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Response of soil biochemical properties and ecosystem function to microplastics pollution

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Microplastics (MPs)-induced changes in soil nutrient cycling and microbial activity may pose a potential risk to soil ecosystem. Although some studies have explored these topics, there is still a large space for exploration and a relative lack of research on the mechanism by which soil health and its functions are affected by these changes. Thus, this study investigated the effects of polyethylene (PE) MPs with two particle sizes (13 μm and 130 μm) at five concentrations (0%, 1%, 3%, 6% and 10%, w/w) on soil biochemical properties and ecosystem function. The findings revealed that the exposure to 13 μm MPs significantly reduced soil respiration (Res) rate, β-glucosidase (Glu) and catalase (CAT) activity, which accompanied with enhanced urease activity and decreased soil pH, available phosphorus (AP), dissolved reactive phosphorus (DRP), dissolved organic carbon (DOC) and available potassium (AK) content in most cases. However, 130 μm MPs exerted negligible influence on the DOC and DRP content, Glu and CAT activity. High concentrations of 130 μm MPs significantly reduced soil pH, total dissolved nitrogen (TDN), AP and AK content, but significantly increased soil Res rate. Overall, soil ecosystem function was significantly reduced by the addition of MPs. The Res rate, soil AP and DRP content and Glu activity were the most important predictors of soil ecosystem function. We found that the risk posed by MPs to soil ecosystem function was dose-dependent and size-dependent. These findings underscore that MPs can alter soil functions related to soil nutrient cycling and provide further insights into MPs behavior in agroecosystems. Supplementary Information The online version contains supplementary material available at 10.1038/s41598-024-80124-8.
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Response of soil biochemical
properties and ecosystem function
to microplastics pollution
Yanan Cheng, Fei Wang, Wenwen Huang & Yongzhuo Liu
Microplastics (MPs)-induced changes in soil nutrient cycling and microbial activity may pose a potential
risk to soil ecosystem. Although some studies have explored these topics, there is still a large space
for exploration and a relative lack of research on the mechanism by which soil health and its functions
are aected by these changes. Thus, this study investigated the eects of polyethylene (PE) MPs with
two particle sizes (13 μm and 130 μm) at ve concentrations (0%, 1%, 3%, 6% and 10%, w/w) on soil
biochemical properties and ecosystem function. The ndings revealed that the exposure to 13 μm
MPs signicantly reduced soil respiration (Res) rate, β-glucosidase (Glu) and catalase (CAT) activity,
which accompanied with enhanced urease activity and decreased soil pH, available phosphorus
(AP), dissolved reactive phosphorus (DRP), dissolved organic carbon (DOC) and available potassium
(AK) content in most cases. However, 130 μm MPs exerted negligible inuence on the DOC and DRP
content, Glu and CAT activity. High concentrations of 130 μm MPs signicantly reduced soil pH, total
dissolved nitrogen (TDN), AP and AK content, but signicantly increased soil Res rate. Overall, soil
ecosystem function was signicantly reduced by the addition of MPs. The Res rate, soil AP and DRP
content and Glu activity were the most important predictors of soil ecosystem function. We found
that the risk posed by MPs to soil ecosystem function was dose-dependent and size-dependent. These
ndings underscore that MPs can alter soil functions related to soil nutrient cycling and provide further
insights into MPs behavior in agroecosystems.
Keywords Microplastics, Polyethylene, Soil biochemical properties, Soil ecosystem function
Microplastics (MPs), dened as plastic particles smaller than 5mm, enter agro-ecosystems through various
pathways. It is estimated that the abundance of MPs in agriculture soils has far exceed that in the ocean, and
soil has become a larger reservoir for environmental MPs pollution1,2. eir widespread presence and potential
impacts on ecosystems make them “pollutants of importance and agents of global change”3,4. Due to their small
particle size and large surface area, MPs can absorb various organic and inorganic pollutants, rendering them
resistant to bio-ingestion and degradation5,6. is resistance allows MPs to persist in the soil for a long time,
resulting in rapid accumulation in the global terrestrial environment, potentially leading to long-term eects on
soil ecosystems7. erefore, MPs in terrestrial ecosystems are of increasing concern.
Recent studies have revealed that a relatively high concentration of MPs in soils8 signicantly altered
the physicochemical properties of the soil9, and directly and adversely aected soil fauna10, plants11 and
microorganisms12. Furthermore, there was a potential risk to human health through the accumulation and
transmission of MPs via the food chain13. Aer inltrating the soil, MPs were bound with organic matter or
minerals and incorporated into the soil matrix, thereby inducing alterations in soil aggregate structure, bulk
density, porosity, permeability, water holding capacity, as well as other physical and chemical properties14. For
instance, the addition of high-density polyethylene (HDPE) at a concentration of 0.1% signicantly decreased
soil pH, whereas polylactic acid (PLA) did not exhibit a signicant impact on soil pH at the same concentration15.
