Front. Plant Sci. Frontiers in Plant Science Front. Plant Sci. 1664-462X Frontiers Media S.A. 10.3389/fpls.2022.1075007 Plant Science Original Research Soil heterogeneity in the horizontal distribution of microplastics influences productivity and species composition of plant communities Zhang Xiao-Mei 1 † Cao Xiao-Xiao 1 2 † He Lin-Xuan 1 Xue Wei 1 Gao Jun-Qin 3 Lei Ning-Fei 2 Chen Jin-Song 4 Yu Fei-Hai 1 * Li Mai-He 5 * 1 Institute of Wetland Ecology & Clone Ecology, Taizhou University, Taizhou, China 2 College of Ecology and Environment, Chengdu University of Technology, Chengdu, China 3 School of Ecology and Nature Conservation, Beijing Forestry University, Beijing, China 4 College of Life Science, Sichuan Normal University, Chengdu, China 5 Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, Switzerland

Edited by: Deliang Kong, Henan Agricultural University, China

Reviewed by: Baoli Fan, Northwest Normal University, China; Qiaoqiao Huang, Chinese Academy of Tropical Agricultural Sciences, China

*Correspondence: Fei-Hai Yu, feihaiyu@126.com; Mai-He Li, maihe.li@wsl.ch

†These authors have contributed equally to this work

This article was submitted to Functional Plant Ecology, a section of the journal Frontiers in Plant Science

 08 12 2022 2022 13 1075007 20 10 2022 23 11 2022 Copyright © 2022 Zhang, Cao, He, Xue, Gao, Lei, Chen, Yu and Li 2022 Zhang, Cao, He, Xue, Gao, Lei, Chen, Yu and Li

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Contamination of soils by microplastics can have profound ecological impacts on terrestrial ecosystems and has received increasing attention. However, few studies have considered the impacts of soil microplastics on plant communities and none has tested the impacts of spatial heterogeneity in the horizontal distribution of microplastics in the soil on plant communities. We grew experimental plant communities in soils with either a homogeneous or a heterogeneous distribution of each of six common microplastics, i.e., polystyrene foam (EPS), polyethylene fiber (PET), polyethylene bead (HDPE), polypropylene fiber (PP), polylactic bead (PLA) and polyamide bead (PA6). The heterogeneous treatment consisted of two soil patches without microplastics and two with a higher (0.2%) concentration of microplastics, and the homogeneous treatment consisted of four patches all with a lower (0.1%) concentration of microplastics. Thus, the total amounts of microplastics in the soils were exactly the same in the two treatments. Total and root biomass of the plant communities were significantly higher in the homogeneous than in the heterogeneous treatment when the microplastic was PET and PP, smaller when it was PLA, but not different when it was EPS, HDPE or PA6. In the heterogeneous treatment, total and root biomass were significantly smaller in the patches with than without microplastics when the microplastic was EPS, but greater when the microplastic was PET or PP. Additionally, in the heterogeneous treatment, root biomass was significantly smaller in the patches with than without microplastics when the microplastic was HDPE, and shoot biomass was also significantly smaller when the microplastic was EPS or PET. The heterogeneous distribution of EPS in the soil significantly decreased community evenness, but the heterogeneous distribution of PET increased it. We conclude that soil heterogeneity in the horizontal distribution of microplastics can influence productivity and species composition of plant communities, but such an effect varies depending on microplastic chemical composition (types) and morphology (shapes).

 environmental heterogeneity experimental plant communities foraging response microplastic heterogeneity soil microplastics Natural Science Foundation of Zhejiang Province10.13039/501100004731 Introduction

Pollution by microplastics is currently a serious environmental problem that receives increasing attention worldwide (Corradini et al., 2019; Roy et al., 2022; Zhang et al., 2022). Studies have shown that microplastics in soils can have profound impacts on survival, growth, morphology and physiology of individual plants (de Souza Machado et al., 2018a; Rillig et al., 2019; Pignattelli et al., 2020), likely via their effects on soil physico-chemical properties and soil microbial communities (de Souza Machado et al., 2018b; Wan et al., 2019). For instance, microplastics have been found to delay seed gemination (Bosker et al., 2019), reduce seed germination rate and seedling survival (Qi et al., 2018; Boots et al., 2019), modify tissue nutrient contents (de Souza Machado et al., 2019), alter root and shoot morphology (Boots et al., 2019; Rillig et al., 2019), and change biomass production and allocation (Cunha et al., 2020). A recent study has shown that microplastics in soils could also influence the productivity of plant communities and lead to changes in the dominant species within the communities (Lozano and Rillig, 2020).

