec ion was
applied o mul iple compa isons. All s a is ical analyses we e pe o med
using Ma lab wi h S a is ics Toolbox.
3. Resul s and discussion
3.1. X- ay omog aphy o willow biocha
The isual obse a ion o he X- ay omog aphy image e ealed ha
he po e s uc u e o willow biocha py olysed a 320 °C e ained he
ini ial s uc u al cha ac e is ics o esh s em wood (Fig. 1). Ob iously,
he ascula issues (xylem) in sapwood and/o hea wood appea as
biocha po osi y. Analogously o li ing wood issue, hese po es a e
Fig. 1. X- ay omog aphic econs uc ion o
willow biocha (le ). The 3D isualiza ion
o omog aphy da a was p oduced wi h
Pa a iew [29] and Blende . The omo-
g aphy da a consis ing o he imaging slices
we e fi s o a ed wi h ImageJ [30] in a way
ha po es a e pe pendicula o he cu ing
ace. A e his a cuboid piece was cu om
a sample. A 57 μm hick slice cu om he
middle o a cuboid piece is shown on igh .
K. Rasa e al. Biomass and Bioene gy 119 (2018) 346–353
348
mainly esponsible o wa e s o age and anspo wi hin he biocha .
The biocha po e sys em was quan ified by image analysis o he X-
ay omog aphy image. The esul s show ha he o al po osi y o
biocha is 0.6 and he po e size dis ibu ion is bimodal wi h local
maxima a ound diame e s 10 and 50 μm(Fig. 2 op panel). The deg ee
o aniso opy was 32.0 (see Hy äluoma e al. [18] o e e ence alues
o deg ee o aniso opy o se e al wood based biocha s) which p o es
ha po osi y o he biocha is highly aniso opic and consis s o pa allel
cylind ical po es wi h minimum numbe o la e al connec ions ( e ified
by HIM, see below). In gene al, hese findings a e well in line wi h
ea lie published 2D scanning elec on mic oscopy images indica ing
bimodal cha ac e is ics o esh willow wood s uc u e e.g. Re s.
[26,27]. In addi ion, a ecen pape [28] discussing he effec o py -
olysis empe a u e on mic ome e -scale po osi y o willow (Salix
schwe inii ‘Amgunskaja') biocha shows simila bimodal po e cha -
ac e is ics as in he p esen pape .
The used imaging esolu ion p e en s obse a ion o po es in sub-
mic ome e size ange. Howe e , he low BET su ace a ea
(6.8 ± 0.43 m
2
g
−1
) sugges s poo ly de eloped nanopo osi y o he
s udied biocha . This sugges ion is in line wi h p e ious s udies e-
po ing de elopmen o py ogene ic nanopo es only in highe py olysis
empe a u es [10,31,32]. To e i y his hypo hesis, we s udied nano-
po osi y o biocha po e walls using HIM, which indica ed ha cell wall
s uc u es emaining in he biocha do no con ain isible nano-scale
po es (Fig. 3). HIM obse a ions and deg ee o aniso opy oge he
sugges ha wa e s o age and flow wi hin willow biocha akes place
in “bundle o cylind ical capilla ies”as is assumed in Eq. (1) (see also
discussion in Sec. 3.3).
3.2. Effec s o biocha amendmen on soil-mois u e cha ac e is ics
Addi ion o biocha educed he bulk densi y o NAT ( om 0.91 o
0.87 g cm
−3
, P = 0.001) and HOM ( om 1.08 o 0.93 g cm
−3
,
P < 0.001) samples compa ed o samples wi hou biocha . The effec
was mo e p onounced in HOM samples, because in HOM samples bio-
cha eplaces mo e mine al soil han in he case o NAT. In NAT samples
biocha alls pa ially in po es be ween agg ega es and hus affec s he
soil bulk densi y less. These indi ec effec s o biocha amendmen ex-
plain changes obse ed in he we -end o SMC ( ela ed o po es wi h
la ge size han he size o in e nal po es o biocha , see below).
