Biochar and Microplastics Affect Microbial Necromass Accumulation and CO₂ and N₂O Emissions from Soil

Biochar (BC) application represents a promising soil management strategy for mitigating CO2 and N2O emissions; however, as concurrent soil pollutants, microplastics may interact with BC. In a 3 month microcosm experiment (25 °C, 60% WHC), we investigated soil C and N dynamics following the addition of polyethylene (PE) microplastics (1 and 5%) to a Calcaric Fluvisol already amended with BC for 1 month. BC alone reduced CO2 and N2O emissions by 11 and 3%, respectively, while PE reduced CO2 and N2O emissions by 11–26 and 4–14%, respectively. The suppression of CO2 emissions by BC and PE microplastics was due to reduced dissolved organic matter (DOM) content as well as increased DOM aromaticity, all of which led to diminished bacterial biomass and β-N-acetyl-glucosaminidase activity. BC decreased N2O emissions by suppressing the nirS and nirK genes while increasing the level of the nifH gene; PE decreased N2O emissions primarily by decreasing the level of the nirK gene. BC alone decreased the microbial necromass carbon content by 35%, primarily due to the suppression of bacterial abundance, thus leading to reduced efficiency in bacterial necromass production. PE had a modest impact, decreasing microbial necromass C by 8–11% in BC-free soil, mainly due to dilution effects. However, in BC-treated soil, PE had a profound influence, as it markedly increased the microbial necromass C by 33–61%. The microbial necromass increased due to the disruption of aggregates, which provided better protection against microbial necromass and a reduction in β-N-acetyl-glucosaminidase activity, which is responsible for necromass mineralization. In summary, the interactive effects of BC and PE microplastics on microbial necromass accumulation, as well as CO2 and N2O emissions, are mainly based on microbial (especially bacterial) necromass and DOM decomposition as well as aggregate destruction. Our findings offer valuable insights for the adaptation and enhancement of soil carbon management strategies in response to the challenge of microplastic contamination.