The distribution of microplastics in soils in the horizontal space is often not uniform but heterogeneous (Rillig et al., 2017; Sun et al., 2022), i.e., microplastics are present in one soil microsite (patch) but absent in its horizontally adjacent soil microsites or microplastics are present in adjacent microsites with different concentrations. For instance, long-term plastic film shedding and random disposal of plastics may create soil patches with microplastics (Steinmetz et al., 2016; Blasing and Amelung, 2018; Zhang and Liu, 2018). Sewage sludge application, wastewater irrigation, human tillage, soil biota activity, atmospheric deposition and wind- or water-mediated movement may redistribute microplastics in soils and create horizontal soil patches with different concentrations of microplastics (Barnes et al., 2009; Nizzetto et al., 2016; Cai et al., 2017; Lwanga et al., 2017; Rillig et al., 2017). As microplastics in soils and their concentrations can influence plant growth (van Kleunen et al., 2020; Lozano et al., 2021; Li et al., 2022), soil heterogeneity in the horizontal distribution of microplastics may have significant ecological impacts on plant communities.

A large number of studies have assessed the ecological impacts of soil heterogeneity in the horizontal distribution of factors other than microplastics, including nutrients (Tsunoda et al., 2014; Xue et al., 2020; Adomako et al., 2021a; Gao et al., 2021), water (You et al., 2016; Wang et al., 2017), heavy metals (Roiloa and Retuerto, 2012; Xu and Zhou, 2017) and particle size of the soil (Baer et al., 2005; Huang et al., 2013; Xue et al., 2016). These studies have shown that soil heterogeneity in the distribution of such factors can affect growth, morphology and physiology of individual plants (Zhou et al., 2012; Tsunoda et al., 2014; Adomako et al., 2021b; Si et al., 2021), influence dynamics of plant populations (Hutchings et al., 2003; Baer et al., 2020), modify intraspecific and interspecific plant-plant interactions (Liang et al., 2020; Xue et al., 2021; Hu et al., 2022), and change plant community structure and ecosystem function (Wijesinghe et al., 2005; Yao et al., 2021). One underlying mechanism is that some plants can grow across patches and allocate more roots and/or shoots in favorable microsites (e.g., high-nutrient patches and patches not contaminated by heavy metals) and less in unfavorable microsites (e.g., low-nutrient patches and patches contaminated by heavy metals), showing foraging responses to increase resource harvesting (Hutchings et al., 2003; Tsunoda et al., 2014; Chen et al., 2019; Cao et al., 2022). Similarly, we hypothesize that soil heterogeneity in the horizontal distribution of microplastics can affect species composition and productivity of plant communities. So far, however, few studies have considered the impacts of soil microplastics on plant communities (Lozano and Rillig, 2020) and none has tested the impacts of soil heterogeneity in the horizontal distribution of microplastics on plant communities.

Microplastics are diverse in their types (chemical composition) and shapes (morphology) (Rillig et al., 2019; Li et al., 2022). Differences in the chemical composition and morphology of microplastics may result in differences in their impacts on soil physico-chemical properties and soil microbial communities (Huang et al., 2021). de Souza Machado et al. (2018b), for instance, have shown that polyester fibers increase water holding capacity, but polyacrylic fibers and polyethylene fragments have no significant effect. Also, microplastic fibers were found to have a larger impact on soil aggregation than other microplastic shapes (Lozano et al., 2021) and polyethylene foams increased soil pH more than polyethylene films (Zhao et al., 2021). Consequently, microplastics of different types and shapes can have different impacts on plant growth (Qi et al., 2018; de Souza Machado et al., 2019; Rillig et al., 2019). If microplastics in soils have a negative effect on plant growth due to their impacts on soil physico-chemical properties and soil microbial communities (de Souza Machado et al., 2019; Rillig et al., 2019; Li et al., 2022), then plants may allocate more shoots and/or roots in soil patches without microplastics than in their horizontally adjacent soil patches with microplastics. Such foraging responses may promote resource harvesting and thus increase the productivity of the whole plant communities. On the other hand, if microplastics in soils have no effect on plant growth, then the heterogeneous distribution of soil microplastics will not influence the species composition and productivity of plant communities. Therefore, we hypothesize that the impacts of soil heterogeneity in the horizontal distribution of microplastics on plant communities may vary depending on the chemical composition and morphology of microplastics.

