Intensive ammonium fertilizer addition activates iron and carbon conversion coupled cadmium redistribution in a paddy soil under gradient redox conditions

While over-fertilization and nitrogen deposition can lead to the enrichment of nitrogen in soil, its effects on heavy metal fractions under gradient moisture conditions remains unclear. Here, the effect of intensive ammonium (NH4+) addition on the conversion and interaction of cadmium (Cd), iron (Fe) and carbon (C) was studied. At relatively low (30-80 %) water hold capacity (WHC) NH4+ application increased the carbonate bound Cd fraction (F2-Cd), while at relatively high (80-100 %) WHC NH4+ application increased the organic matter bound Cd fraction (F4-Cd). Iron-manganese oxide bound Cd fractions (F3-Cd) and oxalate-Fe decreased, but DCB-Fe increased in NH4+ treatments, indicating that amorphous Fe was the main carrier of F3-Cd. The variations in F1-Cd and F4-Cd observed under the 100-30-100 % WHC treatment were similar to those observed under low moisture conditions (30-60 % WHC). The C=O/C-H ratio of organic matter in soil decreased under the 30-60 % WHC treatment, but increased under the 80-100 % WHC treatment, which was the dominant factor influencing F4-Cd changes. The conversion of NH4+ declined with increasing soil moisture content, and the impact on oxalate-Fe was greater at 30-60 % WHC than at 80-100 % WHC. Correspondingly, genetic analysis showed the effect of NH4+ on Fe and C metabolism at 30-60 % WHC was greater than at 80-100 % WHC. Specifically, NH 4 + treatment enhanced the expression of genes encoding extracellular Fe complexation (side- rophore) at 30 - 80 % WHC, while inhibiting genes encoding Fe transmembrane transport at 30 - 60 % WHC, indicating that siderophores simultaneously facilitated Cd detoxification , Fe complexation. Furthermore, biosynthesis of sesquiterpenoid, steroid, butirosin and neomycin was significantly correlated with F4 - Cd, while glycosaminoglycan degradation metabolism and assimilatory nitrate reduction was significantly correlated with F2 - Cd. Overall, this study gives a more comprehensive insight into the effect of NH 4 + on activated Fe and C conversion on soil Cd redistribution under gradient moisture conditions.