Artificial humic acid reshapes microbial C-N metabolism and nitrogen partition in microplastic-contaminated soils

Microplastics pose a serious threat to soil ecosystems, particularly nitrogen cycling. Among the diverse types of microplastics, polypropylene (PP) and polylactic acid (PLA) are frequently detected in agricultural soils. Artificial humic acid (A-HA), a humic-like substance derived from lignocellulosic waste, may regulate soil carbon and nitrogen dynamics, but its effects on nitrogen partitioning and microbial C-N metabolic potential under different microplastic stresses remain unclear. Here, a 90-day microcosm experiment integrating soil physicochemical analyses, dissolved organic matter characterization, bacterial community profiling, and metagenomics was conducted in PLA- and PP-contaminated soils. At day 15, 600 mg kg^-1 A-HA decreased nitrate (NO₃⁻-N) by 11.36% and 6.46% in PLA- and PP-contaminated soils, respectively, while increasing soluble organic nitrogen (SON) by 17.32% and 23.89%. A-HA also enriched Nitrospira and Steroidobacter and altered the abundance of genes associated with nitrogen transformation, assimilation, and carbon metabolism, including nrfA, GLU, gltB, and icd. Increased abundance of nrfA suggested greater dissimilatory nitrate reduction to ammonium (DNRA) potential, while enrichment of NADP⁺-dependent icd indicated altered potential for 2-Oxoglutarate generation in the TCA cycle. Overall, A-HA modified nitrogen partitioning toward soluble and microbial organic pools and was associated with coordinated changes in microbial C-N metabolic potential under microplastic exposure.