Complementary TCA cycle precursor supply enhances ammonia assimilation and nitrogen retention during vegetable waste composting.

Reactive nitrogen loss reduces nutrient recovery efficiency and the environmental performance of organic waste composting. Citric acid and pyruvic acid were evaluated separately and in combination as complementary tricarboxylic acid cycle precursors during vegetable waste composting. All treatments received the same amount of exogenous carbon and were adjusted to the same initial pH. Precursor amendments reduced cumulative NH3 emissions by 60%-76%. The combined treatment produced the lowest final nitrogen loss and increased final glutamate content and humic substance content by 25.2% and 31.7%, respectively, relative to the control. Metabolite profiles showed coordinated α-ketoglutarate and NADPH availability, providing the carbon skeleton and reducing power required for ammonium incorporation through glutamate dehydrogenase (GDH) and the glutamine synthetase (GS) - glutamate synthase (GOGAT) pathways. The combined treatment also produced the strongest coordination among ammonia assimilation gene abundances, enzyme activities, and bacterial and fungal succession. Bacterial communities were more closely associated with ammonia assimilation indicators, whereas fungal communities were more strongly associated with precursor turnover and reducing power availability. Integrated analyses further supported cross-domain coordination between carbon turnover and ammonia assimilation. Collectively, complementary TCA cycle precursor supply directed more ammonium toward glutamate and organic nitrogen formation, enhanced nitrogen retention and humification, and reduced the ammoniacal nitrogen available for NH3 volatilization. This metabolic regulation provides a strategy for improving nutrient recovery during vegetable waste composting.