However, Qi et al.16 found that PLA signicantly increased soil pH and alleviated soil acidication. e contents
of soil available nitrogen and phosphorus were reduced by 10–13% and approximately 30% in a rice paddy soil
amended with 1% polyvinyl chloride (PVC) MPs, respectively17. e changes in soil microhabitats induced
by MPs may aect the structure and diversity of local microbial community15. e addition of 1% and 5% of
polyethylene (PE) and 5% of PVC signicantly declined the richness and diversity of the bacterial communities,
but signicantly increased the abundance of betaproteobacteriales, including the Burkholderiaceae, which were
closely related to nitrogen xation18. Soil microorganisms and the enzymes they produce were sensitive to soil
stresses that could be used as indicators of microbial activity and as environmental biomarkers19. However, it
School of Resources and Environment, Henan Institute of Science and Technology, 90 Eastern Hualan Avenue,
Xinxiang 453003, China. email: yncheng@hist.edu.cn
OPEN
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was found that the addition of low-density polyethylene (LDPE) and HDPE had no obvious inuence on the
activities of urease, CAT and invertase20. In conclusion, MPs can directly or indirectly aect the soil properties
in most cases, varying with MPs type, dose, size, shape, and soil type.
e impact of MPs on soil physicochemical and biological properties is likely to inuence the soil functions,
which refers to the capacity of soil ecosystems to provide and maintain multiple ecosystem functions and services
simultaneously21. For instance, under well-watered conditions, the presence of MPs bers led to a decrease
in soil multifunctionality, indicating their negative eects on the ecosystem22. Furthermore, the activities of
α/β-1, 4-glucosidase, urease, protease and NH4+-N content were the most important predictors of ecosystem
multifunctionality23. Zhou et al.24 revealed that adding 0.03% polystyrene (PS) improved soil ecosystem
multifunctionality by 4–12%, but decreased by 4–11% by adding 0.3% PS. However, the inuence of MPs on soil
functions remains unclear, which is critical to elucidate the ecological consequences of MPs in agroecosystems.
e persistence and non-natural properties of MPs in soils might qualify these particles to be drivers of
soil function change. However, how the presence of MPs shaping soil functions is still an open question, not
to mention the studies on the eects of dierent concentrations and particle sizes of MPs on soil functions.
erefore, in this study, we evaluated on how the concentration and particle size of PE MPs inuenced soil
biochemical properties and soil function related to soil nutrient cycling to enhance the comprehension of soil
ecosystem responses to MPs as a global change factor. We hypothesized that (i) the presence of MPs may aect
the availability and cycling of soil nutrients; (ii) high concentrations and small particle sizes of MPs may have
greater negative impacts on soil system; and (iii) MPs may decrease soil function related to soil nutrient cycling,
with their size mediating the dierentiation of soil function.
Materials and methods
Soil and MPs
e topsoil (0–20cm) used in this study was collected from an experimental eld (113°5024 E, 35°1226N)
established by Henan Xinlianxin Chemicals Group Co. in cooperation with local universities. e experimental
eld sampled were control plots without any treatment and were not covered with mulch lms in history; thus
there is no concern regarding MPs pre-contamination of the soil. e average sand, silt and clay contents in the
0–20cm soil were 35.81%, 44.62% and 19.57%, respectively. e tested soil is a loamy uvo-aquic soil, which
is relatively widespread globally, particularly in alluvial plains adjacent to rivers, lakes, and oceans, as well as in
regions with high groundwater levels, including North America, Europe and Southeast Asia. e following are
the physicochemical properties of the tested soil: pH 7.65 (soil-water ratio 1:2.5), organic matter 10.68g·kg− 1,
available nitrogen 123.90mg·kg− 1, available phosphorus (AP) 26.41mg·kg− 1 and available potassium (AK)
277.0mg·kg− 1. e air-dried soil was sieved through a 2mm mesh and mixed homogenously.
Plastic lm, mainly composed of PE, is one of the direct sources of MPs in farmland soils25. erefore, PE
MPs were chosen because they are commonly detected in farmland soil26. PE MPs with average diameters of
130μm and 13 μm, respectively, were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai,
China) and were white powders composed of spherical particles.
