Metallic micronutrients mitigate herbicide-microplastic stress on soil microbial functions in cornfields

Both herbicides and microplastics disrupt microbial functions in soils, but the pathways through which metallic micronutrients alleviate such disturbances are poorly understood. A nationwide survey in cornfields was conducted to reveal relationships among pollutants, micronutrients and microbial functions, followed by field experiments to verify the mitigating effects of these micronutrients. The survey demonstrated that acetochlor, atrazine and fiber-microplastics were negatively correlated with the functional genes nirS, cbbL and phoD, whereas soil pH was positively correlated with these genes. Structural modeling (SEM) indicated that fiber-shaped microplastics synergistically amplified the suppressive effects of atrazine and acetochlor on cbbL and phoD abundances, primarily through nirS downregulation, soil pH decline, and reduced microbial diversity. Consistently, field experiments showed that the herbicide thiencarbazone-methyl·isoxaflutole, when combined with biodegradable microplastics, inhibited nirS expression, thereby decreasing the abundances of cbbL and phoD. SEM analysis based on the survey suggested that zinc supplementation exerted an antagonistic effect against combined contamination by elevating nirS abundance. This mitigation effect was further verified by field variance analysis, which showed that zinc addition significantly enhanced nirS and cbbL abundances. Overall, this study provides a targeted strategy for alleviating the combined suppressive impacts of herbicides and microplastics on soil microbial functions during maize production.