SOIL, 8, 373–380, 2022
https://doi.org/10.5194/soil-8-373-2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.
S
OIL
What comes after the Sun? On the integration
of soil biogeochemical pre-weathering
into microplastic experiments
Frederick Büks and Martin Kaupenjohann
Chair of Soil Science, Department of Ecology, Technische Universität Berlin, 10587 Berlin, Germany
Correspondence: Frederick Büks (frederick.b[email protected])
Received: 2 July 2021 – Discussion started: 6 September 2021
Revised: 8 April 2022 – Accepted: 5 May 2022 – Published: 23 May 2022
Abstract. Recent studies have been engaged in estimating the adverse effects of microplastic (MP) on soil qual-
ity parameters. Mass concentrations of MP, as found in highly contaminated soils, have been shown to weaken
the soil structure, and parts of the edaphon are adversely affected mainly by the <100 µm MP size fraction.
However, the vast majority of these studies used pristine particles, which have surface characteristics different
from those of environmental MP. Exposed to UV radiation, plastic undergoes photochemical weathering with
embrittlement and the formation of surface charge, leading to an alteration of physiochemical behavior. When
plastic particles then enter the soil environment, further aging factors appear with yet unknown efficacy. This
little explored soil biogeochemical phase includes biofilm cover, decay with enzymes (as shown in laboratory
experiments with both conventional and biodegradable plastics), contact with biotic and abiotic acids, oxidants,
and uptake by the soil fauna that causes physical fragmentation. Such transformation of the surfaces is assumed
to affect soil aggregation processes, soil faunal health, and the transport of plastic colloids and adsorbed sol-
ubles. This perspective article encourages us to consider the weathering history of MP in soil experiments and
highlights the need for reproducing the surface characteristics of soil MP to conduct laboratory experiments with
closer-to-nature results.
1 Did we neglect biogeochemical aging factors?
Since the mass production of plastic articles for daily use
started in the early 1950s (Thompson et al., 2009), a number
of processes have caused the contamination of ecosystems
such as inland and coastal waters, sediments, the open and
deep seas, soils, and even the atmosphere with microplastic
(MP; e.g., Cole et al., 2011; Woodall et al., 2014; Wu et al.,
2018; Büks and Kaupenjohann, 2020; Trainic et al., 2020).
The formation of soil MP pools occurs through littering and
dispersion from landfills, the application of wastewater, con-
taminated surface water, sewage sludge, composts, diges-
tates, mulching foils, seed and fertilizer coatings, road dust,
and atmospheric deposition (Eerkes-Medrano et al., 2015;
Huerta Lwanga et al., 2017a; Weithmann et al., 2018; Cor-
radini et al., 2019; Dierkes et al., 2019; He et al., 2019; Edo
et al., 2020; Huang et al., 2020; Bertling et al, 2021; Katsumi
et al., 2021; Szewc et al., 2021).
Today, we are facing a global contamination of soil
ecosystems with MP that averages 1.7 mg kg−1of dry soil
in agricultures (Büks and Kaupenjohann, 2020), exceeds this
value by several orders of magnitude in heavily contami-
nated soils at road sides and industrial areas (Fuller and Gau-
tam, 2016; Dierkes et al., 2019), and reaches even remote ar-
eas (Abbasi et al., 2021). Several laboratory studies showed
the adverse effects of high MP concentrations on the soil
fauna (Büks et al., 2020a) and soil structure (e.g., de Souza
Machado et al., 2018, 2019; Liang et al., 2019; Lozano et al.,
2021) and underlined the relevance of especially the small-
sized fraction (MP <100 µm) (Büks et al., 2020b).
Although these results are rightly alarming due to the func-
tion of the soil structure and the edaphon as soil fertility pa-
rameters (Bronick and Lal, 2005; Thiele-Bruhn et al., 2012),
Published by Copernicus Publications on behalf of the European Geosciences Union.
374 F. Büks and M. Kaupenjohann: On the integration of soil BG pre-weathering into microplastic experiments
their informative value is limited by the fact that the vast ma-
jority of experiments used pristine plastic and a short runtime
that does not allow for further weathering (e.g., de Souza
Machado et al., 2018, 2019; Liang et al., 2019; Büks et al.,
2020a; Lozano et al., 2021). For a better matching with en-
vironmental conditions, some studies have used photooxida-
tively weathered plastic. In soil, however, the bulk of MP is
additionally exposed to biogeochemical alteration for years.
