Irrigation is a considerable nitrogen input in global cropland

Abstract Irrigation unintentionally delivers reactive nitrogen to croplands via nitrate-rich water (NIrrig), yet this input remains largely absent from nitrogen budgets and policies. Here, we compile over 1300 field observations of NIrrig to quantify its magnitude and agronomic relevance, and upscale its global contribution. While the median inputs reached 19 kg N ha-1 yr-1, 10% of observations exceeded 100 kg N ha-1 yr-1. Globally, we estimate that irrigation supplies 14 Tg N yr-1, equivalent to 14% of synthetic fertilisers in croplands. Hotspots emerge in regions with intensive irrigation and high inputs, highlighting NIrrig as a substantial but underused nitrogen source. Our findings expose a major overlooked component of agricultural nitrogen budgets, offering a pathway to reduce fertiliser overuse, enhance nitrogen use efficiency and promote nitrogen circularity in irrigated systems.