Biocha applica ion influenced soil mois u e cha ac e is ics o e he
en i e ange o measu ed ma ic po en ials. The majo changes occu ed
a ma ic po en ials abo e −316 kPa, while diffe ences le elled ou a
he d y-end o he cu e (Fig. 4). Acco ding o Young-Laplace equa ion,
in ma ic po en ial o −300 kPa wa e emaining in soil is s o ed in
po es wi h EPD < 1 μm. Biocha addi ion hus modifies soil po osi y
especially a po e size egime EPD > 1 μm. In gene al, his finding in-
dica es ha biocha wi h high mic ome e -scale po osi y, obse ed wi h
X- ay omog aphy and 3D image analysis, eflec s di ec ly o soil WHC
in egime impo an o plan wa e up ake (Fig. 4,Table 1).
Close inspec ion o SMC o NAT and HOM samples (Fig. 2) e eals
ha biocha applica ion inc eased po osi y in wo specific egimes. In
bo h soils (i espec i e o soil s uc u e), a clea inc ease in po osi y
occu ed a po e sizes a ound 25 μm (EPD). The o he maximum is
app oxima ely a EPD 5 μm and 10 μm o NAT and HOM samples, e-
spec i ely. These peaks co espond he po e sizes obse ed in he bio-
cha (10 and 50 μm, EPD) mul iplied by ac o 0.5 o la ge po es. This
ac o diffe s sligh ly be ween NAT and HOM samples o smalle po e
class. The ac ha a bimodal shape is obse ed bo h in he po e-size
dis ibu ion de e mined wi h image analysis and de i ed om SMC
p o ide s ong suppo o he in e ence ha biocha affec s he soil
mois u e cha ac e is ics g ea ly ia di ec mechanism. In nex sec ion
some biocha - ela ed p ope ies affec ing how biocha in e nal po osi y
ansla es o changes obse ed in SMC a e discussed in mo e de ails.
3.3. Biocha po e sizes s. changes in soil mois u e cha ac e is ics
The e a e se e al easons why i is unlikely ha a one- o-one
compa ibili y be ween 3D image analysis and SMC esul s will be ob-
ained. Con e sion o he SMC o po osi y esponse cu e was by using
he Young-Laplace equa ion wi h gene alized assump ions which a e
ypically no ully alid o any p ac ical po ous ma e ial including he
po e sys em conside ed he e. One assump ion is ha po e space consis s
o a bundle o cylind ical capilla ies wi h uni o m ci cula c oss sec ion
o diffe en sizes and pe ec ly smoo h su aces. In addi ion i is as-
sumed ha po e walls a e chemically homogeneous and pe ec ly
we ing, i.e., in he Young-Laplace equa ion cos θ= 1 a all po e su -
aces.
Fig. 2. Po e size dis ibu ion de e mined by 3D image analysis o X- ay omo-
g aphy image ( op panel) and he change in he po e size dis ibu ion due o
biocha addi ion o NAT (middle panel) and HOM (bo om panel) samples.
Effec o biocha on soil po osi y (Δ , cm
3
g
−1
soil) was de e mined by sub-
ac ing he soil wa e con en (exp essed as a a io o wa e olume o d y mass
o sample) o pu e soil om ha o soil-biocha mix u e as desc ibed in Sec. 2.8.
Ma ic po en ial was con e ed o equi alen po e size using Eq. (1).
K. Rasa e al. Biomass and Bioene gy 119 (2018) 346–353
349

P ac ically all po e su aces a e ough. Wenzel's model is widely
used o desc ibe he effec o su ace oughness on he con ac angle
[33]. Ano he effec o su ace he e ogenei ies is so-called con ac angle
hys e esis which means ha con ac angle is no unique bu a ies
be ween wo ex eme alues known as eceding and ad ancing con ac
angles. Also, assump ion o pe ec ly we ing po e walls is no ealis ic.