Its surface characteristics and role within soil ecosystems
thereby possibly change compared to solely aboveground
weathering.
2 Underground weathering – an additional phase of
aging?
On a microscopic scale, the surfaces of pristine plastic items
are normally smooth with nearly no surface charge (e.g., Fo-
topoulou and Karapanagioti, 2012, 2015). Exposed to sun-
light, the depletion of UV absorbers and HALS (hindered
amine light stabilizers) leads to enhanced photooxidation
(e.g., Kokott, 1989; Pickett, 2018). From the point of view
of the macroscopic observer, the plastic becomes less hy-
drophobic, stiff, and more prone to fragmentation by wind
and water erosion.
Standardized approaches from materials science are newly
used in soil science to reproduce natural photooxidative ag-
ing characteristics (i.e., the German Federal Ministry of
Education and Research, BMBF, initiative “Plastic in the
Environment”; https://www.bmbf-plastik.de/en, last access:
8 April 2022; e.g., Büks et al., 2021). They recommend
xenon arc lamps with borosilicate filters that adjust the emit-
ted spectrum closer to the natural UV spectrum, or fluores-
cent UV lamps (see DIN EN ISO 4892-2/3, “Plastics – Meth-
ods of exposure to laboratory light sources”). The perfor-
mance of these approaches is enhanced by use of modern
daylight filters, a steady temperature of 38 ◦C, relative air hu-
midity of 25 % to 50 %, and regular washing of the sample
surfaces by artificial rain (Pickett, 2018). Besides the use of
UV, γirradiation treatment is reported to imitate the carbonyl
stretch in PE samples similar to a long-term UVB exposition
(Johansen et al., 2019). Furthermore, Zhou et al. (2020) could
demonstrate that discharged plasma oxidation (DPO) is like-
wise suitable for increasing the surface area, crystallinity, and
carbonyl indices of plastic particles within hours.
However, it is unknown whether these protocols prop-
erly reproduce the additional influence of underground ag-
ing, which occurs under different physiochemical conditions.
When plastic is exposed to the dark world of soil fauna, mi-
croorganisms, roots, and frequent leaching, the composition
of weathering parameters changes significantly (Table 1).
The plastic is now faced to new mechanical stresses such as
(bio-)turbation, largely moist conditions, and is exposed to a
variety of soil biogeochemical processes.
One of these potential aging factors is the diverse and ac-
tive soil fauna that has been shown to ingest, digest, and ex-
crete plastic particles (Büks et al., 2020). It is an ensemble of
small, mobile bioreactors that incubate soil particles, includ-
ing MP, within a habitat of high microbial diversity – their
gastrointestinal tract – and distribute them throughout the soil
by excretion. A well-known example for this multifaceted
functionality is the earthworm. Some taxa like woodlice, ter-
mites, mealworms, and earthworms have been additionally
found to comminute plastic by gnawing and, hence, actively
produce MP (e.g., Lenz et al., 2012; Zhang et al., 2018; Büks
et al., 2020a). There are also indications that the mealworm
microbiome is able to degrade not only additives but also PE
and PS polymers (e.g., Brandon et al., 2018).
While moisture evaporates quickly on sun-exposed, heated
plastic surfaces, in soils it is the ubiquitous condition for mi-
crobial life, extracellular metabolic processes, and the release
and transport of chemical agents that react with the plastic
outside the fauna. Microbial colonization and biofilm forma-
tion on surfaces of MP particles have been shown in stud-
ies on various aquatic ecosystems (e.g., Zettler et al., 2013;
McCormick et al., 2014; Oberbeckmann et al., 2015; Dus-
sud et al., 2018; Jiang et al., 2018). Recent studies on soil
ecosystems have also demonstrated that MP surfaces of dif-
ferent origin are covered with microbial communities. This
could hypothetically cause a masking of plastic surface char-
acteristics by the biofilm matrix. The composition of surface
MP communities is very different from that of the soil ma-
trix (Chai et al., 2020; Zhang et al., 2019). The altered soil
microbial community is thereby not only determined by the
physiochemical properties of the surrounding soil but also
by the type of plastic and its additives (Chai et al., 2020; Ng
et al., 2020; Wang et al., 2020; Wiedner and Polifka, 2020;
Yan et al., 2020; Yi et al., 2020). This might lead to a physio-
chemical behavior of plastic particles that differs not because
of the plastic type but because of its biofilm cover.