Experimental setup
As reported, the environmentally relevant concentration of MPs in soil was found to be 1%, with 5% presenting a
high level of MPs23,26,27. erefore, 1% and 3% were designed to simulate dierent levels of MPs pollution under
environmental conditions, and 6% and 10% to simulate the condition of extreme MPs pollution based on the
previous studies26,28. In this study, two size of MPs with diameters of 13μm and 130μm were selected, and ve
concentrations were established. Nine treatments with three replicates for each were considered for cultivation:
(1) CK: 0% (w/w) MPs (sharing in both MP particle sizes); (2) T1-130: 1% (w/w) 130μm MPs; (3) T2-130: 3%
(w/w) 130μm MPs; (4) T3-130: 6% (w/w) 130μm MPs; (5) T4-130: 10% (w/w) 130μm MPs; (6) T1-13: 1%
(w/w) 13μm MPs; (7) T2-13: 3% (w/w) 13μm MPs; (8) T3-13: 6% (w/w) 13μm MPs; and (9) T4-13: 10% (w/w)
13μm MPs. Approximately 0, 1.5, 4.5, 9.0 and 15g of MPs of both sizes were weighed and added to 150g of
soil (dry weight). e samples were incubated at 25 ± 1°C under a natural photoperiod for 30 days and the soil
water content was maintained at 60% of the maximum water holding capacity throughout the experiment. Aer
30 days of exposure, soil was collected and separated into two subsamples. One subsample was stored at 4°C
for the determination of the activities of CAT, Glu and urease, Res rate, and the contents of dissolved organic
carbon (DOC) and total dissolved nitrogen (TDN); while the other was air-dried at room temperature, ground
and passed through a 1mm sieve for the determination of soil pH, the contents of AP, AK and dissolved reactive
phosphorus (DRP).
Measurement of soil biochemical properties
Soil pH was measured with a pH meter at a ratio of 1:2.5 (w/w soil: water)29. Soil AP30 and DRP31 were extracted
by 0.5M NaHCO3 solution and 0.01M CaCl2 solution respectively, followed by analysis using the molybdenum
blue colorimetric method. Soil AK was extracted by 1M ammonium acetate solution, and its concentration was
subsequently measured via ame photometry32. TDN content was quantied at both 220nm and 275nm using
a UV spectrophotometer following potassium persulfate oxidation33. DOC content was determined using a
TOC analyzer (Vario TOC, Elementar Analysensysteme GmbH, Germany) in a 1: 3 (w/v) soil: water suspension
according to the Kalbitz et al.34. Soil Res rate was determined using the indoor-incubation alkali absorption
method35. Briey, 20g fresh soil was incubated in a sealed container at 25°C for 7 days. e CO2 produced was
trapped in an excess of 0.05M NaOH, and the residual NaOH was titrated with 0.05M HCl. e amount of CO2
is then determined by titrating the remaining NaOH. Soil Res rate was expressed in mg CO2-C·kg− 1 soil·d− 1.
Urease activity was measured by indophenol blue colorimetry according to the Kandeler and Gerber36.
Briey, 5g fresh soil was incubated with 10 mL 10% urea solution and 10 mL citrate buer solution (pH 6.7)
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at 37°C for 24h, followed by quick ltration. 1 mL of the ltrate was then diluted to 20 mL, and treated with 4
mL sodium phenol solution and 3 mL sodium hypochlorite solution. e released ammonium was measured
by colorimetry at 578nm. Urease activity was expressed in mg NH4+-N·g− 1 soil·d− 1. CAT activity was measured
using a titrimetric method37. Briey, 2g fresh soil was homogenized with 40 mL distilled water and 5 mL 0.3%
H2O2 for 20min. en, 5 mL 1.5 M H2SO4 was added to stop the reaction, and the reactants were ltrated. e
amount of surplus H2O2 from 20 mL of the ltrate was measured by titration using 20 mM KMnO4. CAT activity
was expressed in mg H2O2·g− 1 soil·20min− 1. Glu activity was measured using the method of Asensio et al.38. In
brief, 2g fresh soil was incubated with 6 mL sodium acetate buer (0.2M, pH 6.2) and 2 mL 2% hydroquinone-
β-D-glucoside (as substrate) at 37°C for 24h, then diluted with distilled water to 50 mL. 5 mL ltrate was diluted
to 10 mL, and treated with 3 mL 3, 5-dinitrosalicylic acid solution. e amount of glucose was measured by
colorimetry at 578nm. Glu activity was expressed in mg glucose·g− 1 soil·d− 1.