To test these hypotheses, we grew experimental plant communities in soils with either a homogeneous or a heterogeneous distribution of each of six common microplastics. Specifically, we addressed the following questions: (1) Does spatial heterogeneity in the horizontal distribution of microplastics in the soil affect the productivity (biomass) and species composition of the experimental plant communities? (2) Do the impacts of such soil microplastic heterogeneity on plant communities vary depending on the chemical composition and morphology of the microplastics?

 Materials and methods Plant species, microplastics and soil

Experimental plant communities were established by sowing seeds of six perennial grassland species of three functional groups, i.e., two grasses (Elymus dahuricus Turcz., Lolium perenne L.), two legumes (Medicago sativa L. and Trifolium repens L.) and two forbs (Plantago asiatica L. and Taraxacum mongolicum Hand.-Mazz.). Seeds of all plant species were purchased from Jiangsu Leerda Seed Industry Co., LTD., in Xuzhou, Jiangsu Province, China, and stored at 4°C before use to keep their vitality.

We used six types of microplastics, i.e., polystyrene foam (EPS; average diameter: 200 μm), polyethylene fiber (PET; average length: 300 μm; specific gravity: 1.36; diameter: 20 μm ± 4 μm), polyethylene bead (HDPE; average diameter: 150 μm), polyethylene fiber (PP; average length: 300 μm; specific gravity: 0.91; diameter: 18 μm ~ 48 μm), polylactic bead (PLA; average diameter: 150 μm) and polyamide bead (PA6; average diameter: 150 μm). EPS is a foam, PET and PP are fibers, and HDPE, PLA and PA6 are beads. These microplastics vary in size, appearance, physical and chemical properties. They are all common plastic pollutants and have been examined in previous studies (Karamanlioglu et al., 2017; Zhou et al., 2018; Lozano et al., 2021). HDPE, PP, PLA and PA6 were purchased from Guangdong Huachuang Plastic Chemical Co., LTD, and PET and PP were purchased from Hunan Huixiang Fiber Co., LTD.

The soil used was a 1:1 (v:v) mixture of river sand and a local soil collected in Taizhou, Zhejiang, China. The local soil was sieved to pass 2-cm mesh to remove gravels and plant debris. The soil mixture contained organic carbon of 2.11 g kg-1, total nitrogen of 0.07 g kg-1 and total phosphorus of 0.91 g kg-1.

 Experimental design

For each of the six microplastics, we first created three types of soils using the soil mixture described above and the microplastic: (1) a blank soil without any microplastics, (2) a soil containing 0.1% (i.e., 1g kg-1) of the microplastic (low-concentration soil) and (3) a soil containing 0.2% of the microplastic (high-concentration soil). The concentrations of microplastics used in this study were within the range of microplastic concentrations in soils collected in the field (Blasing and Amelung, 2018; Zhang and Liu, 2018) and were also used in previous studies (de Souza Machado et al., 2018b). For each microplastic, we established two soil treatments (homogeneous vs. heterogeneous) in boxes (38 cm long × 28 cm wide × 14 cm deep). Each box was divided into four equal patches (19 cm long × 14 cm wide × 14 cm deep) by a plastic divider. For the homogeneous soil treatment (control), each of the four patches in a box was filled with the low-concentration soil; for the heterogeneous soil treatment, two opposite patches in a box were filled with the blank soil and the other two with the high-concentration soil ( Figure 1 ). After filling the soils, we removed the divider from the box so that plant roots could grow freely across patches. Each treatment was replicated six times, resulting in a total of 72 boxes (6 microplastics × 2 soil treatments × 6 replicates).