Although plastic is unlikely to serve as a major substrate
for microbes due to its large molecular size, high chem-
ical stability, and low bioavailability (Oberbeckmann and
Labrenz, 2020), there is indication that biofilm cover causes
the alteration of its chemical properties. Not only acting as
a viscous matrix that protects bacteria against mechanical
stress, predators, desiccation, and irradiation, biofilm is also
an extracellular reaction space that facilitates the concen-
tration and metabolization of nutrients and the recycling of
dead cell material (Flemming and Wingender, 2010). For
this purpose, manifold extracellular enzymes are produced
by the biofilm community to decompose food sources or
modify the biofilm matrix in face of, e.g., oxygen or nutrient
gradients (Flemming and Wingender, 2010). Among these
are esterases, proteases, and amidases that target substrates
like polysaccharides, proteins, extracellular DNA, lipids, and
urea but also allow the (co)metabolization of artificial poly-
mers such as diverse polyesters, ester-based PU, and PET
SOIL, 8, 373–380, 2022 https://doi.org/10.5194/soil-8-373-2022
F. Büks and M. Kaupenjohann: On the integration of soil BG pre-weathering into microplastic experiments 375
in laboratory experiments (Shimao, 2001; Wei and Zimmer-
mann, 2017; Danso et al., 2019).
Given a poor biodegradability of polymers with C–C back-
bones and no hydrolyzable functional groups such as pris-
tine PE, PP, PS, and PVC, laboratory experiments showed
an unexpected degradation of PE by a bacterial alkane hy-
droxylase (Yoon et al., 2012) and, beyond this, the specific
targeting of PET with a bacterial PETase (Yoshida et al.,
2016). In contrast, neither degrading enzymes nor observed
biodegradation have been reported in the case of PP and PVC
(Danso et al., 2019). Unspecific lignin-degrading enzymes
such as laccases, manganese peroxidases, hydroquinone per-
oxidases, and lignin peroxidases produced by actinomycetes
and other bacteria, as well as fungi, have been further shown
to depolymerize even plastics such as PE, PS, and PA that
are considered recalcitrant (Bhardwaj et al., 2013; Wei and
Zimmermann, 2017). Beside the direct proof of enzymatic
degradation pathways, there are numerous references on the
metabolization of (bio)plastic samples by bacterial and fun-
gal strains (e.g., Bhardwaj et al., 2013; Kale et al., 2015;
Raziyafathima et al., 2016; Roohi et al., 2017). However,
since many studies have applied commercial polymers that
have concealed compositions (Danso et al., 2019), there is
often poor insight to what degree the measured mass loss is
caused by microbial/enzymatic decomposition of the poly-
mer or additives. These findings imply that biodegradation
of plastic surfaces in soil is conceivable.
Beside the soil biome, soil-born acids, bases, and oxidants
are expected to directly influence the belowground alteration
of plastic surfaces. While – to the best of our knowledge –
there has been no systematic examination of the effects of
such agents within natural ranges of concentration and time
of exposure, the treatment of plastic with highly concentrated
reagents caused damaging effects from color leaching and
expansion to total dissolution (Enders et al., 2017). However,
pre- and post-treatment with oxidants such as H2O2are com-
mon parts of the extraction of MP from soil samples with
density fractionation (Büks and Kaupenjohann, 2020).
In winter, when the biotic effects are reduced, freeze–
thaw cycles might be an additional factor of fragmentation.
Studies on the effect of alternating freezing and thawing on
the structure of plastic surfaces are sparse and only focus
on composite materials that include non-plastic components
(Wang et al., 2007; Adhikary et al., 2009; Zhou et al., 2014).
However, water that has entered cracks of brittle plastic most
likely contributes to its fragmentation through freezing and
expansion and, thus, increases the surface area exposed to
other aging factors.
3 Pre-weathering under soil conditions:
a methodology for future approaches?
