Agricultural managed groundwater recharge: interrelation of nitrogen cycling and hydrology

Abstract:

Agricultural managed aquifer recharge (Ag-MAR, on-farm recharge), where farmland is flooded with excess surface water in winter to intentionally recharge groundwater, has received increasing attention by policy makers and researchers in recent years. However, there remain concerns about the potential for Ag-MAR to exacerbate NO₃^- contamination of groundwater, and the public health risks associated with elevated NO₃^- concentrations in groundwater and related greenhouse gas emissions. Here, we conducted replicated winter groundwater recharge experiments to quantify the effect of Ag-MAR on N transformations in the soil and vadose zone. The main objectives were to quantify the amount of NO₃^- leaching and gaseous losses occurring during Ag-MAR events in two different vineyards and to identify the biogeochemical transformations occurring within the soil profile and the shallow vadose zone. The experiment was conducted on two raisin grape vineyards, which were each flooded for 2 weeks and 4 weeks, respectively. Site instrumentation included a suite of soil moisture sensors, water level loggers and flow meters to estimate the amount of surface water applied for recharge and the infiltration and percolation of water to the groundwater table. Other measurements (down to 1-m depth) included: redox probes, oxygen sensors, suction cups for pore water sampling, and static flux chambers for nitrogen (N₂O) and carbon-related (CO₂, CH₄) atmospheric fluxes. Soil samples were taken before and after Ag-MAR events to determine soil N species (TN, DON, TOC, NO₃^-, NH₄⁺), pH, EC, and soil texture. Plant and yield were also measured to determine the effect of Ag-MAR on grape physiology and to close the N-mass balance. This study presents results from the first year. Preliminary results indicate that during flooding NO₃^- leaching was the main process with additional but smaller contribution of denitrification.