Polyethylene microplastics hamper aged biochar’s potential in mitigating greenhouse gas emissions

While biochar effectively reduces greenhouse gas emissions, the coincident microplastics will alter these benefits. To assess the long-term efficacy of biochar application in reducing emissions amidst microplastic interference, we investigated the interactive effects of polyethylene microplastics (1%–5% wt) and decadal biochar addition (aged biochar) on C and N stability in a Fluvic Cambisol. Aged biochar and polyethylene individually reduced CO2 emissions by 49% and 18%, respectively, over 91-day incubation. This was due to decreased soil aggregation, dissolved organic matter (DOM) content, and increased DOM aromaticity, which reduced microbial biomass and chitinase activity associated with soil organic matter (SOM) decomposition. This ultimately led to increased accumulation of microbial necromass carbon (MNC) to soil stable carbon pool. Interestingly, the coexistence of polyethylene diminished the efficacy of biochar in mitigating CO2 emissions (+ 44 ~ 82%) and stabilizing MNC (-18 ~ 23%). This was because the interaction between polyethylene and biochar facilitated macroaggregate formation and DOM accumulation and decreased DOM aromaticity, which increased microbial biomass and chitinase activity for SOM decomposition. Similar to soil C dynamics, aged biochar largely reduced N2O emissions by 54%, due to decreased nirK but increased nifH genes. Polyethylene increased N2O emissions by 5% and 25% in biochar-free and aged biochar soil, respectively, likely through upregulation of nirS and nirK but downregulating nifH gene expression. Thus, polyethylene microplastics may undermine the benefits of biochar in mitigating climate change, highlighting the necessity to recognize microplastics as a global change factor and to incorporate their role in elemental cycling alongside their global transport. Graphic Abstract: (Figure presented.) © The Author(s) 2025.