Schematic representation of the experimental design. For each of the six types of microplastics, we established a homogeneous and a heterogeneous soil treatment. For the homogeneous treatment, each of the four equal patches in a box was filled with the soil uniformly mixed with a low concentration (0.1%) of microplastics (grey); for the heterogeneous treatment, two opposite patches in a box were filled with the soil without microplastics (white) and the other two with the soil uniformly mixed with a higher concentration (0.2%) of microplastics (black). The total amounts of microplastics in the homogeneous and heterogeneous treatments were the same.

Plant communities were established by directly sowing seeds into each box. For each of the four patches in a box, we sowed about 54 seeds for each of the six species, resulting in a total density of 2000 seeds/m2 in the whole box. Within each patch, the seeds of each of the six species were roughly evenly distributed.

All boxes were randomly placed in a greenhouse of Taizhou University in Taizhou, Zhejiang Province, China. The experiment started on 1 September 2020, and ended on 1 March 2021. Sufficient water was supplied to each box every 1-2 days to keep the soil moist.

 Measurements and analyses

At the end of the experiment, we harvested the aboveground parts of each species in each patch in each box. Plant roots in each patch were also harvested, but it was not possible to sort them into species. After clearing, all plant parts were oven-dried at 70 °C for 72 hours and weighed. In the heterogeneous treatment, in each box plants in the two opposite soil patches without microplastics (referred to as high-quality patches) were pooled and those in the two soil patches with microplastics (referred to as low-quality patches) were pooled. In the homogeneous treatment, plants were treated in the similar way as those in the heterogeneous treatment for the purpose of analysis, i.e., in each box two opposite patches were referred to as imagined high-quality patches and the other two as imagined low-quality patches.

We calculated evenness of the plant community in each box based on aboveground biomass of each plant species (Pielou, 1966; Xue et al., 2021). Two-way ANOVA was used to test the effects of microplastic type (EPS, PET, PP, HDPE, PLA and PA6), soil heterogeneity (homogeneous vs. heterogeneous) and their interaction on root, shoot and total biomass and species evenness of the plant communities at the whole box level. Following two-way ANOVA, linear contrasts were used to test whether mean values differed significantly between the homogeneous and the heterogeneous treatment within each type of microplastics (Sokal and Rohlf, 1995). Three-way ANOVA was used to examined the effects of microplastic type, soil heterogeneity and patch type (low- vs. high-quality patches) on root, shoot and total biomass of the plant communities at the patch level. Box identity was included as a random factor as the data from the two types of patches in a box were not independent. Following three-way ANOVA, linear contrasts were used to test whether mean values differed significantly between the high- and the low-quality patches within each of 12 combinations of the microplastic type and soil heterogeneity treatments.

We also analyzed the effects of microplastic shape (foam, fiber and bead) and soil heterogeneity on biomass and evenness of the plant communities at the whole box level, and the effects of microplastic shape, soil heterogeneity and patch quality on biomass of the plant communities at the patch level. In these analysis, microplastic type (EPS, PET, PP, HDPE, PLA and PA6) and/or box identity were included as random factors. Before analysis, the data were tested for normality and homogeneity. The analyses were implemented using IBM SPSS 23.0 (IBM Corp., Armonk, NY, USA) and R (version 4.1.2; http://www.r-project.org) in RStudio (version 2021.09.1 Build 372; https://www.rstudio.com/).

 Results Effects on biomass of plant communities at the whole box level

Total biomass and root biomass of the plant communities in the whole boxes were significantly higher in the homogeneous than in the heterogeneous treatment when the microplastic type was PET and PP, were significantly smaller when the microplastic type was PLA, but were not significantly different when the microplastic type was EPS, HDPE or PA6 ( Figure 2A, B , Table S1 ). Shoot biomass of the whole communities was significantly higher in the homogeneous than in the heterogeneous treatment when the microplastic type was PP, but showed no significant difference between the homogeneous and the heterogeneous treatment for the other five types of microplastics ( Figure 2C , Table S1 ). Total, root and shoot biomass of the whole plant communities were all significantly higher in the homogeneous than in the heterogeneous treatment when the microplastic shape was fiber, but did not differ between the two soil treatments when the microplastic shape was foam or bead ( Figure 3 , Table S2 ).