To accoun o bo h non-pe ec we ing and oughness, one can w i e
=
∗
cos θ cos θ
(3)
whe e is he oughness ac o defined as he a io o he ue su ace
a ea o he appa en (p ojec ed) a ea and θ* is so-called in insic con-
ac angle. In eali y biocha s a e subc i ically hyd ophobic
(0° < θ* < 90°) and po e su aces ough, whe eby cos θ* < 1 and
> 1. The e o e, depending on he physical and chemical p ope ies o
po e walls, po e sizes can be ei he la ge o smalle han hose deduced
om SMC. The impo ance o we ing p ope ies on he hyd ologic
effec s o biocha on soil has been demons a ed by Suliman e al. [34].
I mus also be emphasized ha simple app oaches o we abili y
(including he discussion abo e) do no conside complica ed ime-de-
penden beha io obse ed in many na u al sys ems. We abili y o
many na u al ma e ials, including soil and biocha , is a highly complex
phenomenon which is no ye p ope ly unde s ood.
In p ac ice, i is difficul o de e mine he con ac angle needed o
p ope ly use he Young-Laplace equa ion. The commonly used me hod
ha is based on op ical de e mina ion o he con ac angle o a sessile
d op does no measu e in insic con ac angle as he measu emen is
pe o med on a ough su ace o ma e ial. Sessile d op me hod has been
used o de e mine con ac angle o biocha s [15,35]. I is impo an o
no e ha his la ge-scale oughness does no co espond o he small-
scale oughness used in Eq. (2). The in insic con ac angle should be
de e mined a he h ee-phase con ac line wi hin he po e which is no
possible in p ac ice. Fu he mo e, he de i ed oughness ac o in Eq.
(2) is he oughness a po e walls which also is p ac ically impossible o
de e mine.
The discussion abo e explains he ac o s why he e is no one- o-
Fig. 3. Helium ion mic oscopy images o willow
biocha . Size o he images (a) 1 mm, (b) 50 μm, (c)
4μm, and (d) 2 μm. In image (a) o e all po osi y o
biocha is shown while image (b) is ocused on po es
wi h EPD o app ox. 5–10 μm. Image (c) shows po e
wall be ween he biocha po es and (d) is he same
po e wall wi h highe magnifica ion indica ing ha
he e is no isible connec ions be ween indi idual
po es.
Fig. 4. Soil mois u e cha ac e is ics cu e o each s udied soil s uc u e and
biocha combina ions. P esen ed esul s a e a e age o each ea men s (n = 4
and n =3 o samples wi h and wi hou biocha addi ion, espec i ely). E o
ba s deno e s anda d de ia ions.
K. Rasa e al. Biomass and Bioene gy 119 (2018) 346–353
350
one co espondence be ween he po e sizes de e mined by image ana-
lysis and hose de i ed om SMC. We also obse ed ha he e is di -
e en ans o m coefficien be ween he la ge po es and small po es.
One explana ion could be he exis ence o cons ic ions o h oa s in he
biocha po es ha could be o med, e.g., by a compounds which could
pa ially clog he po es.
Image analysis and po osi y esponse de i ed om SMC can be di-
ec ly compa ed only i all po es a e connec ed o he sample su ace
di ec ly o h ough la ge po es. The Image analysis de e mines po e
size dis ibu ion while SMC me hod is ela ed o he size dis ibu ion o
po e h oa s. E en hough he po es in he used biocha we e ubula
and did no ha e ob ious h oa s (see Secs. 3.1 and 3.2), we ne e -
heless es ima ed he effec o po e h oa s on he compa ison by de-
e mining he h oa size o po e size a io using he me hod desc ibed
in Appendix B.