Assessment of soil ecosystem function
Referring to the soil multi-nutrient cycling index proposed by Jiao et al.39, which is analogous to the
multifunctionality, we employed the soil function index in this study to quantify the impact of MPs on the
nutrient cycling within soil ecosystems. Ten soil properties including soil pH, DOC, TDN, AK, AP, DRP, Res
rate, CAT, Glu and urease were considered to assess soil function index. ese variables are closely related to soil
nutrient cycling and microbial activity, and can well reect multiple functions of soil ecosystem, such as nutrient
retention and utilization, soil fertility and biogeochemical cycles40. e soil function index was calculated by
the common averaging approach. Firstly, each function was standardized separately by Z-score transformation
ranging from 0 to 1, and then averaged to obtain the function index values using the averaging approach41.
Statistical analysis
All statistical analyses were performed using SPSS 26.0 (IBM SPSS Statistics 26). Data were rst tested for
normality using the Shapiro-Wilk test (P < 0.05) and homogeneity of variance using the Levene test (P < 0.05).
en the data were analyzed using one-way ANOVA with particle size as a xed factor to evaluate the dierences
between dierent MPs concentrations. Mean values were then compared using the Duncan test at P < 0.05.
Signicant dierences between particle sizes for a given MP concentration were determined via a t test with
95% condence intervals. A detrended correspondence analysis (DCA) was conducted on ten soil biochemical
properties in relation to MPs concentration and particle size. Given that all the ordination axes were less than 3,
the redundancy analysis (RDA) was performed by Canoco 5.0 to reveal the correlation between soil biochemical
properties and microplastic concentration and particle size. Random forest analysis was conducted using the
“randomForest” package in R (version 4.3) to evaluate the credible predictors of soil function related to the
cycling of multiple nutrients. Soil biochemical properties were served as predictors for soil function index in
random forest analysis. e mean decrease in accuracy (IncMSE%) was used to estimate the importance of these
predictors. And the cluster heat map analysis was conducted using the “pheatmap” package in R (version 4.3).
All gures were generated using Origin 2018.
Results
Soil chemical properties
In the present study, no signicant changes of soil DOC content were observed in 130 μm MPs treatments
relative to the CK (Fig.1a). In contrast, except for T1-13, the addition of 13μm MPs led to a signicant decrease
in DOC content (9.15–59.07% dierence, P < 0.05) with the increase of MPs concentration. Soil TDN was
signicantly aected by 130μm MPs (Fig.1b), specically, compared to CK, TDN content was signicantly
higher in T1-130 treatment (increased by 5.34%, P < 0.05), but was signicantly lower in other treatments
(decreased by 5.39–8.55%, P < 0.05). For the 13μm group, signicant decrease in TDN content was observed
only in T4-13 with the highest concentration, compared to CK (19.26% decrease, P < 0.05). Soil AK was not
signicantly altered by low-dose MPs of both sizes (P > 0.05), but signicantly reduced by high-dose MPs
(average decreased by 9.21% and 55.51% for 130μm MPs and 13μm MPs, respectively, P < 0.05 in both groups;
Fig.1c). AP content was decreased by both sizes of MPs, with the highest decrement observed in T4-13 (58.53%
decrease, P < 0.05; Fig.1d). Similarly, soil pH was signicantly reduced in both the 130μm and 13μm treatments
(average reduction 10.59%, P < 0.05; and 11.47%, P < 0.05 respectively; Fig.1e). DRP content did not exhibit
signicant changes due to the addition of 130μm MPs (P > 0.05), but decreased signicantly aer the addition of
13μm MPs (5.01–41.85% dierence, P < 0.05; Fig.1f). Overall, there were signicant dierence in the inuence
of MPs of both sizes on soil chemical properties at the concentrations of 3%, 6% and 10% in addition to soil pH
(P > 0.05, Fig.1). Among these chemical properties, TDN, AK, AP and pH were negatively correlated with the
concentration of 130μm MPs (Fig. S1). In 13μm group, all measured soil properties except pH did decrease with
increasing MPs concentration (Fig. S2).
