Nitrous oxide emission from croplands: biogeochemical drivers, nitrification inhibitors, policy implications, and future directions

Abstract

Nitrous oxide (N₂O) emissions from croplands are a distinct environmental challenge because N₂O is both a potent greenhouse gas and a major contributor to stratospheric ozone depletion. Emissions arise largely from agricultural nitrogen (N) inputs and microbially mediated nitrification and denitrification as N pools are transformed under fluctuating soil oxygen and moisture conditions. Yet N inputs are central to maintaining yield stability, ensuring an adequate food protein supply, and sustaining farm profitability. Therefore, mitigation strategies that rely predominantly on reducing N use are difficult to scale without compromising agricultural productivity. When expressed as chlorofluorocarbon-11–equivalent ozone-depleting emissions, N₂O (179.0 kt yr⁻¹) exceeds the combined emissions of legacy ozone-depleting substances and transitional hydrochlorofluorocarbons (145.7 kt yr⁻¹), strengthening the case for prioritizing agricultural N₂O within climate and ozone-protection frameworks. In this review, we synthesize current understanding of nitrification- and denitrification-derived pathways of N₂O formation and the biological sink of N₂O reduction to dinitrogen (N₂), emphasizing how fertilizer-driven shifts in N pools interact with soil to regulate pathway dominance and the N₂O:N₂ product ratio. We then place these mechanisms in the context of 21st-century fertilizer-use trajectories and rising atmospheric N₂O and summarize the stratospheric reactions that couple agricultural N₂O to ozone loss. Building on these mechanistic foundations, we evaluate direct nitrification-targeted mitigation by contrasting synthetic nitrification inhibitors with emerging biological nitrification-inhibition traits and their potential integration into the cropping system. Finally, we highlight how recent Conference of the Parties negotiations and the 2024 Global N₂O Assessment are elevating N₂O as a priority on the climate and ozone protection agenda. We argue that durable scaling of mitigation will require outcome-based quantification, including models and measurement strategies that deliver credible seasonal-to-annual estimates with transparent uncertainty, so that management changes translate into verifiable N₂O reductions and eligible incentives.