In he analysis, we sepa a ed he wo po e size classes p esen in he
s udied biocha by using po e diame e 25 μm as h eshold be ween
la ge and smalle po es. The a e age h oa size o po e size a ios
we e 0.97 and 0.78 o he la ge and smalle po es, espec i ely. Thus
o la ge po es he h oa effec was p ac ically negligible while o
smalle po es a clea effec was obse ed. This explains a leas pa ly
why he posi ions o modes ela ed o smalle and la ge po es a e no
shi ed by same ac o when compa ing SMC esul s o po e size dis-
ibu ion de e mined by image analysis (Fig. 2).
Al hough he hypo he ical conside a ion o biocha di ec effec on
he SMC is well es ablished, he me hodological limi a ions ha e hin-
de ed s udies on his issue. Howe e , he SMC esul s o sandy soils and
SEM images o biocha p esen ed by Abel e al. [36] could possibly be
in e p e ed in simila way han in ou s udy. Liu e al. ound ha bio-
cha amendmen shi ed soil po e size dis ibu ion owa d smalle
po es, bu biocha effec was no unambiguously es ablished [5].
In addi ion o he di ec influence o biocha in e nal po e sys em,
biocha affec s soil mois u e cha ac e is ics ia indi ec mechanisms,
i.e. soil-biocha in e ac ions affec ing soil s uc u e (agg ega e o ma-
ion) and/o changes in soil pa icle size dis ibu ion [6,37]. The effec
o soil s uc u al changes p obably explains why he modes o HOM
ea men a e wide han hose o NAT ea men (Fig. 2). In e ac ions
be ween soil-soil and soil-biocha a e mo e common in HOM ea men
wi h des oyed soil agg ega es han in NAT ea men whe e biocha
ac s mo e independen ly in ca i ies be ween exis ing soil agg ega es.
In any case, obse ed bimodal po e size dis ibu ion o willow
biocha and consequen inc ease in soil wa e e en ion p ope ies a
co esponding po e size egimes (mul iplied by ac o 0.5) p o ides
s ong e idence ha biocha in e nal mic ome e -scale po osi y con-
ibu es di ec ly o soil wa e s o age. This ou come encou ages u he
s udies and me hodological de elopmen in o de o imp o e unde -
s anding on ela ionship be ween biocha mic on scale po osi y and i s
di ec effec on soil mois u e cha ac e is ics.
3.4. Effec s o biocha amendmen on soil plan a ailable wa e
Biocha applica ion inc eased he plan a ailable wa e (PAW) by
17% and 32% o HOM and NAT samples in compa ison o samples
wi h no biocha addi ion (Table 1). I is no able ha in bo h s udied soil
s uc u es he biocha induced inc ease in PAW occu ed mos ly ma ic
po en ial egime om −10 o −316 kPa (Fig. 2), conside ed he e as
eadily plan a ailable wa e (RPAW), which co esponds o po es wi h
EPD > 1 μm.
Wi h espec o c op p oduc ion a d y condi ions and/o du ing
igo ous g ow h, inc eased s o age o RPAW helps plan s o main ain
po en ial anspi a ion a e o longe ime. I is well documen ed ha
d y ma e p oduc ion o plan s i ually ceases when hey lose u go
p essu e and his may occu a be o e pe manen wil ing poin (PWP)
[38]. Meye and G een epo ed ha whea could use 52–57% o PAW
be o e educ ion in g ow h became e iden [39]. In wide pe spec i e,
measu es like biocha amendmen o inc ease wa e p oduc i i y in
ag icul u e (i.e. ou pu yield pe uni wa e used) a e o impo ance in
he s uggle agains wa e sca ci y h ea ening global ood secu i y
[40].
The p esen s udy highligh s he impo ance o a di ec link be ween
he biocha mic ome e -scale po osi y and RPAW. I espec i ely o soil
s uc u e, biocha amendmen inc eased soil po osi y a he same po e
size egimes (EPD 5–10 and 25 μm, Fig. 2), which e e o biocha in-
duced changes ia di ec mechanism (biocha in e nal po osi y). I
could be hypo hesized ha his specific biocha would esul in simila
changes in o he soil ypes as well. I he hypo hesis holds, he app oach
p esen ed abo e could p o ide a ascina ing ool-box enabling de el-
opmen o ailo ed biocha s ha could modi y soil mois u e cha -
ac e is ics a well-defined mois u e egimes o e he ange o soil ypes.
