Biochar enhances microbial nitrogen retention in polylactic acid-contaminated soil under freeze-thaw cycles

In the context of global climate change and the increasing use of biodegradable plastics, the freeze-thaw cycles and the residual microplastics from polylactic acid (PLA) pose combined stress on soil nitrogen cycling. However, research on the interactive effects of these two factors and the regulatory mechanisms of biochar remains insufficient. This study conducted an indoor simulated freeze-thaw cycle, establishing control (CK), biochar (3%, B), PLA (2%, L), and their composite (BL) treatments to systematically investigate the response mechanisms of biochar to PLA-contaminated soil microbial nitrogen cycle. The results indicated that the BL treatment significantly increased soil pH by 8.39%-11.12% (P < 0.05) after the 20th freeze-thaw cycle and alleviated the decrease in electrical conductivity caused by PLA. Additionally, the BL treatment significantly reduced the ammonium nitrogen content in soil (lower by 48.65%-61.32% compared to other treatments) and the nitrate nitrogen content, while increasing urease activity. Moreover, the catalase activity was enhanced by 5.94%-7.53% compared to L treatment (P < 0.05). Microbial community analysis showed that BL treatment alleviated the inhibition of the phylum Proteobacteria by PLA, promoted the enrichment of Actinobacteria, and upregulated the expression of nitrogen assimilation genes (such as nasA and nrtD), while suppressing the activity of denitrification functional genes (such as nirS and norB). Structural equation modeling further elucidated that biochar primarily enhanced nitrogen retention via indirect pathways by improving soil chemical properties and reshaping microbial community structure and function. This research provides a theoretical basis for ecological risk assessment of biodegradable film residues in cold-region farmland and biochar remediation.