Soil biological properties
e eects of MPs of two sizes on soil Res were completely dierent (Fig.2a). Specically, Res rate was promoted
by the addition of 130μm MPs (average increased by 18.55%, P < 0.05), and increased with the increase of
MPs concentration, but signicantly suppressed with the increase of 13 μm MPs concentration (average
decreased by 52.69%, P < 0.05). e activity of Glu in the soil was decreased signicantly with increasing 13μm
MPs concentration (6.30-31.35% dierence, P < 0.05), however, no obvious change was observed by adding
130μm MPs (Fig.2b). Similarly, CAT activity was signicantly suppressed as the concentration of 13μm MPs
increased (decreased by 3.56–55.59%, P < 0.05; Fig. 2c). For 130μm MPs, a signicant decrease of 6.04% in
CAT activity was observed only at the highest concentration (P < 0.05, Fig. 2d). Compared with CK, urease
activity was signicantly promoted by low concentration of 130μm MPs (increased by 23.05% in T1-130 and
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40.65% in T2-130, respectively, P < 0.05; Fig. 2d). Urease activity showed an increasing trend with the increase
of 13μm MPs concentration, however, signicant increase was observed when concentration was reached to
6% (P < 0.05). In addition, the biological properties in 130μm MPs treatments were signicantly higher than
those in 13μm MPs treatments under the same concentration (P < 0.05, Fig. 2). Generally, the Res rate and
CAT activity were positively and negatively correlated with 130μm MPs concentration, respectively. However,
13μm MPs decreased the soil Res rate, Glu and CAT activities, showing a decreasing trend with increasing MPs
concentration, which was exactly the opposite of urease (Fig.2d).
Key factor determining soil biochemical properties in the presence of MPs
Both MPs concentration and particle size signicantly inuenced soil biochemical properties (Table S1),
however, it remained unclear which factor was the primary driver of these changes. To address this, the RDA was
conducted, which accounted for 63.8% of the total variation in soil biochemical properties (Fig.3, P = 0.002).
e percentages of variance explained by the rst and second axes were 62.64% and 1.14%, respectively. e
contribution rates of MPs concentration and particle size to the variations in soil biochemical properties
were 76.7% and 23.3%, respectively, indicating that MPs concentration was the primary factor aecting
these properties. As shown in Fig.3, except for urease, other indicators were negatively correlated with MPs
concentration. Similarly, most of these indicators were also negatively correlated with the MPs size, which were
consistent with the correlation analysis (Fig. S1 and Fig. S2).
To further investigate the impact of MPs concentration on soil properties, we constructed a cluster heat map,
which revealed that the nine treatments could be grouped into two major clusters (Fig.4). e rst major cluster,
consisted of CK, T1-13, T2-13, T1-130, T2-130, T3-130 and T4-130, was further subdivided into two minor
clusters. One minor cluster was composed exclusively of CK, in which soil pH, contents of AK and AP, and CAT
activity were the highest among the two major clusters. e other minor cluster comprised T1-13, T2-13, T1-
130, T2-130, T3-130 and T4-130, in which soil biochemical properties (except urease) were lower than CK but
higher than the second major cluster. Similarly, all the biochemical indicators besides urease in the second major
cluster composed of T3-13 and T4-13 were lower than the rst major cluster. Overall, soil biochemical indicators
in the second major cluster were generally low, indicating that high concentrations of small-sized MPs had a
greater negative impact on soil nutrient content and cycling.
Fig. 1. e eect of MPs addition on soil physicochemical properties. DOC: dissolved organic carbon (a);
TDN: total dissolved nitrogen (b); AK: available potassium (c); AP: available phosphorus (d); pH: soil pH (e);
DRP: dissolved reactive phosphorus (f). Data are represented as mean ± SD (n = 3). Dierent letters indicate
signicant dierence among MPs concentrations with same particle size, and asterisks indicate signicant
dierence between two particle sizes of MPs for a given concentration at the level of P < 0.05.
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Soil function index
Soil function index varied signicantly across dierent MPs concentrations and sizes (P < 0.05, Fig.5). Except
for the T1-130, other concentrations of MPs signicantly reduced the soil function index, which decreased
signicantly with the increasing concentration of MPs (Fig.5). Furthermore, the soil function index in 13μm
MPs group was signicantly lower than that in 130μm MPs group. e random forest model explained 88.8% of
the overall variance in soil function index and soil Res rate was the most prominent predictor (Fig.6). Soil AP,
DRP content and Glu activity were also reliable indicators (P < 0.05).
Discussion
In this study, the results demonstrated that MPs exposure altered soil biochemical properties and function index,
and these impacts depended on the MPs concentrations and particle sizes. As hypothesized, soil ecological
functions linked with nutrient content and cycling were aected by MPs with dierent concentrations and
particle sizes. ese results suggested that high MPs concentrations, especially small sizes, had signicant
negative impact on soil functions.
