Carbon biodegradability governs nitrogen retention through microbial ammonia assimilation during composting

Nitrogen retention in composting is critically limited by the asynchrony between carbon and nitrogen transformations. While the carbon-to-nitrogen ratio is well-known, the biodegradability of carbon sources fundamentally determines microbial energy acquisition and the supply of carbon skeletons for ammonia assimilation. Here, we investigated how a gradient of carbon biodegradability governs microbial ammonia assimilation and nitrogen fate. Four treatments with distinct biodegradability gradients (G, LG2, LG1, and L) were established by varying the lignin-to-glucose ratio. The results showed that the LG2 treatment achieved the highest nitrogen retention efficiency, evidenced by a 36.9% increase in organic nitrogen and a glutamate content of 2.6 g·kg^-1 DM. The balanced co-existence of labile and recalcitrant carbon in LG2 maintained a continuous carbon flow through the tricarboxylic acid cycle, enhanced dissolved organic carbon degradation (53.3%), and promoted α-ketoglutarate generation, thereby supporting efficient ammonia assimilation. Functional predictions further showed that LG2 enriched ammonia assimilation genes while suppressing nitrification-associated genes, reflecting a metabolic shift that favored biosynthetic NH₄⁺ utilization over oxidative loss pathways. The partial least squares path model identified carbon-source biodegradability as the primary factor regulating ammonia assimilation, where hemicellulose and cellulose promoted α-ketoglutarate-mediated carbon skeleton supply, whereas lignin constrained this routing and suppressed glutamate formation. Overall, our findings demonstrate that moderate carbon biodegradability enhances microbial energy metabolism and assimilation capacity, offering a mechanistic basis for engineering carbon quality to improve nitrogen preservation in composting systems.