The combination of metagenomics and metabolomics reveals the effect of nitrogen fertilizer application driving the remobilization of immobilization remediation cadmium and rhizosphere microbial succession in rice

The remobilization of cadmium (Cd) in contaminated farmland soil due to nitrogen fertilizer addition has raised significant concerns regarding the effectiveness of immobilization remediation. This study investigated the effects of ammonia nitrogen (NH₄⁺-N) and nitrogen (NO₃^--N) application (100 kg/ha) on the remobilization of immobilization of remediation Cd (bound to clay palygorskite) during various growth stages of rice through field experiments. Our findings revealed that increased organic acid secretion (e.g., benzoic acid and malic acid) from rice roots, induced by NH₄⁺-N, significantly enhanced the NH₄NO₃-extractable Cd content. Consequently, the concentration of Cd in brown rice varied from 39.84 to 43.25 μg/kg to 78.31–90.44 μg/kg. While NO₃^--N exhibited a relatively weaker capacity for Cd remobilization (Cd content in brown rices: 50.17–65.23 μg/kg). Meanwhile, the organic acid secretion in roots inhibited the expression of most functional genes (e.g., nifK and napA), leading to shifts in microbial communities and functional metabolism (e.g., Cd²⁺ exporting). According to the results of metagenome-assembled genome (MAG) composition, specific MAGs with fewer functional annotations were enriched under NH₄⁺-N treatment, may further increased risk of Cd exposure in rice by stimulating amt expression. Interaction analysis of metabolic products and microbial communities indicated acids linked to branched-chain amino acid (BCAA) metabolism and urea cycle might serve as a potentially key process influencing microbial dynamics.