Impacts of MPs addition on soil chemical properties
Soil DOC is the most chemically bio-available and easily inuenced carbon by microorganism in soil. A minor
eect of 130μm MPs on soil DOC content was observed at all concentrations, however, signicant reductions
were found in 13μm treatments (Fig.1a). is negative eect may be attributing the inhibition of Glu activity by
13μm MPs, which attenuated the decomposition of carbohydrate and nally decreased soil DOC content26. is
was consistent with our speculation that MPs with smaller particle sizes have a greater impact on the soil. Due to
the addition of 13μm MPs, soil bioavailable carbon (such as DOC, Fig.1a) content was reduced, which inhibited
microbial activity and consequently suppressed the consumption of soil TDN. Conversely, 130μm MPs had no
signicant eects on soil DOC content. However, as shown by the signicant activation of soil Res rate (Fig.2a),
it could be hypothesized that microbial activity was promoted, which might lead to the microbial consumption
of TDN19. erefore, the TDN content was signicantly lower than that of CK (Fig.1b). High application rates of
13 and 130μm MPs at 6% and 10% signicantly reduced the AK content (Fig.1c). Interestingly, the smaller size
Fig. 2. e eect of MPs addition on soil respiration (a) and enzyme activity. Res: soil respiration (a); Glu:
β-glucosidase (b); CAT: catalase (c); urease (d). Data are represented as mean ± SD (n = 3). Dierent letters
indicate signicant dierence among MPs concentrations with same particle size, and asterisks indicate
signicant dierence between two particle sizes of MPs for a given concentration at the level of P < 0.05.
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of MPs also signicantly reduced the AK content, which might suggest that high application rates and small sizes
of MPs would produce negative inuences on soil fertility42. MPs could also inuence biochemical cycle of P by
altering microbial processes43. For instance, Feng et al.44 reported that the addition of biodegradable MPs (e.g.
PLA and polyhydroxybutyate) diminished the availability of soil AP, which may be attributed to inhibition of soil
phosphatase activity by MPs43. Furthermore, the increase in soil carbon-to-nitrogen ratio caused by degradable
MPs might stimulate microbial assimilation of inorganic P, thus exacerbating P deciency45. Two particle size
MPs with dierent concentrations signicantly reduced the AP content (except for T1-130), which might be
attributed to the changes in microbial abundance and activity, and the inhibition of soil enzyme activity caused
by MPs43. Due to their relatively large, possibly reactive surface area and charged properties, MPs could inuence
cation exchange in soil, selectively adsorb substances with negatively or positively charges, and allow free proton
exchange in soil water, ultimately causing changes in soil pH15. Boots et al.15 found that soil pH experienced
a signicant decrease when exposed to HDPE, which is in line with our results. e smaller the MPs size, the
larger the specic surface area, resulting in an increased number of unoccupied adsorption sites on the surface46.
is enhances the probability of solid-liquid two-phase contact and facilitates improved adsorption capacity
of DRP in solution by MPs46. Consequently, there were signicant reductions in DRP content in soil solution
under high 13μm MPs concentrations. Overall, the presence of MPs could elicit declines in the availability of
soil inherent nutrients, which is consistent with our hypothesis.
Impacts of MPs addition on soil biological properties
For 130μm MPs, the soil Res rate was signicantly increased, potentially attributed to the enhanced soil aeration
that stimulated microbial activity by supplying oxygen content47. Instead, the addition of 130μm MPs did
not produce a statistically signicant impact on the activities of Glu and CAT. is could be attributed to the
functional resistance of microbial communities to the exposure of 130μm MPs that exhibited no signicant
harmful eects on microbes48. Conversely, signicant suppression in Res rate by 13μm MPs may be due to the
Fig. 3. Redundancy analysis (RDA) of soil properties and the concentration and particle size. C: MP
concentration; D: MP particle size; AK: available potassium; AP: available phosphorus; CAT: catalase; DOC:
dissolved organic carbon; DRP: dissolved reactive phosphorus; Glu: β-glucosidase; pH: soil pH; Res: soil
respiration; TDN: total dissolved nitrogen. e length and direction of the arrow represent the degree of
inuence of particle size and concentration of MPs on soil properties, and the positive and negative correlation
between the two.
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lower DOC content (Fig.1a), which may serve as a potential nutrient and/or substrate for microorganisms22.
