Effects of microplastics on soil C and N cycling with or without interactions with soil amendments or soil fauna

Abstract Microplastics (MPs) enter the global soil ecosystem in significant quantities, and in agricultural lands, interact with soil amendments (e.g, fertilisers, pesticides, biochar), pollutants (e.g., heavy metals and acid rain) and soil fauna (e.g., earthworms and microbial biomass) which impacts carbon (C) and nitrogen (N) cycling in soil in ways that are largely unexplored. Here, we analysed the difference in the responses of soil C and N contents, greenhouse gas emissions and soil enzyme activities in experiments where MPs alone or MPs in combination with other soil amendments or soil fauna had been explored, by conducting a global meta‐analysis of 2543 observational data extracted from 84 published studies. The results show that MPs alone are associated with significantly increased soil C storage and altered soil N pool composition, increased soil carbon dioxide (CO 2 ) and nitrous oxide (N 2 O) emissions, reduced soil ammonia (NH 3 ) emissions, increased activity of one of the five major enzymes related to C cycle, and increased activity of three of the four major enzymes related to N cycling in soil. Experiments that explored the combined effects of MPs combined with fertilisers on soil C and N contents and enzyme activities indicated greater effects than MPs alone. However, the effects of MPs combined with soil fauna, heavy metals, acid rain, glyphosate, and carbon nanotubes on soil C and N cycling differed from findings of studies that considered the effects of MPs alone. The results of this meta‐analysis provide a theoretical basis for further study of the effects of MPs on C and N cycling in fertilized agricultural soil, compound‐contaminated soil and acidified soil. We also show that, despite the global importance of MPs in soil, the number of studies of their effects on soil C and N cycling is relatively few, and further research on the mechanisms of interactions of MPs with multiple soil amendments in soil systems is urgently needed.