Unaccounted N2O source to the atmosphere from agriculture through groundwater denitrification and use for irrigation

Nitrous oxide (N₂O), a potent greenhouse gas, is produced in cultivated soils and this production is enhanced by the application of reactive nitrogen in the form of fertilizers. Part of fertilizers applied is not taken up by crops, leading to their accumulation in the groundwater where they can evolve into N₂O through nitrification/denitrification. Based on chemical and isotopic measurements of nitrate (NO₃^-) and N₂O in Indian groundwater, we report for the first-time evidence for the widespread occurrence of denitrified NO₃^- and of high concentrations of N₂O (>2000% of saturation). The accumulation of N₂O is high in regions where groundwater is intensely abstracted for irrigation, releasing this accumulated N₂O into the atmosphere. Using annual mean groundwater abstraction for irrigation in India, which is the highest in the globe, we estimated that 0.06 to 20 GgN₂O/y is effluxed to the atmosphere, which adds 0.1 to 20% of the N₂O flux emitted by agricultural soils in India. This so far unidentified source around the globe may thus contribute significantly to the imbalance in the global N₂O budget.