e result of the signicant inhibition of Res rate by 13μm MPs was contrary to the 130μm MPs and the results
of most existing studies4851. Due to the current preliminary understanding of the impact of MPs on soil Res,
the exact mechanisms leading to the opposite eects of dierent MPs sizes on soil Res remain unclear and
require further investigation. Similarly, the microbial activity was inhibited due to the low DOC concentration,
leading to a signicant inhibition in the activities of Glu and CAT involved in the soil carbon cycle52. e
signicant and positive correlations between the Glu activity and DOC content, as well as CAT activity and DOC
content, further conrm this (Fig. S3, R = 0.94, P < 0.001 and R = 0.98, P < 0.001, respectively). In contrast to the
suppression eect of Glu and CAT activities, the urease activity was stimulated by 13μm MPs. e presence of
MPs in soil has been found to enhance the abundance of diazotrophs, which play a major role in stimulating
urease activity18. is may account for the positive eect of 13μm MPs on urease activity. In addition, several
parameters (such as TDN, AK, AP, Res rate, CAT) exhibited a dose-eect relationship with MPs concentration
(Fig. S1, Fig. S2). is indicated that the eect of MPs on soil ecological environment might have a cumulative
eect53, which means that as the MPs concentration increases, their eects on the soil ecosystem may become
increasingly signicant. In particular, except for soil pH, all other indicators demonstrated a signicant dose-
eect relationship with 13μm MPs (Fig. S2), suggesting that the cumulative eect (negative impact) of small-
sized MPs on soil biochemical properties might be more pronounced. However, the inuence of MPs size on the
soil ecological environment remains poorly understood and necessitates more research in the future.
Fig. 4. Cluster heat map analysis of soil properties under dierent MPs treatments. AK: available potassium;
AP: available phosphorus; CAT: catalase; DOC: dissolved organic carbon; DRP: dissolved reactive phosphorus;
Glu: β-glucosidase; pH: soil pH; Res: soil respiration; TDN: total dissolved nitrogen.
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Impacts of MPs addition on soil function index
Soil function index has been designated to evaluate soil ecosystem functions and services under various external
disturbances20; however, research on the impact of MPs on soil ecosystem functioning is still in its infancy. In
this study, soil function index was found to be diminished by the addition of MPs. is trend was consistent
with the observations in nutrient availability (i.e. AP, DRP) and nutrient cycling function (i.e. soil Res, Glu),
as they were the primary contributors to soil function index. is nding highlighted that the soil ecosystem
had the ability to tolerate a certain amount of MPs pollution, albeit with a negative impact on soil functions.
MPs inuences on soil ecosystem functions can be related with their concentration and particle size. Small-
sized MPs, due to their large active surface area, provided more adsorption sites for soluble nutrients, thereby
directly reducing the content of available nutrients46. As the substrate of soil microorganisms, this decrease in
available nutrients would inhibit the activity of enzymes involved in nutrient recycling and transformation by
suppressing the abundance and activity of soil microorganisms19,21,22,25. is further intensies the obstruction
of soil nutrients cycling and transform processes ultimately diminishes soil functions. Indeed, adding MPs leads
to a decline in soil DOC, available N and AP contents due to the inhibition of biochemical process driven by
associated microorganisms upon MPs incorporation17,43. Yi et al.54 reported that the addition of PE and PP MPs
decreased the bacteria abundance and changed the physicochemical properties of soils, subsequently reducing
the resistance of soil microorganisms against pollutants. Similarly, the richness and diversity of soil bacterial
communities were reduced by the addition of PE and PVC MPs, and PE had more severe eects than PVC19.
Soil Res largely depended on soil microbial activity and was highly susceptible to variations in soil conditions,
which explained why soil Res was the most signicant predictor of soil function index in the presence of MPs19.
Key factors aecting the impacts of MPs on soil properties and soil function index
Our ndings demonstrated that MPs had an impact not only on single soil property but also on soil function
index. Specially, MPs concentration was the primary inuencing factor for soil properties and also had signicant
impacts on the soil function index, with higher MPs concentrations associated with lower soil function index.
MPs size also had signicant inuence on soil function index. e soil function index treated with small
Fig. 5. Changes of soil function index across dierent MPs concentrations. Data are shown as mean ± SD.
Dierent lowercase letters indicate signicant dierences among treatments (P < 0.05).
