The interaction between cadmium and water induces a nonlinear response in nitrogen cycling in paddy soils by...

Cadmium (Cd) contamination threatens agricultural soil health by disrupting soil nitrogen (N) supply capacity through the disruption of microbially mediated N cycling processes. Water management plays a critical role in regulating Cd bioavailability; however, the interactive effects of Cd stress and water regimes on N transformation and microbial mechanisms remain poorly understood. This study investigates the coupled "Cd-water-microbe-N cycle" framework in red paddy soils with Cd concentrations ranging from 0 to 10 mg kg^-1 under flooded and non-flooded conditions. A ¹⁵N paired isotope labeling approach combined with a Markov chain Monte Carlo (MCMC) model was employed to quantitatively estimate the rates of major N transformation processes. Microbial community structure and functional gene responses were analyzed using HT-qPCR, high-throughput sequencing, and co-occurrence network analysis. Results showed that Cd effects were strongly water-dependent and concentration-specific. Under flooded conditions, Cd (0.3-10 mg kg⁻¹) reduced gross mineralization and immobilization rates by up to ∼20-40% while increasing nitrification and denitrification rates, resulting in negative net N supply and higher NO₃^- loss risk. In contrast, low-to-moderate Cd levels (0.3-3.0 mg kg⁻¹) under non-flooded conditions enhanced N transformation rates by ∼10-30%, maintaining a positive N supply. Microbial network analysis revealed that bacteria dominated functional differentiation, with low-dose Cd (≤1.5 mg kg⁻¹) promoting synergistic interactions and high Cd levels (≥6.0 mg kg⁻¹) causing network fragmentation and stronger inhibition of N cycling functions. This study provides mechanistic insights into the interactions among Cd contamination, water regimes, microbial networks, and N cycling, which may help inform management strategies for Cd-contaminated farmland.