Is irrigation water an overlooked source of nitrogen in agriculture?

The increase of agricultural nitrogen (N) inputs since the 1960s is a key driver in surface- and groundwater nitrate pollution. The water abstracted from these sources can input substantial amounts of reactive nitrogen (N Irrig ) if used for crop irrigation. This input is often not included in N related agricultural policies and studies, which are likely underestimating the magnitude of N pollution hotspots and overestimating the N use efficiency. In this study, we provided prima facie evidence that N Irrig is a neglected source of N in irrigated systems. The N Irrig was computed for 278 municipalities in mainland Portugal along the period 1995–2019 based on the gross irrigation requirements and nitrate concentration in ground- and surface water sources. The former was derived using two complementary approaches, using the AquaCrop and GlobWat models, while the latter were computed following spatially explicit approaches. N Irrig showed annual large fluctuations (6–11 Gg N yr -1 ), of which 91% was from groundwater sources. Results show that N Irrig averaged 14 ( ± 11) kg N ha -1 yr -1 , which is equivalent to 3 ( ± 4) % of the N in synthetic fertilisers. This input was higher in the municipalities that simultaneously present high irrigation demand and the nitrate-contaminated groundwater as an irrigation source. In these cases, located in Nitrate Vulnerable Zones, N Irrig reached up to 95 kg N ha -1 yr -1 and more than 80% of the N in synthetic fertilizers. This study highlights the importance of linking water and nutrient policies to better gain insight on N Irrig , for which the current study provided for a simple modelling framework.