DOI: 10.6084/m9.figshare.25476508
title: Ammonia-derived pathways as a major contributor to N2O production from global upland soils

description: The concentration of atmospheric N<sub>2</sub>O 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 N<sub>2</sub>O 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 (NH<sub>4</sub><sup>+</sup>) 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 N<sub>2</sub>O production in agricultural soils have not yet been well investigated.Here, we report that NH<sub>4</sub><sup>+</sup>-derived pathways (ammonia oxidation as the first step, with ambient NH<sub>4</sub><sup>+</sup> as direct substrate), rather than NO<sub>3</sub><sup>-</sup>-derived pathway (heterotrophic denitrification, with ambient NO<sub>3</sub><sup>-</sup> as the direct substrate), are estimated to be the dominant microbial N<sub>2</sub>O 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 N<sub>2</sub>O production along the soil profiles (0–200 cm depth), of which 95 ± 3 % may attributed to NH<sub>4</sub><sup>+</sup>-derived pathways. These global estimations are based on a comprehensive array of methods using 0.01% C<sub>2</sub>H<sub>2</sub>-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 <sup>15</sup>N-<sup>18</sup>O slurry experiments distinguished between various NH<sub>4</sub><sup>+</sup>-derived N<sub>2</sub>O production pathways, revealing that nitrifier denitrification (ND), one of several NH<sub>4</sub><sup>+</sup>-derived pathways, contributes to most of the N<sub>2</sub>O production. Site-scaled agricultural soil profile (0–100 cm depth) studies, employing <sup>15</sup>N <i>semi-in situ</i> soil core incubation, corroborated the 0.01% C<sub>2</sub>H<sub>2</sub>-inhibitor results. Metagenomic binning and transcriptomic analyses identified higher abundances of N<sub>2</sub>O-producing genes in nitrifying bacteria and a lack of N<sub>2</sub>O-reducing genes. These results reveal the mechanism of microbial N<sub>2</sub>O production and emphasize the leading role of the NH<sub>4</sub><sup>+</sup>-derived pathway in N<sub>2</sub>O production. Our results indicate that ammonia loading plays a critical role in controlling N<sub>2</sub>O production from agricultural upland soils. Previously reported global N<sub>2</sub>O emission may have been significantly underestimated agricultural N<sub>2</sub>O release because the potential contribution of NH<sub>4</sub><sup>+</sup>-derived N<sub>2</sub>O production has been overlooked. Global models of agricultural soil ecosystems need to better represent NH<sub>4</sub><sup>+</sup>-derived pathways for accurately estimating and predicting agricultural N<sub>2</sub>O emissions. Due to their superior efficiency, applications of NH<sub>4</sub><sup>+</sup> or urea-based fertilizers and nitrification inhibitors in agro-ecosystems should be considered as an effective option for mitigating global N<sub>2</sub>O emission and hence climate change. With rapidly increasing NH<sub>4</sub><sup>+</sup> or urea production and application in agro-ecosystems, it is imperative that any novel applications of NH<sub>4</sub><sup>+</sup> such as green-ammonia should be integrated into strategies for mitigating the risks of NH<sub>3</sub> release and N<sub>2</sub>O emissions.