Ammonia-derived pathways as a major contributor to N2O production from global upland soils

The concentration of atmospheric N₂O has increased by more than 22% since 1750, with the fastest growth observed in the past five decades. The latest comprehensive assessment indicates that this rapid growth attributes to human-induced emissions which have increased by 30%, whereby the agricultural sector contributes 70% of these anthropogenic N₂O emissions, particularly from fertilized soils. Ammonium or urea-based fertilization (can be rapidly converted into ammonium ions by urease in soils and is also considered as ammonium (NH₄⁺) fertilizer) in agricultural soils has increased rapidly and constituted about 90% of total nitrogen (N) fertilizer in the recent decade. However, the mechanisms and pathways contributing to N₂O production in agricultural soils have not yet been well investigated.Here, we report that NH₄⁺-derived pathways (ammonia oxidation as the first step, with ambient NH₄⁺ as direct substrate), rather than NO₃^--derived pathway (heterotrophic denitrification, with ambient NO₃^- as the direct substrate), are estimated to be the dominant microbial N₂O sources (88 ± 2 %) in agricultural upland soils around the world. The cultivated soil horizons (0–20 cm depth) is thought to contribute 54 ± 5 % of N₂O production along the soil profiles (0–200 cm depth), of which 95 ± 3 % may attributed to NH₄⁺-derived pathways. These global estimations are based on a comprehensive array of methods using 0.01% C₂H₂-inhibitor technologies, from of a wide range of soil types, and from multi-scale studies of soil horizons across a wide variety of environmental conditions (i.e. temperature, oxygen, and N fertilization). Isotopic ¹⁵N-¹⁸O slurry experiments distinguished between various NH₄⁺-derived N₂O production pathways, revealing that nitrifier denitrification (ND), one of several NH₄⁺-derived pathways, contributes to most of the N₂O production. Site-scaled agricultural soil profile (0–100 cm depth) studies, employing ¹⁵N <i>semi-in situ</i> soil core incubation, corroborated the 0.01% C₂H₂-inhibitor results. Metagenomic binning and transcriptomic analyses identified higher abundances of N₂O-producing genes in nitrifying bacteria and a lack of N₂O-reducing genes. These results reveal the mechanism of microbial N₂O production and emphasize the leading role of the NH₄⁺-derived pathway in N₂O production. Our results indicate that ammonia loading plays a critical role in controlling N₂O production from agricultural upland soils. Previously reported global N₂O emission may have been significantly underestimated agricultural N₂O release because the potential contribution of NH₄⁺-derived N₂O production has been overlooked. Global models of agricultural soil ecosystems need to better represent NH₄⁺-derived pathways for accurately estimating and predicting agricultural N₂O emissions. Due to their superior efficiency, applications of NH₄⁺ or urea-based fertilizers and nitrification inhibitors in agro-ecosystems should be considered as an effective option for mitigating global N₂O emission and hence climate change. With rapidly increasing NH₄⁺ or urea production and application in agro-ecosystems, it is imperative that any novel applications of NH₄⁺ such as green-ammonia should be integrated into strategies for mitigating the risks of NH₃ release and N₂O emissions.