While plastics are considered persistent, the above in vitro
experiments indicate that degradation in soil is possible. The
difference in factors leading to photooxidative and biochem-
ical weathering makes it plausible for MP surface character-
istics to develop differently in above- and belowground envi-
ronments. If the physicochemical behavior of the microplas-
tic is significantly affected by this, then the effects must be
considered in the design of laboratory and field experiments.
The focus on surfaces is particularly important in studies on
(1) soil aggregation and structure that strongly depend on
biofilm cover and surface charge/polarity of the involved pri-
mary particles, (2) adverse effects on the soil fauna that might
be influenced by particle shape and sorption of (in)organic
pollutants, (3) interactions with plants and microorganisms,
and (4) the transport of colloidal MP within the soil pore
space.
However, there are currently no studies that evaluate the
efficacy of specific soil biogeochemical aging mechanisms.
Recent work only showed the alteration of plastic surfaces
during environmental weathering, indicating that future ex-
periments have to be conducted with pre-weathered instead
of pristine MP. It is still an open question as to whether
there is effective soil biogeochemical aging beyond the pho-
tooxidative phase or whether the DIN EN ISO 4892-2/3 ap-
proach, as applied in recent work, is sufficient to imitate soil
weathering conditions in future studies.
Only a few studies have integrated soil biogeochemical
factors into pre-weathering approaches of artificial MP so far
(Table 2; Büks et al., 2020a), which are, alas, fragmentary,
heterogeneous, and often directly applied to pristine plastic.
Tsunoda et al. (2010) heated plastic items within a water bath
at 90 ◦C for 3 weeks and abraded the surface prior to feed-
ing experiments with termites. This treatment attempted to
make the surface more accessible for gnawing and also to
extract soluble additives from the pristine plastic. In another
experiment, the formation of biofilms on MP surfaces was
induced by 4 weeks of incubation in seawater to make the
material more attractive as a food source for the lugworm
Arenicola marina (Gebhardt and Forster, 2018), an approach
that can be likewise applied with soil solution. In order to
remove soluble substances and fine particles from artificial
MP, pristine plastics have been treated with organic solvents
(Huerta Lwanga et al., 2016, 2017b; Rodrigues-Seijo et al.,
2018, 2019; Wang et al., 2019; Yang et al., 2019). If the plas-
tic type is prone to the solvents, the surface is roughened by
the dissolution of oligomers and, thus, increased. However,
these techniques are not assumed to increase the number of
carbonyl groups and surface charge. Therefore, they do not
change the interaction with the soil matrix and the soil fauna
and have never been tested for the similarity to natural weath-
ering.
In contrast, some authors avoided artificial aging and in-
stead applied natural weathering over shorter periods be-
tween 2 weeks and 12 months, which can be used as a kind
of plastic breeding (e.g., Martin-Closas et al., 2016; Zhang et
al., 2018). This treatment changes the physiochemical char-
acteristics of plastics similar to environmental short-term
https://doi.org/10.5194/soil-8-373-2022 SOIL, 8, 373–380, 2022
376 F. Büks and M. Kaupenjohann: On the integration of soil BG pre-weathering into microplastic experiments
Table 1. Development of surface characteristics during the three phases of aging (pristine, photooxidative, and soil biogeochemical phase).
Data of soil biogeochemical weathering are only known from aquatic systems. The question mark (?) indicates assumptions based on soil
biogeochemical processes found in soils. Some references are from 1Fotopoulou and Karapanagioti (2012), 2Fotopoulou and Karapanagi-
oti (2015), 3ter Halle et al. (2017), 4Dong et al. (2020), 5Pickett (2018), 6Andrady et al. (1993).
Characteristic Pristine phase Photooxidative
phase
Soil biogeochemical
phase
Topography Smooth1,2,4Rough5Rough1,2,4
Surface charge, carbonyl index No1,2,3,4Yes6Increasing1,2,3,4
Crystallinity, cross-links,
chain scissions
Low3High5Increasing3,4
Biofilm cover Low Low Growing or mature2,5
Aging factors No UV radiation5
Blue/violet spectrum5
Frequent leaching5
Wind/water erosion
Enzymes(?)
Organic acids(?)
Inorganic acids(?)
Bases(?)
Oxidants(?)
Bioturbation(?)
Feeding by the edaphon(?)
Frequent leaching(?)
