Influences and underlying mechanisms of substituting chemical nitrogen fertilizer with different organic amendments on soil nitrogen mineralization in red soil

The partial substitution of chemical nitrogen fertilizer with organic amendments is a key strategy for sustainable agriculture. However, the effects of this practice on nitrogen mineralization in acidic red soil remain unclear. This study combined field and soil incubation experiments to investigate the influences of different organic substitution practices on soil nitrogen mineralization, by substituting 20% chemical nitrogen fertilizer with high C/N ratio organic amendments including maize straw (MS) and biochar (CB) and with low C/N ratio amendments including cow manure (CD) and biogas residue (BR). The total nitrogen application rate was equal across all treatments (300 kg N ha −1 ). The results revealed dichotomous impacts on the net nitrogen mineralization rate (Rm) under different nitrogen substitution practices. Compared to applying chemical fertilizer alone (CK), CD and BR increased the Rm by 160% and 385%, respectively. In contrast, MS and CB decreased the Rm by 70% and 113%, indicating net nitrogen immobilization. We found that net nitrogen mineralization was governed by a dual-control mechanism involving soil C/N ratio and soil pH, both of which were influenced by the organic amendments. Metagenomic analysis confirmed that organic substitution profoundly altered the genetic potential for nitrogen cycling. Crucially, these shifts were adaptive responses to the modified soil environment, driven primarily by changes in soil pH and microbial biomass stoichiometry (MBC:MBN). Specifically, low-C/N amendments that increased soil pH (CD and BR) stimulated net nitrogen mineralization, while the high-C/N amendment (CB) and the acidifying amendment (MS) inhibited this process, promoting net immobilization. This study elucidated the mechanisms by which different types of organic amendments regulated nitrogen mineralization in acidic red soil, providing actionable guidance for selecting appropriate organic amendments to enhance soil fertility while avoiding unintended consequences.