Fulvic Acid Enhances\nNitrogen Fixation and Retention\nin Paddy Soils through Microbial-Coupled Carbon and Nitrogen Cycling

Fulvic acid, the most soluble and active humic substance,\nis widely\nused as an agent to remediate contaminated soils and improve soil\nfertility. However, the influence of fulvic acid (FA), as a microbial\ncarbon source, on carbon and nitrogen cycles in paddy soils remains\nelusive. Therefore, to investigate it, an incubation experiment was\nconducted. Gas analyses indicated that the carbon dioxide and methane\nemissions were enhanced in FA treatment, which increased up to 94.08-fold\nand 5.06-fold, respectively. <sup>15</sup>N-labeling experiments revealed\nthat nitrogen fixation capability was promoted (1.2-fold) to reduce\nthe carbon and nitrogen imbalance due to fulvic acid amendment. Metagenomic\nanalysis further revealed that gene abundances of degradation of lignin-like\ncompounds, gallate degradation, methanogenesis, nitrogen fixation,\nand urea hydrolysis increased, while the bacterial ammonia oxidation\nand anaerobic ammonium oxidation decreased, caused by FA application.\nMetabolic reconstruction of metagenome-assembled genomes revealed\nthat Azospirillaceae, Methanosarcinaceae, and Bathyarchaeota, with\nhigher abundance in FA treatment, were the key microorganisms to maintain\nthe carbon and nitrogen balance. The metabolic pathways of fulvic\nacid degradation and coupled nitrogen fixation and retention were\nconstructed. Collectively, our results provided novel insights into\nthe theoretical basis of the use of humic substances for reducing\nnitrogen fertilization and climate change.