Abstract
Microplastics are increasingly recognized as a factor of global change. By altering soil inherent properties and processes, ripple-on effects on plants and their symbionts can be expected. Additionally, interactions with other factors of global change, such as drought, can influence the effect of microplastics. We designed a greenhouse study to examine effects of polyester microfibers, arbuscular mycorrhizal (AM) fungi and drought on plant, microbial and soil responses. We found that polyester microfibers increased the aboveground biomass of Allium cepa under well-watered and drought conditions, but under drought conditions the AM fungal-only treatment reached the highest biomass. Colonization with AM fungi increased under microfiber contamination, however, plant biomass did not increase when both AM fungi and fibers were present. The mean weight diameter of soil aggregates increased with AM fungal inoculation overall but decreased when the system was contaminated with microfibers or drought stressed. Our study adds additional support to the mounting evidence that microplastic fibers in soil can affect the plant-soil system by promoting plant growth, and favoring key root symbionts, AM fungi. Although soil aggregation is usually positively influenced by plant roots and AM fungi, and microplastic promotes both, our results show that plastic still had a negative effect on soil aggregates. Even though there are concerns that microplastic might interact with other factors of global change, our study revealed no such effect for drought.

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Acknowledgments
MCR acknowledges funding from an ERC Advanced Grant (694368). EFL acknowledges funding from the Deutsche Forschungsgemeinschaft (LE 3859/1-1). JB acknowledges funding from the Deutsche Forschungsgemeinschaft (RI 1815/22-1).
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A.L. designed the study and set up the experiment, with the help of L.F., E.F.L. and A.W.; A.L. performed analysis; L.F., provided experimental data; A.L., E.F.L., J.B. and M.C.R. wrote the manuscript; all authors contributed to the final version of the manuscript.
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Highlights
• Polyester fibers increased aboveground biomass.
• Under drought conditions the AM-fungal-only treatment had the highest biomass.
• Colonization with AM fungi increased under microfiber addition.
• The mean weight diameter of soil aggregates decreased under microplastic contamination and drought stress, respectively.
• Under drought conditions AM fungi increased litter decomposition.
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Lehmann, A., Leifheit, E.F., Feng, L. et al. Microplastic fiber and drought effects on plants and soil are only slightly modified by arbuscular mycorrhizal fungi. Soil Ecol. Lett. 4, 32–44 (2022). https://doi.org/10.1007/s42832-020-0060-4
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DOI: https://doi.org/10.1007/s42832-020-0060-4
Keywords
- Arbuscular mycorrhizal fungi
- Microplastic
- Soil aggregation
- Organic matter decomposition
- Drought
- Root traits