Distinctive Patterns and Controls of Nitrous Oxide Concentrations and Fluxes from Urban Inland Waters

Abstract

Inland waters are significant sources of nitrous oxide (N 2 O), a powerful greenhouse gas. However, considerable uncertainty exists in the estimates of N 2 O efflux from global inland waters due to a lack of direct measurements in urban inland waters, which are generally characterized by high carbon and nitrogen concentrations and low carbon-to-nitrogen ratios. Herein, we present direct measurements of N 2 O concentrations and fluxes in lakes and rivers of Beijing, China, during 2018−2020. N 2 O concentrations and fluxes in the waters of Beijing exceeded previous estimates of global rivers due to the high carbon and nutrient concentrations and high aquatic productivity. In contrast, the N 2 O emission factor (N 2 O-N/DIN, median 0.0005) was lower than global medians and the N 2 O yield (ΔN 2 O/(ΔN 2 O + ΔN 2 ), average 1.6%) was higher than those typically observed in rivers and streams. The positive relationship between N 2 O emissions and denitrifying bacteria as well as the Michaelis−Menten relationship between N 2 O emissions and NO 3 − −N concentrations suggested that bacteria control the net production of N 2 O in waters of Beijing with N saturation, leading to a low N 2 O emission factor. However, low carbon-to-nitrogen ratios are beneficial for N 2 O accumulation during denitrification, resulting in high N 2 O yields. This study demonstrates the significant N 2 O emissions and their distinctive patterns and controls in urban inland waters and suggests that N 2 O emission estimates based on nitrogen loads and simple emission factor values are not appropriate for urban inland water systems. KEYWORDS: nitrous oxide (N 2 O), greenhouse gas, N 2 O emission factor, N 2 O yield, urban inland waters, wastewater treatment plants

Distinctive Microbial Processes and Controlling Factors Related to Indirect N2O Emission from Agricultural and Urban Rivers in Taihu Watershed

“…N 2 O is also the primary cause of stratospheric ozone damage. Its atmospheric concentration has increased from 270 ppb before the industrial revolution to 329 ppb in 2019. − Anthropogenic sources contribute an estimated 38.5% (2.7–11.1 Tg N 2 O–N a –1 ) of global N 2 O emissions, 10–17% of which come from inland rivers. , However, compared to the well-documented N 2 O emissions from terrestrial ecosystems, N 2 O emissions from inland rivers have received little attention . Human activities, especially agricultural utilization of synthetic fertilizers and discharge of domestic sewage, have the potential to indirectly enhance N 2 O fluxes from agricultural and urban rivers .…”

“…Hence, the Intergovernmental Panel on Climate Change (IPCC) used dissolved nitrate and the indirect emission factor (EF 5r ) to estimate the dissolved N 2 O concentration in surface water (i.e., N 2 O–N = NO 3 – –N × EF 5r ) for riverine N 2 O emission estimation. , This IPCC method is convenient for regional and global N 2 O emission estimation as dissolved NO 3 – –N concentration data are more commonly available than N 2 O data. However, the default EF 5r value (0.0026) given by IPCC in 2019 may skew the estimations of riverine N 2 O fluxes with different land use types. , In fact, the EF 5r of freshwater ecosystems remains unspecified, especially for agricultural and urban rivers. ,, The IPCC have refined EF 5r from 0.0075 (1998) to 0.0025 (2006) and to 0.0026 (2019) . Hence, many studies have estimated the EF 5r (EF 5r‑e ) with different landscapes using field-measured dissolved N 2 O and NO 3 – –N concentrations and provide EF 5r‑e for the refinement of the default EF 5r value or the establishment of a regionally-specific EF 5r . ,, Previous studies that investigated the IPCC’s predictions concluded that they only accounted for about 76% of the measured N 2 O emissions in urban rivers .…”

“…Nitrate is the substrate and organic carbon serves as an electron donor for denitrification. Hence, high organic carbon-to-nitrate ratios can ensure complete denitrification and consequently reduce N 2 O production . In addition, trophic state, suspended solids, algae accumulation and decomposition, temperature, dissolved oxygen (DO) concentration, pH, and many other environmental factors have been reported to significantly affect N 2 O emissions in inland rivers. ,,, Together, the influences of these environmental factors on N 2 O production are very complex and interconnected.…”

Stream Transport and Substrate Controls on Nitrous Oxide Yields From Hyporheic Zone Denitrification

“…Comparisons of alternative kinetic rate models (Supporting Information S1) suggest that this relationship reflects a common maximum denitrification rate across sites (μ max term in Equation 3) in contrast to first-order kinetic models used in a number of studies (Gu et al, 2007;Quick et al, 2016;Zarnetske et al, 2011). This approach to maximum denitrification rates is supported by a recent multi-site study of stream denitrification across Beijing, China (Wang et al, 2021). Despite the link between nitrate concentrations and denitrification efficiencies, nitrate was not statistically correlated with N 2 O yields in LINX II experiments.…”

“…where K in is the inhibition constant specified for individual reactions. This modeling framework is supported by a recent large-scale survey of denitrification dynamics across Beijing, China (Wang et al, 2021).…”