Shifts in the sources and fates of nitrate in shallow groundwater caused by agricultural intensification intensity: Revealed by hydrochemistry, stable isotopic composition and source contribution

Groundwater nitrogen (N) pollution in intensive agricultural regions is increasing globally, and understanding the effects of increasing agricultural intensity on the sources and fates of N is critical for the effective control of groundwater pollution. Using multiple stable isotopes (δ15N-NO3—, δ18O-NO3—, δ18O-H2O, δD-H2O), hydrochemistry and the Bayesian stable isotope mixing model, the sources and fates of N were expounded, and their contributions were quantified in shallow groundwater (SG, n = 325) from an intensive agricultural region, a facility agricultural region and a cropland fallow region. We found that NO3— accounted for 60% of total N, and 52% of SG samples exceeded the WHO NO3— drinking water threshold (10 mg N L−1). Each N form concentrations and NO3— exceeding the standard rate in the intensive and facility agricultural regions were significantly higher than those in the cropland fallow region. Soil N, N fertilizer and manure and sewage were the dominant NO3— contributors in the SG of intensive agricultural regions, contributing to 66%, 21% and 10% of NO3—, respectively. The highest contributions of manure and sewage occurred in the cropland fallow region (46%), followed by the soil N (33%) and N fertilizer (13%), while the N sources in the SG from the facility agricultural region were dominated by manure and sewage (35%), soil N (31%) and N fertilizer (25%). Denitrification was the primary N biogeochemical cycle process in SG from the intensive and facility agricultural regions, whereas nitrification mainly occurred in the cropland fallow region. These results indicated that the shifts in the sources and fates of N in SG were controlled by agricultural intensification intensity, and soil N and manure and sewage, rather than N fertilizer, may become the most dominant N sources in SG in intensive agricultural regions. The N contamination control in SG should pay attention to improving septic tanks and sewage pipelines, the scientific and reasonable stacking of manure, and reducing the soil N pool in intensive agricultural regions. [Display omitted] •Increasing agricultural intensity changed the N sources and fates in groundwater.•Soil N and manure & sewage were the main NO3— contributors in groundwater.•Denitrification in intensive and facility agricultural regions was the primary N cycle process in groundwater.•Nitrification in the cropland fallow region was the primary N cycle process in groundwater.