Summary
Agricultural soils contribute ~52% of global anthropogenic nitrous oxide (N<sub>2</sub>O) emissions, predominantly from nitrogen (N) fertilizer use. Global N<sub>2</sub>O emission factors (EFs), estimated using IPCC Tier 1 methodologies, largely rely on short-term field measurements that ignore legacy effects of historic N fertilization. Here we show, through data synthesis and experiments, that EFs increase over time. Historic N addition increases soil N availability, lowers soil pH, and stimulates the abundance of N<sub>2</sub>O producing microorganisms and N<sub>2</sub>O emissions in control plots, causing underestimates of EFs in short-term experiments. Accounting for this legacy effect, we estimate that global EFs and annual fertilizer-induced N<sub>2</sub>O emissions of cropland are
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