Global analysis of nitrogen inputs from irrigation water to cropland

Irrigation unintentionally delivers reactive nitrogen to croplands via nitrate-rich water, yet this input is rarely quantified and remains largely absent from nitrogen budgets and fertilization policies. Here we compile over 1,300 field observations and upscale them using machine learning to quantify the nitrogen delivered through irrigation and map its contribution across irrigated croplands for 2010–2019. While the median input was 19 kg N ha−1 yr−1, 10% of observations exceeded 100 kg N ha−1 yr−1. Globally, we estimate that irrigation supplied 14 Tg N yr−1, equivalent to 13% of synthetic fertilizers used on croplands. Important clusters appear in regions with intensive irrigation and high nitrogen inputs, indicating that irrigation represents a substantial but underused nitrogen source. Our findings expose an overlooked component of agricultural nitrogen budgets that can reduce fertilizer overuse, improve nitrogen-use efficiency and promote nitrogen circularity in irrigated croplands.