Ammonium-derived nitrous oxide is a global source in streams

Ammonium-derived nitrous oxide is a global source in streams

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The Paris Agreement’s target of limiting global warming to 1.5 °C is projected to be breached in the near future. Extreme regional heatwaves also cause immediate and marked temperature spikes, sometimes exceeding 10 °C above normal levels. Over the past four decades, global emissions of nitrous oxide (N2O), a long-lived and powerful greenhouse gas depleting stratospheric ozone, have increased at a rate of 2% per decade, making N2O the most rapidly increasing greenhouse gas.

N2O emissions from rivers, estuaries and continental shelves have been the subject of debate for many years. The 5th IPCC Assessment Report proposed that, together, rivers, estuaries and coastal zones emit 0.6 Tg N2O-N yr-1 (based on IPCC’s 2006 guidelines). This corresponds to approximately 3% of all N2O emissions and approximately one-third of the IPCC’s previous estimate of 1.7 Tg N yr-1 in the 4th Assessment Report for the same systems. Several studies have highlighted that emissions from rivers might be underestimated or significantly overestimated in IPCC assessments. In 2020, Prof. Tian Hanqin’s group developed a riverine N2O model within the framework of a dynamic land ecosystem model and reported a fourfold increase in global riverine N2O emissions from 70.4±15.4 Gg N yr-1 in 1900 to 291.3±58.6 Gg N yr-1 in 2016. The small rivers in headwater zones (lower than fourth-order streams) contributed up to 85% of the global riverine N2O emissions, most of which are produced in hyporheic zones (beneath streambeds where stream waters exchange with adjacent sediments). Currently, the microbial mechanisms underlying N2O production in hyporheic exchange zones are largely unknown. These processes may significantly influence global N2O budgets and be potentially severely underestimated.

Recently, Wang et al. (2024) reported that ammonia-derived pathways (nitrifier nitrification, nitrifier denitrification, and nitrification-coupled denitrification), rather than nitrate-derived pathways (heterotrophic denitrification), are the dominant hyporheic N2O sources (69.6 ± 2.1%) in agricultural streams worldwide, using complementary methods involving 15N-18O dual-isotope tracing, 15N tracing of semi-in situ sediment cores, quantitative reverse transcription PCR (RT‒qPCR), and metagenomic assembly and binning analysis for a wide range of sample types and temperature zones globally. The potential N2O metabolic pathways of metagenome-assembled genomes (MAGs) provides evidence that nitrifying bacteria contain greater abundances of N2O production-related genes than denitrifying bacteria. This study highlights the importance of mitigating agriculturally derived ammonium in low-order agricultural streams in controlling N2O emissions. Global models of riverine ecosystems need to better represent ammonia-derived pathways for accurately estimating and predicting riverine N2O emissions.

Microbial processes (a) and mechanism (b) of N2O production with two major N-substrates in global streams. Ammonia-derived N2O production pathways with ambient NH4+ as substrate, included three nitrifier nitrification (NN, ①, Orange arrow), nitrifier denitrification (ND, ②, Green arrow), and nitrification-coupled denitrification (NCD, ③, Purple arrow). Nitrate-derived N2O production pathway with ambient NO3- as substrate, represents the only one pathway - lumped heterotrophic denitrification (HD, ④, Blue arrow). Enzymes conducted the important transformations including ammonia monooxygenase (AMO), hydroxylamine oxidoreductase (HAO), nitrite oxidoreductase (NXR), nitrite reductase (NIR), nitrate reductase (NAR/P), nitric oxide reductase (NOR), and nitrous-oxide reductase (NOS). The N2O reductase (encoded by nosZ gene) is the only known biological N2O sink.

This study reveals that ammonia-derived pathways mainly control agricultural stream N2O production, which is in contrast to the current opinion that nitrate-derived pathways is the main N2O production pathway. This result also provides substantial evidence for the need to reassess N2O emissions in global models because in current process-based N2O models, the N2O/N2 ratio is used to represent the N2O production rate during denitrification. Model parameters associated with microbially mediated N2O production are poorly represented. Although the IPCC methods for estimating and reporting greenhouse gas emissions to the United Nations include N2O from nitrification, previous studies have been focused on quantifying denitrification and N2O emissions resulting from nitrate. The current models may severely underestimate N2O budgets in the IPCC assessments because the contribution of ammonia-derived N2O production has hardly directly been considered, especially in streams adjacent to agricultural area which contributed 70% of global anthropogenic N2O emissions that have increased by 30% in the past four decades.

These results also show the critical role of global fertilizer management in mitigating N2O emission of agricultural system. In order to increase grain production, the application of ammonia-based fertilizer continues to increase and has reached 90% of total nitrogen fertilizer application to agricultural soils since the 2010s. Our results emphasize that the critical issue in N2O mitigation is to reconsider using ammonia-based fertilizers and to improve management for ammonium nitrogen. The excessive use of ammonia fertilizer, especially in Asia, would deteriorate the local aquatic environment and jeopiodize drinking water supply, but also lead to increasing global atmospheric N2O in agricultural system, thus destroying the ozone layer and promoting global warming. The 2021 United Nations Food System summit also called for the establishment of a sustainable global food system which emphasizes the balance between food and ecosystem security. Therefore, mechanistic understanding of the pathways of N2O emission in agricultural system will facilitate better prediction of the impact of N fertilization on climate change and will help improve the management of N fertilization for sustainable global food systems.

I am in the research group of the Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, mainly focusing on biogeochemical mechanisms and processes. This study is our first time considering and organizing from the perspective of global change modeling science. Here, I would like to express my sincere gratitude to Professor Tian Hanqin of Boston College. Professor Tian is the important guide and mentor of my study. Sincerely thank you.

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