Heavy metal contamination threats carbon sequestration of paddy soils with an attenuated microbial anabolism

As a global environmental concern, heavy metal pollution significantly impacts soil organic carbon (SOC) dynamics. Nevertheless, the microbial mechanisms governing SOC persistence under heavy metal contamination remain unclear, as previous research primarily focused on microbial catabolism. This study elucidated SOC variation induced by heavy metal contamination from the perspective of microbial anabolism, a key contributor to SOC sequestration according to recent theory. Herein a field survey was conducted at 13 sampling sites in polluted rice paddies, determining both SOC content and key microbial parameters. Nemerow index (a comprehensive index of pollution level) ranged from 0.48 to 2.93, with cadmium and copper as the primary contaminants. SOC content ranged between 14.56 and 23.97g kg⁻¹ across sampling sites and showed a negative relationship with nemerow index (R² = 0.46, P < 0.001). Variation partitioning and random forest analyses indicated that SOC reduction was primarily driven by the combined effects of microbial factors and heavy metal pollution, with dominant role of microbial factors. Nemerow index negatively correlated with microbial C use efficiency (CUE) (R² = 0.42, P < 0.001) and microbial biomass turnover (R² = 0.12, P = 0.017). Structural equation modeling further suggested that heavy metal pollution reduced SOC by decreasing microbial biomass carbon (MBC) formation and microbial residue accumulation through negative effects on microbial CUE and soil nitrogen availability. Collectively, our research provided robust evidences that heavy metal pollution could threat C sequestration of paddy soils by attenuating microbial anabolism with reduced accumulation of microbial-derived carbon.