Such ailo ed biocha p oduc s could imp o e biocha pe o mance in
c op p oduc ion a diffe en clima ic a eas, soil ypes and p oduc ion
sys ems, which is essen ial when sus ainabili y o biocha is conside ed
in global scale [1]. In any case, his hypo hesis calls o u he ca e ul
s udies. Howe e , i should also kep in mind ha while biocha effec s
on SMC ia di ec mechanism may be independen o soil ype, he
biocha induced changes in soil bulk densi y, ex u e and s uc u e a e
s ongly depend on he p ope ies o ecei ing soil and he ype o
biocha used.
4. Conclusions
The p esen s udy add esses he impo ance o biocha mic ome e -
scale po osi y when used as soil amendmen . I was shown ha biocha
in e nal po osi y de ec ed by X- ay omog aphy modifies SMC ia di ec
mechanism. Bimodal po osi y o willow biocha ac ed as aceable
ma ke when added o a clay soil. Howe e , a co ec ion ac o o
p edic ela ionship be ween biocha po e size dis ibu ion and i s ob-
se ed effec s on SMC was needed (in his case close o 0.5) due o e.g.
oughness and hyd ophobici y o biocha po es. Because biocha was
shown o inc ease soil po osi y due o i s in e nal po osi y (di ec me-
chanism) i could be hypo hesized ha his effec is independen om
soil ype. On he o he hand, biocha induced changes in soil ex u e
and s uc u e, and modified SMC simul aneously (indi ec mechanism).
The la e mechanism is known o be s ongly dependen on soil and
biocha p ope ies, which may explain why esul s on biocha effec on
SMC a e o en con o e sial.
F om he p ac ical poin o iew, he willow biocha used in his
expe imen can g ea ly inc ease amoun o eadily plan a ailable wa e
(be ween ma ic po en ials −10 and −316 kPa) in clay soil i espec i e
o soil ini ial s uc u e. The esul sugges s ha biocha amendmen
Table 1
Wa e olume ac ion a ma ic po en ial o −10 kPa (Field capaci y, FC) and −1500 kPa (Pe manen wil ing poin , PWP), plan a ailable wa e (PAW) be ween
hese ma ic po en ials and eadily plan a ailable wa e (RPAW) be ween ma ic po en ials o −10Kpa and −316 kPa.
FC (cm
3
cm
−3
) PWP (cm
3
cm
−3
) PAW (cm
3
cm
−3
) RPAW (cm
3
cm
−3
)
NAT 0% 0.36 p < 0.001 0.13 p = 0.01 0.22 p < 0.001 0.08 p = 0.02
5% 0.41 0.12 0.29 0.14
HOM 0% 0.45 p = 0.04 0.15 p < 0.001 0.30 p < 0.001 0.07 p < 0.001
5% 0.48 0.12 0.35 0.18
K. Rasa e al. Biomass and Bioene gy 119 (2018) 346–353
351
would help plan s o wi hs and mois u e s ess du ing d y pe iods and/
o igo ous g ow h s a e. Wi h espec o biocha use in g owing media
o ag onomic applica ions, we s ongly ecommend u iliza ion o e-
sea ch me hods capable o e eal mic ome e -scale po e s uc u e o
biocha s.
Acknowledgemen s
Riikka Keskinen, Johanna Nikama and Ilkka Sa ikka a e wa mly
hanked o ca ying ou he labo a o y wo k ela ed o he soil
mois u e cha ac e is ic cu e. We also wish o hank Mi ja Muhola and
A e Mikkelson om VTT Technical Resea ch Cen e o Finland L d o
ca ying ou BET su ace a ea and CHNSO analysis. This p ojec has
ecei ed unding om he Eu opean Union's Ho izon 2020 esea ch and
inno a ion p og amme unde g an ag eemen No 637020 −MOBILE
FLIP.
