Irrigation as a coupled water-nitrogen flux: Correcting a blind spot in N$_2$O emissions

In irrigated regions, water does not arrive alone but also carries dissolved nitrogen recycled from past losses to groundwater and surface waters. Yet the nitrogen in irrigation water remains largely invisible when designing field experiments, planning fertilization, simulating and interpreting soil N$_2$O emissions, and developing mitigation strategies. By overlooking irrigation as a coupled water-nitrogen flux, we miss a key link between legacy pollution, food production, and agricultural emissions. Without accounting for it, part of today’s emissions may reflect nitrogen pollution accumulated decades ago.