The impact of microplastics on soil micro-food webs is regulated by biochar and earthworms

Microplastic contamination in agricultural soils is becoming increasingly severe worldwide. Microplastic may impact soil organisms and their functions through both toxic and non-toxic effects, thereby threaten soil and human health. However, to date, numerous studies have focused solely on the effects of microplastics on individual taxa, neglecting the composition and function of the soil micro-food web. Biochar and earthworms as mitigation agents are constantly employed to improve microplastic-contaminated farmland soil. It remains unclear whether the presence of biochar and earthworms influences the effects of microplastics on the soil micro-food web. Hence, we conducted an extensive microcosm experiment to investigate the effects of microplastics, biochar, earthworms, as well as their interactions on soil microbial and nematode communities, carbon cycle associated with the energy flux, and co-occurrence network of the micro-food web. Our results showed that microplastics increased microbial biomass and nematode abundance by 2 and 8 times, respectively, and changed nematode community composition. Microplastics elevated the relative abundance of bacterivores while decreasing that of fungivores, shifting the energy pathway towards a bacteria-dominated channel, subsequently accelerating bacterial and total energy flux in the soil food web. Moreover, microplastics reduced nematode diversity but increased the number of edges and average degree of the co-occurrence network, thereby enhancing food web complexity. Biochar application reduced network complexity and interacted with microplastics, antagonistically diminishing the role of microplastics in rapid energy flux. Earthworm addition improved network complexity and synergistically amplified the preference of microplastics for bacterial and fast energy channels. Our study provides new perspectives on the effects of microplastics, biochar and earthworms on the function of the soil micro-food web, offering a basis for soil management in global agricultural ecosystems.