Appendix A
Image p ocessing
G ey-scale X- ay omog aphy image was fil e ed wi h a h ee-dimensional median fil e ( adius 2). Fil e ed image was segmen ed in o po e and
solid oxels by global h esholding. Th eshold alue was selec ed wi h he modified O su's me hod [41] desc ibed Hy äluoma e al. [18]. This
modified e sion u ilizes ideas p esen ed ea lie by Hapca e al. [42]. Segmen ed image was hen fil e ed wi h a majo i y fil e ( adius 2) and finally
isola ed (“floa ing”) objec s wi h olume less han 1000 oxels we e emo ed om he image. A c oss sec ion o he g ey-scale image and co e-
sponding c oss-sec ion o he segmen ed and fil e ed image is shown in Fig. A1.
Figu e A1. C oss sec ions o he x- ay omog aphy images o he willow biocha . On le , g ey-scale image wi h g ey-scale his og am in he inse . Ligh e colo in he
his og am shows he g ey-scale alues in e p e ed as po e space and da ke hose in e p e ed as solid ma e ial. On igh , final segmen ed and fil e ed image used in
analyses.
Appendix B
Po e h oa analysis
The a io o po e size and he smalles h oa leading o ha po e was es ima ed in he ollowing way. As a s a ing poin , a omog aphic image
ha was segmen ed in o po es and solids was used. Me hod u ilizes wo scala fields and one ec o field de i ed om he image. Fi s is dis ance
ans o m which de e mines he sho es dis ance o he po e walls o each po e oxels. Dis ance ans o m was compu ed using so-called d [3–5,7]
Cham e me ics wi h a escaling p ocedu e as explained in de ail by S ensson and Bo ge o s [43]. Secondly, o each oxel a po e size alue was
de e mined by mo phological opening, e.g. Re . [24]. Finally, we compu ed po e-scale flow field o sa u a ed p essu e-d i en flow wi h d i ing
p essu e pa allel wi h he po e di ec ion. He e he la ice Bol zmann me hod was u ilized o flow compu a ion [44]. D3Q19 la ice Bol zmann model
wi h wo- elaxa ion ime collision ope a o was used and he no-slip bounda y condi ion a fluid-solid bounda ies was en o ced wi h bounce-back
bounda y condi ion. Sample size in he simula ion was 454 × 400 × 638 μm
3
. Fu he de ails o he used me hod can be ound om, e.g., [45].
Th oa size was hen de e mined by u ilizing dis ance ans o m, po e size and flow fields.
Inspec ion poin s we e selec ed as local maxima o dis ance ans o m which we e de e mined using a 5 × 5 × 5 mo ing window. Fo hese
poin , he s eamlines we e in eg a ed bo h ups eam and downs eam, and he smalles po e size ound ollowing he s eamline was hen de-
e mined in bo h di ec ions. As poin s along s eamline do no coincide wi h he g id nodes, ilinea in e pola ion was u ilized o de e mine he
po e-size alues. This me hod p oduces wo h oa sizes, one ups eam and ano he downs eam om he inspec ion poin . La ge o hese h oa s
was selec ed as i is he easie ou e o he po e o be d ained. The h oa size hus de e mined is compa ed o he po e size a he inspec ion poin o
de e mine he h oa o po e size a io.
I should be no ed ha , as he omog aphy image used in his analysis is smalle han he g ain size used in he expe imen , he esul s ob ained
a e only app oxima e. In eal biocha g ains ou es o su ace a e longe and hus smalle h oa s could be p esen . The e o e, he esul s ob ained
can sligh ly unde es ima e he effec o po e h oa s.
K. Rasa e al. Biomass and Bioene gy 119 (2018) 346–353
352
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