Insight into nitrate-contaminated shallow groundwater using nitrate isotopes and microbial community: A case study of vegetable cultivation area in the Huaibei Plain, China

High-intensity agricultural activities frequently lead to elevated nitrate (NO 3 − ) in groundwater. However, limited research has been conducted on the sources of NO 3 − in groundwater and its biotransformation processes in vegetable growing areas (VGA) of the Huaibei Plain in China. This study investigated groundwater samples from a typical VGA of the Huaibei Plain and compared them with samples from surrounding village area (VA), analyzing hydrogeochemical parameters, multiple isotopes (δ 15 N-NO 3 , δ 18 O-NO 3 , δ 18 O-H 2 O, and δ 2 H-H 2 O), and performing 16S rRNA sequencing. The source and biotransformation process of NO 3 − were quantitatively analyzed using a Bayesian isotope mixing model (MixSIAR) and PICRUSt2 functional gene prediction. The results showed that the mean nitrate nitrogen (NO 3 − -N) concentration in VGA groundwater was 35.78 mg/L, whereas in the VA, the mean NO 3 − -N concentration of groundwater was 3.43 mg/L, and NO 3 − -N was the most dominant N form. In VGA groundwater, the sources of NO 3 − were manure and sewage (M&S) (57.6 %), followed by chemical fertilizer (CF) (19.6 %), soil nitrogen (SN) (18.2 %), and atmospheric deposition (AD) (4.6 %). In contrast, the sources of NO 3 − in VA groundwater were predominantly M&S (77 %), with contributions from AD (8.8 %), SN (7.2 %), and CF (7.0 %). Moreover, nitrification emerged as the dominant mechanism driving nitrogen transformation in both VGA and VA groundwater. PICRUSt2 analysis indicates that VGA groundwater exhibits a higher gene abundance of denitrification and dissimilatory reduction of NO 3 − to ammonia nitrogen (DNRA). NO 3 − , Na + , Cl − , and δ 18 O-NO 3 − were the main environmental factors affecting the microbial community ( p < 0.05), while F − and SO 4 2− were the main factors significantly affecting the functional genes of the nitrogen cycle ( p < 0.05). To mitigate NO 3 − contamination in groundwater, strategies such as enhancing domestic sewage treatment efficiency, optimizing irrigation practices for better water resource management, and reinforcing the denitrification process are recommended. These findings provide a valuable reference for effectively managing groundwater resources and restoring groundwater quality in comparable areas within the plain groundwater zone. • Intensive vegetable cultivation can lead to elevated nitrate levels in groundwater. • Quantitative identification of nitrate using dual nitrate isotope with Bayesian modeling. • VGA groundwater contains high proportions of DNRA-capable microbes. • Intensive denitrification processes are the cause of nitrate-rich groundwater.