Unveiling hidden threats: Nitrate pollution in agricultural catchments with deep vadose zone

Abstract

Identifying dominant nitrogen sources in groundwater and surface water is critical for controlling non-point source (NPS) pollution in agricultural watersheds. However, in regions with deep vadose zones (>4 m) and limited precipitation. However, in regions with deep vadose zones (>4 m) and limited precipitation, the hidden threats of agricultural NPS pollution remain critically understudied due to prolonged pollution retention and insufficient transport dynamics. Here, we investigated the transport routes of surplus N and the driving mechanism of NPS pollution in a semi-humid agricultural catchment with vadose zone thickness exceeding 80 m and large-scale apple orchards, through a combined approach of questionnaire survey, hydrochemistry, multi-isotope tracer technology (δ¹⁵N-NO₃, δ¹⁸O-NO₃, δ²H-H₂O, δ¹⁸O-H₂O) and MixSIAR model. The results indicated that the high nitrogen input and surplus in apple orchards led to an annual potential nitrogen loss in runoff up to 257.88 kg·ha⁻¹, 3.7 times that of croplands. Heavy precipitation was the main driving force for the loss of residual soil nitrogen. As the main form of inorganic nitrogen in water, the nitrate concentration in surface water peaked at 8.63 mg L⁻¹ during heavy rainfall, approaching the drinking water safety threshold of World Health Organization. Simultaneously, precipitation contribution to surface water increased sharply to 66.2 %. This resulted in a rapid rise in the proportion of nitrate derived from fertilizer and soil nitrogen, rising from 2 % to 90 %. Thus, the surface water nitrate mainly originated from agricultural surplus nitrate during heavy precipitation. In contrast, during periods without heavy precipitation, groundwater was the dominant source (>80 %) of surface water recharge, making groundwater the main source of surface water nitrate. Critically, the deep vadose zone has effectively impeded the leaching of agricultural surplus nitrate into groundwater so far. Therefore, groundwater nitrate and surface water nitrate without heavy precipitation both mainly (> 90 %) originated from manure and sewage. This study emphasized that heavy precipitation periods were the high-risk intervals for nitrogen NPS pollution in agricultural area with deep vadose zone and limited precipitation, providing scientific support for targeted NPS pollution prevention and control strategies.