Characteristics of the Water Environment and the Mechanism of Nitrogen Metabolism in the Xisha River

The nitrogen cycle is the key to the healthy operation of river ecosystems and plays an important role in maintaining the ecological balance, purifying water quality, and promoting the circulation of material. The Xisha River was chosen as the research object to analyze the water quality condition from 2021 to 2023, and the microbial diversity of nitrogen metabolism, functional genes, and metabolic pathways in the water body were analyzed using macro-genomics technology. The results showed that total nitrogen (TN) was the main exceedance factor in the water body, and ammonia nitrogen (NH 3 -N), TN, and total phosphorus (TP) were the key factors affecting the water quality. The downstream station (W2) exhibited the most significant water quality changes, while the upstream station (W5) showed the highest biodiversity and abundance. The top five genera in abundance in the water body were unclassified__c__Actinomycetia , unclassified__p__Bacteroidota , Paenisporosarcina , Candidatus_Planktophila , and unclassified__c__Betaproteobacteria . The five most abundant nitrogen metabolism genes were K01915 (nitrate reductase), K00265 (nitrite reductase), K01673 (ammonium transporter), K00266 (nitrite reductase), and K02575 (nitrate reductase), each contributing to critical nitrogen cycling processes such as denitrification, nitrification, and nitrogen assimilation. The six major nitrogen metabolism pathways were denitrification (M00529), anisotropic nitrate reduction (M00528), anisotropic nitrate reduction (M00529). anisotropic nitrate reduction (M00530), complete nitrification (M00804), nitrate assimilation (M00615), methylaspartate cycling (M00740), and assimilatory nitrate reduction (M00531). TN was identified as the primary environmental factor influencing both microbial communities and nitrogen metabolism genes. Co-occurrence network analysis identified K01915 (nitrate reductase), K00459 (ammonium transporter), K01673 (ammonium transporter), and K00261 (nitrate reductase) as pivotal genes involved in nitrogen metabolism. This study reveals the microbial-driven nitrogen cycle and lays the foundation for mitigating nitrogen pollution in the Xisha River.