Freeze–thaw cycles(?)
Table 2. Approaches of surface (pre-)weathering in recent experiments with soil microplastic. The abbreviations used in this table are as
follows: UV – ultraviolet; TBBPA – tetrabromobisphenol A; FE – feeding experiment; BD – biodegradable plastic; OP – oxo-degradable plas-
tic, PA – polyamide; PE – polyethylene; PO – polyolefin; PP – polypropylene; PVC – polyvinyl chloride; TCE – thermoplastic copolyester
elastomer. NA denotes that information was not available.
Aging factor Applied plastic type Aging time (d) Resulting characteristics Experimental focus Reference
UV radiation (climate chamber) Diverse Variable Photooxidative aging Diverse DIN EN ISO 4892-2,
DIN EN ISO 4892-3
Gamma irradiation (60Co source) PE, PP NA Photooxidative aging Cation adsorption Johansen et al. (2019)
Discharged plasma oxidation (DPO) PVC 0.02 Photooxidative aging TBBPA adsorption of and
toxicity to algae
Zhou et al. (2020)
Water bath (90 ◦C) +abrasion PO, PA, PE, TCE 21 Extraction of additives,
increased accessibility
for feeding organisms
Feeding experiment
with termites
Tsunoda et al. (2010)
Incubation in seawater PA, PS 28 Surface biofilm formation FE lugworms Gebhardt and Forster (2018)
Incubation in aquatic systems PE, PP 19 Surface biofilm formation Cation adsorption Johansen et al. (2019)
Methanol treatment PE, PS NA Extract soluble additives FE earthworms Wang et al. (2019)
Ethanol treatment PE NA Extract soluble additives FE earthworms Rodrigues-Seijo et al. (2018)
PE NA Extract soluble additives FE earthworms Rodrigues-Seijo et al. (2019)
Pentane +octane treatment PE NA Extract soluble additives FE earthworms Huerta Lwanga et al. (2016)
NA Extract soluble additives FE earthworms Huerta Lwanga et al. (2017b)
NA Extract soluble additives FE earthworms Yang et al. (2019)
Plastic nursing (soil) BD, OD, PE ∼150 Belowground weathering Mulch foil degradation
experiment
Martin-Closas et al. (2016)
Plastic nursing (soil, compost) BD, PE 14–365 Belowground weathering Feeding experiment
with earthworms
Zhang et al. (2018)
SOIL, 8, 373–380, 2022 https://doi.org/10.5194/soil-8-373-2022
F. Büks and M. Kaupenjohann: On the integration of soil BG pre-weathering into microplastic experiments 377
weathering belowground and is suitable for the alteration of
large amounts of plastic. But, it is very costly in terms of time
when the production of strongly weathered MP is needed.
Once we know the important biogeochemical aging fac-
tors, long-term weathering experiments will be extremely
helpful to understand the dynamics of surface alteration of
soil MP. These experiments must take into account not only
ecosystem parameters (e.g., humidity, edaphon activity, and
soil organic carbon) and starting conditions such as plas-
tic type, particle surface, and protection by specific addi-
tives. The increase in surface area and charge density over
time might cause a non-linear aging, while the biofilm cover
cloaks the real MP surface characteristics – issues that should
also be carefully included in the experimental design.
There is a great incentive to develop pre-weathering ap-
proaches to create designer MP for laboratory experiments.
Those closer-to-nature weathering protocols might contain
full chains of aboveground and in-soil aging factors and can
be diversified according to actual material and environmental
conditions. When applied to coming experiments, they will
help us to better understand and predict the short- and long-
term effects of soil MP, the concentration of which is the re-
sult of decades of contamination and is still increasing.
Data availability. All of the data are published within this paper.
Author contributions. FB developed the article concept, col-
lected data, and prepared the paper. MK supervised the study by
participating in structural discussions on the idea and concept of the
paper and the final corrections.
Competing interests. The contact author has declared that nei-
ther they nor their co-author has any competing interests.
Disclaimer. Publisher’s note: Copernicus Publications remains
neutral with regard to jurisdictional claims in published maps and
institutional affiliations.
Financial support. This open-access publication was funded by
Technische Universität Berlin.
Review statement. This paper was edited by Peter Fiener and re-
viewed by two anonymous referees.
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