Mechanistic insights into polylactic acid and cadmium co-regulation of nitrogen dynamics in the pea-soil system via microbial drivers.

The increasing coexistence of microplastics and metals in agricultural soils poses a profound threat to ecosystem health, yet the mechanisms through which combined pollution affects nitrogen cycling remain poorly understood. This study investigated the influence of three polylactic acid (PLA) particle sizes (4 mm, 150 μm, and 500 nm) at a concentration of 1%, individually and in combination with cadmium (Cd, 3.0 mg/kg), on nitrogen transformation in the pea-soil system. Results showed that nano-PLA significantly promoted pea growth, for example, increasing biomass by 11.6% (3.19 g), compared to Control. Additionally, the combination of 4 mm PLA and Cd significantly altered the distribution of nitrogen in the soil-plant system, enhancing the accumulation of NO₃⁻-N in both peas (15.28 g/kg, 68.2%) and soil (4.36 mg/kg, 4.4%). PLA increased Cd bioaccumulation in peas (1.76-4.06 mg/kg) relative to Control (0.62 mg/kg), showing a significant negative correlation between soil Cd availability and PLA particle size (r = -0.9, p < 0.01). PLA-Cd altered microbial community assembly through stochastic processes, increased the abundance of nitrogen-cycling genes (e.g., nirK and amoA) by 1.8-7.5 fold, and enhanced gene co-occurrence network complexity. Mechanistically, PLA-Cd mitigated Cd-induced oxidative stress and photosynthetic inhibition in peas, and modified soil properties (e.g., available phosphorus) to influence microbial communities. This microbial shift stimulated the expression of nifH gene, enhancing nitrogen fixation. This soil-microbe-plant interplay clarifies the mechanism by which PLA-Cd co-exposure regulates the nitrogen cycling, providing critical insights for ecological risk assessment of compound pollution in agricultural systems.