(A) Total, (B) root and (C) shoot biomass and (D) species evenness of the plant communities in the homogeneous and the heterogeneous treatment for each of the six types of microplastics. Bars and vertical lines are mean and SE. Symbols (*** P < 0.001, ** P < 0.01 and * P < 0.05) indicate significant differences between the homogeneous and the heterogeneous treatment.

(A) Total, (B) root and (C) shoot biomass and (D) species evenness of the plant communities in the homogeneous and the heterogeneous treatment for each of the three shapes of microplastics. Bars and vertical lines are mean and SE. Symbols (*** P < 0.001 and ** P < 0.01) indicate significant differences between the homogeneous and the heterogeneous treatment.

 Effects on biomass of plant communities at the patch level

We observed significant three-way interaction effects of both microplastic type × soil heterogeneity × patch quality and microplastic shape × soil heterogeneity × patch quality on all three biomass measures at the patch level ( Table S3-S4 ). At the patch level, biomass (total, root and shoot) of the plant communities generally did not differ significantly between the imagined two types of patches in the homogeneous treatment in any of the six microplastic types or in any of the three microplastic shapes (foam, fiber and bead); the only exception was shoot biomass which was higher in the low- than in the high-quality patches in the homogeneous treatment when the microplastic type was PET ( Figure 4C ).

(A) Total, (B) root and (C) shoot biomass of the plant communities in the (imagined) high- and low-quality patches in the homogeneous and the heterogeneous treatment for each of the six microplastics. Bars and vertical lines are mean and SE. Symbols (*** P < 0.001, ** P < 0.01 and * P < 0.05) indicate significant differences between the high- and low-quality patches.

In the heterogeneous treatment, however, total biomass and root biomass of the plant communities were significantly smaller in the low-quality patches (with the higher concentration of microplastics) than in the high-quality patches (without microplastics) when the microplastic type was EPS, but greater when the microplastic type was PET or PP ( Figures 4A, B ). In addition, in the heterogeneous treatment, root biomass was significantly smaller in the low- than in the high-quality patches when the microplastic was HDPE ( Figure 4B ), and shoot biomass was also significantly smaller when the microplastic was EPS and PET ( Figure 4C ). In the heterogeneous treatment, total and root biomass of the plant communities were significantly smaller in the high- than in the low-quality soil patches when the microplastic shape was fiber, but did not differ between the two types of patches when the microplastic shape was foam or bead ( Figures 5A, B ). Shoot biomass of the plant communities was significantly higher in the high- than in the low-quality patches when the microplastic shape was foam, but showed no difference between the patch types when the microplastic shape was fiber or bead ( Figure 5C )

(A) Total, (B) root and (C) shoot biomass of the plant communities in the (imagined) high- and low-quality patches in the homogeneous and the heterogeneous treatment for each of the thee microplastic shapes. Bars and vertical lines are mean and SE. Symbols (*** P < 0.001 and * P < 0.05) indicate significant differences between the high-and low-quality patches.

 Effects on species diversity of plant communities at the whole box level

Species evenness of the plant communities was significantly higher in the homogeneous than in the heterogeneous treatment when the microplastic type was EPS, was significantly lower when the microplastic type was PET, but showed no significant difference when the microplastic type was HDPE, PP, PLA or PA6 ( Figure 2D , Table S1 ). Species evenness was also significantly higher in the homogeneous than in the heterogeneous treatment when the microplastic shape was foam, but showed no difference between the two soil treatments when it was fiber or bead ( Figure 3D , Table S2 )

 Discussion

Contamination of soils by microplastics can have profound ecological impacts on terrestrial ecosystems (Rillig, 2012; de Souza Machado et al., 2018a; Roy et al., 2022). While previous studies have shown that microplastics in soils may influence plant growth and community productivity and composition (de Souza Machado et al., 2019; Rillig et al., 2019; Lozano and Rillig, 2020), no study has tested the impact of spatial heterogeneity of soil microplastics. Our study showed for the first time that spatial heterogeneity of soil microplastics could influence productivity and species composition of experimental plant communities, but such effects varied depending on microplastic types and shapes.

