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 semi-in situ 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.