Linking Microbial Communities and Nitrogen Cycling in Agricultural Groundwater of a Semiarid Region

Abstract This study investigated the hydrogeochemical conditions and bacterial community structure in a semiarid agricultural region of Dianbu Town, Shandong Province, China. The results revealed severe nitrate pollution in groundwater, with concentrations as high as 472.20 mg/L and an exceedance rate of 85.71% relative to the Class III national groundwater quality standard. Hydrochemical and microbial analyses identified distinct redox zonation within the aquifer systems: oxidizing conditions in the western region favored nitrification and aerobic denitrification, while reducing environments in the central area promoted anaerobic processes such as denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Bacterial communities were dominated by Proteobacteria and Bacteroidota at the phylum level. Key genera including Flavobacterium, Limnohabitans and Vogesella were significantly correlated with nitrogen transformation pathways and hydrochemical parameters (e.g., dissolved oxygen, oxidation–reduction potential, sulfate, and total dissolved solids). These findings highlighted the potential of microbial indicators as bioindicators of redox state and nutrient cycling dynamics in contaminated aquifers. The study provides critical insights for region-specific water quality monitoring and sustainable groundwater management, supporting the development of biologically informed remediation strategies to mitigate agricultural pollution and protect drinking water resources in water-scarce regions.