When growing in spatially heterogeneous environments consisting of favorable and unfavorable patches, many plants are able to allocate more roots and/or shoots in favorable patches and less in unfavorable patches (Hutchings et al., 2003; Dong et al., 2015; Liu et al., 2020), and such a foraging response may help them take up more resources and increase their growth (Rajaniemi and Reynolds, 2004; Questad and Foster, 2008; Xue et al., 2020). We also found that the plant communities showed patch-level foraging responses in the environments with the spatially heterogeneous distribution of soil microplastics. However, such an effect varied with microplastic types and shapes likely due to their different effects on soil physico-chemical properties and soil microbial abundance and activities (de Souza Machado et al., 2018b; Qi et al., 2018; Rillig et al., 2019). Additionally, the effect of soil microplastic heterogeneity on community productivity was not related to the patch-level foraging responses of the plant communities.

EPS and HDPE commonly have a negative effect on plant growth because they can induce cytogenotoxicity by aggravating reactive oxygen species generation (Jiang et al., 2019; Maity and Pramanick, 2020; Pignattelli et al., 2020). We observed that, in the environment with the spatially heterogeneous distribution of soil microplastics, plant communities produced more root, shoot and total biomass in the soil patches without microplastics (high-quality patches) than in the soil patches with the higher (0.2%) concentration of microplastics (low-quality patches) when the microplastic was EPS and more root biomass when the microplastic was HDPE ( Figure 4 ), demonstrating root and/or shoot foraging responses (2015; James et al., 2009; Giehl and von Wirén, 2014; Keser et al., 2014). However, such foraging responses at the patch level did not cascade to influence biomass of the plant communities at the whole box level ( Figures 2A-C ), as also reported in some previous studies testing the effect of spatial heterogeneity in the distribution of other soil factors (Dong et al., 2015; Liu et al., 2017; Xue et al., 2018; Adomako et al., 2021b; Yao et al., 2021).

PET and PP used in this study are both microplastic fibers. Adding plastic fibers to the soil can reduce soil bulk density (Rillig et al., 2019), increase soil porosity (Zhao et al., 2021) and permeability (de Souza Machado et al., 2019), which can facilitate plants to take roots into the soil (Zimmerman and Kardos, 1961). Thus, in the heterogeneous treatment with PET and PP, root biomass was greatly improved when plants grew in the patches with a higher concentration (0.2%) of PET or PP than in the patches without microplastics ( Figure 4B ), resulting in higher total biomass of the plant communities in the PET and PP patches ( Figure 4A ). Consequently, total and root biomass of the plant communities also differed greatly between the two types of soil patches when the microplastic shape was fiber ( Figures 5A, B ). These results suggest that the plant communities also demonstrated foraging responses in the soil with the spatially heterogeneous distribution of microplastic fibers such as PET and PP.

However, such foraging responses did not result in promoted growth of the whole plant communities (at the whole box level). Instead, the spatially heterogeneous distribution of microplastic fibers (PET and PP) in the soil decreased biomass of the whole plant community biomass ( Figures 2A-C , 3A-C ). The decreased growth was because plants grew better when the soil contained 0.1% of PET and PP in the homogeneous treatment than when the soil did not contain any microplastics or contained the higher concentration (0.2%) of microplastics in the heterogeneous treatment (all P < 0.5; Figures 3A, B ). This result suggests that the positive effect of microplastic fibers on plant growth can vary depending on their concentrations in the soil (Lozano et al., 2021).

