Citric acid-driven metabolic regulation strategy for coordinating nitrogen retention and phosphorus activation in...

In this study, citric acid was evaluated as a metabolic modulator to mitigate nitrogen volatilization and phosphorus fixation during aerobic composting. Results indicate that this approach can synergistically enhance nitrogen retention and phosphorus activation by influencing microbial-enzymatic interactions. The optimal outcomes were observed with the addition of 100 mmol/L citric acid (Acid-100), which significantly increased total nitrogen by 6.93%, available nitrogen by 72.60%, and available phosphorus (H₂O-P and NaHCO₃-P) by 16.42% compared to the control. NaOH-P may be the main part of phosphate dissolution. The relative abundances of Pseudomonas, Bacillota, and Streptomyces increased by 14.71%, 46.15%, and 282%, respectively, compared to CK. The addition of citric acid also increased the activities of nitrogen and phosphorus-related enzymes in composting, such as increasing the activities of ACP and Phytase by 133% and 125%, which may contribute to promote the dissolution of phosphorus. At the same time, AMO, HAO activity and AOB abundance increased by 27.27%, 19.03% and 69.76%, respectively, which may promote nitrification and thus strengthen the fixation of nitrogen. Furthermore, Citric acid lowers the pH of the composting system and may release phosphorus by binding with metal phosphates (such as aluminum phosphate). The addition of exogenous citric acid to the composting process has been shown to alleviate the nitrogen-phosphorus trade-off through microbially mediated biochemical processes, thereby providing a theoretical basis and technical approach for the production of high-nutrient organic fertilizers.