Microbial-enzymatic coupling drives nitrogen stabilization during static pile composting of rice straw amended with...

Nitrogen loss during straw composting substantially undermines nutrient recycling efficiency and the agronomic value of finished compost products. Clarifying how different nitrogen sources mediate nitrogen transformation and nitrogen retention in organic fractions is therefore essential for improving compost maturity and nitrogen retention. This study evaluated how three nitrogen-rich amendments with divergent chemical characteristics (chicken manure, fish meal, and soybean powder) influenced nitrogen transformation, organic nitrogen fractionation, enzyme dynamics, and microbial succession during static pile composting of rice straw. Compost maturity varied significantly among treatments (P < 0.05), with germination indices of 100% for soybean powder, 90.1% for fish meal, and 84.9% for chicken manure. Chicken manure primarily promoted a mineralization-oriented nitrogen transformation pattern characterized by rapid ammonium accumulation and subsequent nitrification, accompanied by elevated urease activity and enrichment of Firmicutes. In contrast, fish meal and soybean powder were associated with nitrogen transformation patterns involving stronger proteolytic and oxidative enzyme activities and greater nitrogen retention in relatively stable organic fractions, resulting in significant increases in amine nitrogen and hydrolysable unknown nitrogen (HUN), a relatively stable organic nitrogen fraction, by 45.1-139% relative to the control (P < 0.05). Coordinated proteolytic and oxidative enzyme activities, together with enrichment of Thermobifida, Actinobacteriota, and Aspergillus, were strongly associated with HUN formation. Overall, protein-rich nitrogen sources were more conducive to microbial-enzymatic interactions associated with organic nitrogen stabilization, whereas chicken manure favored nitrogen mineralization. These findings demonstrate that nitrogen sources can shape nitrogen transformation toward greater inorganic nitrogen production or higher organic nitrogen retention, thereby providing practical insights for improving nitrogen retention and compost quality in straw composting systems.