At the whole box level, we observed that the plant communities produced more total and root biomass in the soil with the heterogeneous distribution of PLA than in the soil with the homogeneous distribution ( Figures 2A, B ). However, the plant communities did not show patch-level foraging responses in the soil with the heterogeneous distribution of PLA ( Figure 4 ) or in the soil with the heterogeneous distribution of microplastic beads in general ( Figure 5 ), suggesting that this benefit of soil microplastic heterogeneity was not related to foraging responses (Adomako et al., 2020; Liu et al., 2020; Cao et al., 2022). PLA is one type of biodegradable microplastics, and can change soil physico-chemical properties such as soil pH in the initial stage of polylactic acid degradation, which may affect soil microbial communities (Karamanlioglu et al., 2017; Chamas et al., 2020). Previous studies showed that PLA can negatively affect plant growth (Souza et al., 2013; Boots et al., 2019). In this study, the promoted community growth in the soil with the heterogeneous distribution of PLA was solely because the plant communities grew worse when the soil contained 0.1% of PLA (in the homogeneous treatment) than when it did not contain PLA or contained the higher concentration (0.2%; in the heterogeneous treatment), particularly for root growth (all P < 0.05; Figure 4B ). This result suggests that the negative impact of PLA on plant growth can vary depending on its concentration in the soil, as reported for other microplastics (van Kleunen et al., 2020; Lozano et al., 2021).

A plant community commonly comprises species of different foraging abilities and consequently environmental heterogeneity may alter its species composition because it may benefit species with a higher foraging ability more than those with a low foraging ability or those do not demonstrate the foraging response (Hutchings et al., 2003; Xue et al., 2020). In our study, spatial heterogeneity of soil microplastics decreased species evenness when the microplastic was EPS (with a foam shape), promoted it when the microplastic was PET (with a fiber shape), and had no effect when the microplastic was one of the other four microplastics (with either a fiber or a bead shape; Figure 2D ). These results suggest that spatial heterogeneity in the horizontal distribution of microplastics in the soil can affect species composition of plant communities, but such an effect varies depending on microplastic types and likely also microplastic shapes. It is well-known that different types and shapes of microplastics may differentially affect plant growth because they may differ in phytotoxicity (Dong et al., 2020; Pignattelli et al., 2020; Li et al., 2022) and in the effect on soil physio-chemical properties and soil microbial communities such as soil microbial activity and mycorrhizal binding in plant roots (de Souza Machado et al., 2019; Rillig et al., 2019; Lozano and Rillig, 2020; Lozano et al., 2021; Zhao et al., 2021). The promoted evenness of soil heterogeneity in the horizontal distribution of microplastics may be due to the increased microhabitat diversity, as observed in studies examining effects of soil heterogeneity in other factors (Liu et al., 2021; Helbach et al., 2022). However, it is unclear what resulted in the decreased species evenness in the soil with the heterogeneous distribution of EPS, which seemed not to be related to the difference in the patch-level responses of individual plant species ( Figure S1 , Table S5 ). Further studies could be designed to resolve this question.

In our experiment, there were no physical barriers between patches with and without microplastics. This setup mimicked the situation in the field that allowed plant roots to grow freely across adjacent patches. In a long run, plant growth, soil biota activity and water movement will eventually homogenize the soil in the container that is heterogeneous at the beginning. However, this process usually will take a much long time compared to the shorter experimental duration in the greenhouse, which will not influence the treatment effect.

We conclude that soil heterogeneity in microplastics can influence productivity and species composition of plant communities, but such an effect varies depending on microplastic types and likely also shapes (e.g., chemical composition and morphology). However, our results fail to support the idea that foraging responses of plant communities can result in promoted productivity. Further studies could combine soil microbial and physico-chemical analyses to explore the mechanisms underlying the effect of soil microplastic heterogeneity on community productivity (Baer et al., 2020; Lozano et al., 2021; Xue et al., 2021; Zhao et al., 2021). The impacts of patch scale and patch contrast of soil microplastic heterogeneity should also be considered in future (Adomako et al., 2021b; Li et al., 2022).

 Data availability statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

 Author contributions

X-MZ drafted the manuscript; X-XC conducted the experiment and analyzed the data; L-XH reanalyzed the data; WX designed the experiment and helped analyze the data; J-QG, N-FL, J-SC, and M-HL contributed substantially to manuscript revision; F-HY designed the experiment and contributed substantially to manuscript revision. All authors contributed to the article and approved the submitted version.

 Funding

This study was supported by the Joint Fund of Zhejiang Provincial Natural Science Foundation (grant LTZ20C030001) and NSFC (32071527, 31761123001). Open access funding provided by WSL - Swiss Federal Institute For Forest, Snow And Landscape Research.

 Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

 Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

 Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fpls.2022.1075007/full#supplementary-material

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