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particle size MPs was signicantly lower than that of large particle size MPs, which indicated that under the
same concentration, small particle size MPs might have more serious negative eects on soil nutrients content,
availability and cycling. Previous studies have demonstrated that the presence of MPs signicantly aected the
ecosystem multiple functions, with this eect being highly dependent on MPs concentration23. Specially, the
addition of relatively low-concentration MPs promoted soil biological properties; however, this impact was
shied from positive to negative as MPs concentration increase23,24,28. Instead, our RDA analysis indicated
that soil biochemical properties were primarily inuenced by MPs concentration, whereas the inuence of
MPs size was relatively minor. Furthermore, based on the clustering analysis results, when MPs concentration
falls below a certain threshold (3% in this study), there was no signicant dierence in their impact on soil
biochemical properties between the two MPs sizes. A signicant size eect occurred when MPs concentration
was reached to 6%; specically, smaller-sized MPs exhibited more substantial negative eects on soil ecosystem
function. It is important to note that 6% and 10% are not environmentally relevant concentrations for MPs in
soils; however, such high concentrations can be observed in certain highly polluted areas, including urban and
industrial discharge zones, oceans, and regions surrounding municipal wastewater treatment plants, where MPs
concentrations may reach or even exceed 6%27,55. erefore, although MPs concentrations of 6% and 10% are
relatively rare under normal circumstances, they remain plausible in specic contaminated environments. ese
two high concentrations were selected as hypothetical extreme scenarios to explore the potential impacts of high
MPs loads on soil ecosystems and determine the threshold at which soil ecosystem functions may be aected,
which is crucial for understanding the potentially serious harm that MPs can inict on soil ecosystems. Overall,
the inuence of MPs on the soil ecosystem may be cumulative with a more pronounced negative eect observed
for small-sized MPs. It should be noted that these outcomes may be attributed to the limited selection of only one
soil type and two MPs sizes in this study. Dierent soil types vary in physical, chemical, and biological properties,
which can signicantly inuence the behavior of MPs in soil and their ecological eects. In future research, it
would be benecial to include multiple types of soil and MPs to comprehensively evaluate the ecological impacts
of MPs in dierent soil environments.
Fig. 6. Random forest analysis to identify relative importance of soil variables drivers on soil function index.
AK: available potassium; AP: available phosphorus; CAT: catalase; DOC: dissolved organic carbon; DRP:
dissolved reactive phosphorus; Glu: β-glucosidase; pH: soil pH; Res: soil respiration; TDN: total dissolved
nitrogen. e mean decrease in accuracy (IncMSE%) was used to indicate the relative importance of each
variable for predicting soil function index. Signicance levels of each predictor are as follows: *P < 0.05 and
**P < 0.01.
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Conclusions
e presence of MPs can aect soil biochemical properties and ecosystem function. In this study, 13μm MPs
treatment reduced soil pH, soil DOC, AP and DRP contents, as well as soil Res rate, Glu and CAT activities.
However, the addition of high-concentration MPs promoted the urease activity, while reduced the soil AK
content. Dierently, 130μm MPs treatment had no signicant eect on soil DOC, DRP contents, Glu and CAT
activities. Meanwhile, it reduced soil pH, TDN and AP contents but signicantly promoted the soil Res rate.
MPs signicantly decreased soil function index. ese ndings highlight the profound inuence of MPs on soil
biochemical properties and ecosystem function, emphasizing the pressing need to address and control the MPs
pollution in agroecosystems.
Data availability
All data generated or analyzed during this study are available from the corresponding author on reasonable
request.
Received: 7 August 2024; Accepted: 15 November 2024
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Acknowledgements
is research was funded by the Henan Provincial Science and Technology Research Project (242102320092),
the National Natural Science Foundation of China (31872184), and the Natural Science Foundation of Henan
Province (222300420044).
Author contributions
Yanan Cheng contributed to conceptualization, data curation, methodology, writing – original dra, writing –
review & editing. Fei Wang contributed to conceptualization, methodology, writing – review & editing. Wenwen
Huang contributed to conceptualization, formal analysis, writing– review & editing. Yongzhuo Liu contributed
to data curation, formal analysis, writing – review & editing. All authors have read and agreed to the published
version of the manuscript.
Declarations
Competing interests
e authors declare no competing interests.
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Additional information
Supplementary Information e online version contains supplementary material available at h t t p s : / / d o i . o r g / 1
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Microplastics (MPs) are emerging pollutants of terrestrial ecosystems. The impacts of MP particle size on terrestrial systems remain unclear. The current study aimed to investigate the effects of six particle sizes (i.e., 4500, 1500, 500, 50, 5, and 0.5 μm) of polyethylene (PE) and polyvinyl chloride (PVC) on soil respiration, enzyme activity, bacteria, fungi, protists, and seed germination. MPs significantly promoted soil respiration, and the stimulating effects of PE were the strongest for medium and small-sized (0.5–1500 μm) particles, while those of PVC were the strongest for small particle sizes (0.5–50 μm). Large-sized (4500 μm) PE and all sizes of PVC significantly improved soil urease activity, while medium-sized (1500 μm) PVC significantly improved soil invertase activity. MPs altered the soil microbial community diversity, and the effects were especially pronounced for medium and small-sized (0.5–1500 μm) particles of PE and PVC on bacteria and fungi and small-sized (0.5 μm) particles of PE on protists. The impacts of MPs on bacteria and fungi were greater than on protists. The seed germination rate of Brassica chinensis decreased gradually with the decrease in PE MPs particle size. Therefore, to reduce the impact of MPs on soil ecosystems, effective measures should be taken to avoid the transformation of MPs into smaller particles in soil environmental management.
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