Fulvic Acid Enhances Nitrogen Fixation and Retention in Paddy Soils through Microbial-Coupled Carbon and Nitrogen Cycling

Fulvic acid, the most soluble and active humic substance,
is widely
used as an agent to remediate contaminated soils and improve soil
fertility. However, the influence of fulvic acid (FA), as a microbial
carbon source, on carbon and nitrogen cycles in paddy soils remains
elusive. Therefore, to investigate it, an incubation experiment was
conducted. Gas analyses indicated that the carbon dioxide and methane
emissions were enhanced in FA treatment, which increased up to 94.08-fold
and 5.06-fold, respectively. 15N-labeling experiments revealed
that nitrogen fixation capability was promoted (1.2-fold) to reduce
the carbon and nitrogen imbalance due to fulvic acid amendment. Metagenomic
analysis further revealed that gene abundances of degradation of lignin-like
compounds, gallate degradation, methanogenesis, nitrogen fixation,
and urea hydrolysis increased, while the bacterial ammonia oxidation
and anaerobic ammonium oxidation decreased, caused by FA application.
Metabolic reconstruction of metagenome-assembled genomes revealed
that Azospirillaceae, Methanosarcinaceae, and Bathyarchaeota, with
higher abundance in FA treatment, were the key microorganisms to maintain
the carbon and nitrogen balance. The metabolic pathways of fulvic
acid degradation and coupled nitrogen fixation and retention were
constructed. Collectively, our results provided novel insights into
the theoretical basis of the use of humic substances for reducing
nitrogen fertilization and climate change.