Microbial metabolic mechanisms underlying organic nitrogen formation mediated by the coupling between the tricarboxylic acid cycle and nitrogen-transformation processes.

Organic nitrogen (ON) transformation is critical for nitrogen retention during composting. Here, pilot-scale windrow composting experiments were conducted using food waste digestate composting (FW) and chicken manure composting (CM) to investigate associations between microbial functional potential and ON dynamics through glutamate-centered carbon-nitrogen metabolism. The results supported a potential pathway in which α-ketoglutarate from the tricarboxylic acid cycle (TCA) coupled with ammonium nitrogen (NH4+) through glutamate metabolism and was associated with ON dynamics. Core functional genes (FW: e.g., glnA, GDH2, GLUD1_2; CM: e.g., gltB, glnA, ureC) were identified, and their associated microbes (FW: e.g., Novibacillus, Planifilum; CM: e.g., Dietzia, Brevibacterium) were further determined using gene-taxon association analysis. Different regulatory patterns were observed between the two practical composting systems The CM microbial network was more connected than the FW network, with 10,732 versus 9,089 edges and graph densities of 0.194 versus 0.148. Additionally, In FW, functional genes made the largest independent contribution to ON variation (25.3%), and ON dynamics were more closely associated with the glutamate dehydrogenase (GDH) pathway and carbon-skeleton availability. In CM, gene-associated microbes made the largest independent contribution (14.2%). Although the glutamine synthetase/glutamate synthase (GS/GOGAT) pathway and urease-related processes exhibited efficient metabolic coupling, peptide-like DON molecular signatures remained relatively abundant during most composting stages. These findings provide a mechanistic framework for optimizing nitrogen transformation and retention during composting.