Tracing Microbial Production and Consumption Sources of N₂O in Rivers on the Qinghai-Tibet Plateau via Isotopocule and Functional Microbe Analyses

Abstract

Nitrous oxide (N 2 O), a potent greenhouse gas, is produced in rivers through a series of microbial metabolic pathways. However, the microbial source of N 2 O production and the degree of N 2 O reduction in river systems are not well understood and quantified. This work investigated isotopic compositions (δ 15 N-N 2 O and δ 18 O-N 2 O) and N 2 O site preference as well as N 2 O-related microbial features, thereby differentiating the importance of nitrification, denitrification, and N 2 O reduction in controlling N 2 O emissions from five rivers on the eastern Qinghai-Tibet Plateau (EQTP). The average N 2 O concentration in overlying water (15.2 nmol L −1 ) was close to that in porewater (17.5 nmol L −1 ), suggesting that both overlying water and sediment are potentially important sources of N 2 O. Canonical and nitrifier denitrification dominated riverine N 2 O production, with contribution being approximately 90%. Nitrification is a nonnegligible source of N 2 O production, and N 2 O concentration was positively correlated with nitrification genetic potential. The degree of N 2 O reduction ranged from 78.1 to 94.1% (averaging 90%), significantly exceeding the reported values (averaging 70%) in other freshwaters, which was attributed to the higher ratios of organic carbon to nitrogen and lower ratio of (nirS + nirK)/nosZ in EQTP rivers. This study indicates that a combination of isotopic and isotopocule values with functional microbe analysis is useful for quantifying the microbial sources of N 2 O in rivers, and the intense microbial reduction of N 2 O significantly accounts for the low N 2 O emissions observed in EQTP rivers, suggesting that both the production and consumption of N 2 O in rivers should be considered in the future.

Isotopic Constraints on Sources and Transformations of Nitrate in the Mount Everest Proglacial Water

“…Such change in water supply from the glacier melt could cause a distinct shift in physicochemical parameters in the glacial lake, such as increased water temperature, reduced flow velocity, longer water retention time, decreased water turbidity, and increased microbial diversity . All these factors combined with the previous accumulated N could shift the nitrogen cycling condition in waters, e.g., NO 3 – nitrification, which could turn proglacial lakes and glacier-fed rivers into sources of N 2 O emissions. ,, Studies in some glacial lakes and glacier-fed rivers have indicated the occurrence of nitrification process in NO 3 – facilitated by bacterial activities. ,, Recent research has also indicated that nitrification is a non-negligible source of riverine N 2 O production on the Tibetan Plateau . Therefore, the cycling of NO 3 – in fast-changing proglacial waters is expected to play a more important but currently undetermined role in NO 3 – cycling over the Tibetan Plateau and other high-mountain regions.…”

“…Glacial lakes and glacier-fed rivers in proglacial environments serve as major recipients and distributors of glacier-derived NO 3 – and nitrification-related microorganisms. ,, The highly hydrological connectivity and unique topography of high-mountain glacier regions facilitate the rapid transport of glacier-derived NO 3 – . Previous studies have demonstrated that the nitrification process is a non-negligible source of N 2 O production in high-mountain river systems and should be considered in the global nitrogen budget. , Study on a high-mountain-glacierized basin illustrated the contribution of uncycled atmospheric NO 3 – and the increasing significance of microbially produced NO 3 – . The triple-oxygen isotopic values of NO 3 – in a subalpine headwater catchment also demonstrate the noteworthy contribution of atmospheric NO 3 – in the watershed and the substantial release of NO 3 – during snowmelt .…”

“…Nitrate (NO 3 – ) is a prominent form of nitrogen compound in natural waters and plays a crucial role in aquatic ecosystems. , The transformation of NO 3 – (e.g., nitrification and denitrification) could produce nitrous oxide (N 2 O), a potent greenhouse gas with a warming capacity 300 times higher than that of carbon dioxide. , Rivers are considered to be complex systems that facilitate NO 3 – cycling and serve as main conduits for NO 3 – transport. − Studies have demonstrated that river systems substantially contribute to N 2 O emissions. ,, Therefore, understanding the sources and transformations of NO 3 – in river systems has been drawing increasing attention. , In high-altitude mountain regions where rivers originate, glacier melting is one of the primary drivers for the increase in NO 3 – concentration and flux in river water. ,− Atmospheric NO 3 – can be deposited in snow via wet and dry deposition and preserved in glaciers, , and NO 3 – will be transported into aquatic ecosystems with meltwater runoff when the glaciers melt. ,,, Glacier melting leads to the exposure of previously ice-covered terminal areas, the expansion of glacial lakes, and alterations in water flow. These changes can modify the environment for nitrogen cycling and impact the NO 3 – supply, potentially affecting downstream ecosystems. − Therefore, studying the sources and transformations of NO 3 – in glacier-covered regions can provide scientific evidence for a better understanding of regional and even global nitrogen cycles under a warming climate.…”

Black Soldier Fly Larvae Mitigate Greenhouse Gas Emissions from Domestic Biodegradable Waste through Carbon-Nitrogen Redistribution and Microbial Reconstruction

“…3c). In addition, to further characterize the emission intensity of N 2 O, we calculated (nirK + nirS + nor)/nosZ (Chen et al 2023). The ratio of (nirK + nirS + nor)/nosZ continuously decreased in all samples with the progress of bioconversion (Fig.…”

“…Researchers found that the emission intensity of N 2 O was positively correlated with the functional gene abundance of nirK, nirS, and nosZ. (Chen et al 2023). In addition, methanosarcina, methanoculleus, and methanobacterium were the most abundant methanogenic genera in the aerobic composting of livestock manure (Sun et al 2022).…”