Biochar-microplastics interaction modulates soil nitrous oxide emissions and microbial communities

Biochar has been proposed as a soil amendment in vegetable fields, where the widespread use of plastic film leads to significant retention of microplastics (MPs) in the soil. However, the interactive effect of biochar and MPs on plant growth and soil functions remains poorly understood. Here, we conducted a pot experiment to examine the effects of biochar application in the presence of conventional and biodegradable microplastics (0.05% w/w ) on the growth of coriander, soil nitrogen (N) cycling processes, and microbial communities. The results showed that biochar application increased aboveground biomass by increasing plant available N of NH 4 + , regardless of the presence of MPs. Biochar also significantly reduced soil nitrous oxide (N 2 O) emissions by an average of 16% without MPs. However, when MPs were present, the effect of biochar on N 2 O emissions was lessened depending on the MP type. Polylactic acid consistently reduced soil N 2 O emissions and the abundance of N 2 O production genes, irrespective of biochar application. Conversely, polyethylene without biochar reduced N 2 O emissions primarily by inhibiting N-related functional genes responsible for nitrification and denitrification. This inhibitory effect was reversed when biochar was applied, leading to a 26% increase in N 2 O emissions due to increased nifH and nirK gene abundance. Although biochar and MPs did not significantly alter microbial α-diversity, they altered the composition and structure of bacterial and fungal communities, linked to changes in soil N turnover. Our study underscores the critical role of MP type in assessing the effects of biochar on soil N cycling and N 2 O emissions. Consequently, plastic pollution may complicate the ability of biochar to improve plant growth and soil functions, depending on the characteristics of the MPs. Graphical Abstract Highlights Biochar significantly increased the aboveground biomass of coriander, independent of microplastics. Polylactic acid addition reduced N 2 O emissions by inhibiting nitrification and denitrification with and without biochar addition. Inhibition of soil N 2 O emissions by polyethylene was reversed to promotion with biochar addition due to the increasing abundance of nifH and nirK. Effects of microplastics on the diversity and composition of soil microbial communities were related to their type and biochar addition.