Tracing the sources and fate of NO3– in the vadose zone-groundwater system of a thousand-year cultivated region

Excess nitrate (NO3−) loading in terrestrial and aquatic ecosystems can result in critical environmental and health issues. NO3−-rich groundwater has been recorded in the Guanzhong Plain in the Yellow River Basin of China for over 1000 years. To assess the sources and fate of NO3− in the vadose zone and groundwater, numerous samples were collected via borehole drilling and field surveys, followed by analysis and stable NO3− isotope quantification. The results demonstrated that the NO3− concentration in 38% of the groundwater samples exceeded the limit set by the World Health Organization. The total NO3− stock in the 0−10 m soil profile of the orchards was 3.7 times higher than that of the croplands, suggesting that the croplandto-orchard transition aggravated NO3− accumulation in the deep vadose zone. Based on a Bayesian mixing model applied to stable NO3− isotopes (δ15N and δ18O), NO3− accumulation in the vadose zone was predominantly from manure and sewage N (MN, 27−54%), soil N (SN, 0−64%), and chemical N fertilizer (FN, 4−46%). MN was, by far, the greatest contributor to groundwater NO3− (58−82%). The results also indicated that groundwater NO3− was mainly associated with the soil and hydrogeochemical characteristics, whereas no relationship with modern agricultural activities was observed, likely due to the time delay in the thick vadose zone. The estimated residence time of NO3− in the vadose zone varied from decades to centuries; however, NO3− might reach the aquifer in the near future in areas with recent FN loading, especially those under cropland-to-orchard transition or where the vadose zone is relatively thin. This study suggests that future agricultural land-use transitions from croplands to orchards should be promoted with caution in areas with shallow vadose zones and coarse soil texture.