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Microplastics (MPs), plastic particles <5 mm, are found in environments, including terrestrial ecosystems, planetwide. Most research so far has focused on ecotoxicology, examining effects on performance of soil biota in controlled settings. As research pivots to a more ecosystem and global change perspective, questions about soil-borne biogeochemical cycles become important. MPs can affect the carbon cycle in numerous ways, for example, by being carbon themselves and by influencing soil microbial processes, plant growth, or litter decomposition. Great uncertainty surrounds nano-sized plastic particles, an expected by-product of further fragmentation of MPs. A major concerted effort is required to understand the pervasive effects of MPs on the functioning of soils and terrestrial ecosystems; importantly, such research needs to capture the immense diversity of these particles in terms of chemistry, aging, size, and shape.

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Microplastics are a diverse and ubiquitous contaminant, a global change driver with potential to alter ecosystem properties and processes. Microplastic-induced effects in soils are manifold as microplastics differ in a variety of properties among which the shape is of special interest. Our knowledge is limited regarding the impact of various microplastic shapes on soil processes. Therefore, we conducted this two-part research comprising a meta-analysis on published literature and a lab experiment focusing on microplastic shapes- and polymer-induced effects on soil aggregation and organic matter decomposition. We here focus on fibers, films, foams and particles as microplastic shapes.In the meta-analysis, we found a strong research focus on fibrous and particulate microplastic materials, with films and foams neglected.Our experiment showed that microplastic shapes are important modulators of responses in soil aggregation and organic matter decomposition. Fibers, irrespective of their chemistry, negatively affected the formation of aggregates. However, for other shapes like foams and particles, the polymer identity is an important factor co-modulating the soil responses.Further research is needed to generate a data-driven foundation to permit a better mechanistic understanding of the importance and consequences of microplastics added to soils.

With growing populations and climate change, assuring food and nutrition security is an increasingly challenging task. Climate-smart and sustainable agriculture, that is, conceiving agriculture to be resistant and resilient to a changing climate while keeping it viable in the long term, is probably the best solution. The role of soil biota and particularly arbuscular mycorrhizal (AM) fungi in this new agriculture is believed to be of paramount importance. However, the large nutrient pools and the microbiota of subsoils are rarely considered in the equation. Here we explore the potential contributions of subsoil AM fungi to a reduced and more efficient fertilization, carbon sequestration, and reduction of greenhouse gas emissions in agriculture. We discuss the use of crop rotations and cover cropping with deep rooting mycorrhizal plants, and low-disturbance management, as means of fostering subsoil AM communities. Finally, we suggest future research goals that would allow us to maximize these benefits.

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.