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 (NIrrig) 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 NIrrig is a neglected source of N in irrigated systems. The NIrrig 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. NIrrig showed annual large fluctuations (6–11 Gg N yr-1), of which 91% was from groundwater sources. Results show that NIrrig 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, NIrrig 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 NIrrig, for which the current study provided for a simple modelling framework.