Effects of Hydrochemistry and Hydrodynamics on Nitrogen Transformation Processes and Functional Gene Abundance in Sediments of Shahu Lake, China

ABSTRACT Lakes receive nitrogen (N)‐contaminated groundwater discharges, which can lead to algal blooms. Therefore, investigating the transformation and transport of inorganic nitrogen within the infiltration zone (IZ) is essential. This study combined field monitoring and sediment column experiments to elucidate N transformation mechanisms within the IZ under various hydrochemical and hydrodynamic conditions. Factors influencing N transformation included water chemistry, hydraulic residence time, water flow direction and abundance of genes involved in the N cycle. The results revealed that denitrification and dissimilatory nitrate reduction to ammonium (DNRA) were the dominant processes for NO 3 − ‐N reduction in both upwelling and downwelling flow‐controlled IZ. Notably, the efficiency of nitrate‐nitrogen removal via denitrification or DNRA was higher under low flow rates and long hydraulic residence time. Moreover, increased hydraulic residence time increased the abundance of functional genes associated with N transformation. NO 3 − ‐N concentrations exhibited cyclic fluctuations owing to frequent interactions between groundwater and lake water. Additionally, the positive and negative feedback effects induced by inorganic N and dissolved oxygen concentrations on the abundance of functional genes may offset each other under equivalent hydraulic retention periods. This study demonstrated that the removal rate of NO 3 − ‐N ranged from 45% to 54% in upwelling flow and from 51% to 71% in downwelling flow. Ultimately, these findings provide a scientific foundation for comprehensive lake management.