Hydroperiodic dynamics of microbial-mediated nitrogen cycling and its multi-element coupling effect in the Weihe...

The aquatic ecosystem is affected by both human activities and climate change, and the excessive input and imbalance of nitrogen (N) have become an environmental issue of global concern. In this study, based on metagenomic sequencing, network analysis, random forest and structural equation model were used to investigate the hydroperiod dynamics, driving mechanisms of N cycling and the coupling effects of N, carbon (C) and sulfur (S) cycling in the Weihe River. The results showed that the nitrogen concentration and pathways exhibited heterogeneity during the high, normal and low water periods. Nitrogen fixation and organic nitrogen mineralization were abundant during the high water period, and nitrification, anammox were dominant during the low water period. The co-occurrence network of bacteria and nitrogen cycling genes was dominated by positive correlations, and Limnohabitans, Flavobacterium, and Polynucleobacter showed diverse metabolic potentials. Redundancy analysis showed that nitrogen cycling genes were more sensitive to substrate changes in the high and low water periods, and dependent on basic physicochemical conditions during the normal water period. A structural equation model revealed that climatic conditions, water properties, and microbial communities had positive effects on nitrogen cycling genes (standard effects of 0.154, 0.347, 0.603), with microbial-gene collaboration driving core functionality. The strong positive correlation of the co-occurrence network of C, N, and S functional genes revealed the elemental synergies. The multi-element coupled cycling networks showed more complex and stronger interactions during the normal water periods, and the efficiency of cross-element metabolism was reduced during low water periods. This study holds significance in understanding the ecological effects of microbial-mediated nitrogen cycling and multi-element coupled cycling in rivers, and provides a theoretical basis for nutrient control in micro-polluted rivers.