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Nitrogen is a requisite and highly demanded element for living organisms on Earth. However, increasing human activities have greatly altered the global nitrogen cycle, especially in rivers and streams, resulting in eutrophication, formation of hypoxic zones, and increased production of N2O, a powerful greenhouse gas. This review focuses on three aspects of the nitrogen cycle in streams and rivers. We firstly introduce the distributions and concentrations of nitrogen compounds in streams and rivers as well as the techniques for tracing the sources of nitrogen pollution. Secondly, the overall picture of nitrogen transformations in rivers and streams conducted by organisms is described, especially focusing on the roles of suspended particle-water surfaces in overlying water, sediment-water interfaces, and riparian zones in the nitrogen cycle of streams and rivers. The coupling of nitrogen and other element (C, S, and Fe) cycles in streams and rivers is also briefly covered. Finally, we analyze the nitrogen budget of river systems as well as nitrogen loss as N2O and N2 through the fluvial network and give a summary of the effects and consequences of human activities and climate change on the riverine nitrogen cycle. In addition, future directions for the research on the nitrogen cycle in river systems are outlined.

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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

Ammonia-oxidizing bacteria (AOB) and archaea (AOA) as well as comammox catalyze ammonia oxidation. The distribution and biogeography of these ammonia oxidizers might be distinctive in high-elevation rivers, which are generally characterized by low temperature and low ammonium concentration but strong solar radiation; however, these characteristics have rarely been documented. This study explored the abundance, community, and activity of ammonia oxidizers in the overlying water of five rivers in the Qinghai-Tibet Plateau (QTP). Potential nitrification rates in these rivers ranged from 5.4 to 38.4 nmol N liter−1 h−1, and they were significantly correlated with ammonium concentration rather than temperature. Comammox were found in 25 of the total 28 samples, and they outnumbered AOA in three samples. Contrary to most studied low-elevation rivers, average AOB amoA gene abundance was significantly higher than that of AOA, and AOB/AOA ratios increased with decreasing water temperature. The Simpson index of the AOA community increased with elevation (P < 0.05), and AOA and AOB communities exhibited high dissimilarities with low-elevation rivers. Cold-adapted (Nitrosospira amoA cluster 1, 33.6%) and oligotrophic (Nitrosomonas amoA cluster 6a, 31.7%) groups accounted for large proportions in the AOB community. Suspended sediment concentration exerted significant effects on ammonia oxidizer abundance (r > 0.56), and owing to their elevational variations in source and concentration, suspended sediments facilitated distance-decay patterns for AOA and AOB community similarities. This study demonstrates distinctive biogeography and distribution patterns for ammonia oxidizers in high-elevation rivers of the QTP. Extensive research should be conducted to explore the role of these microbes in the nitrogen cycle of this zone. IMPORTANCE Ammonia-oxidizing archaea (AOA) and bacteria (AOB) as well as comammox contribute to ammonia oxidation, which plays significant roles in riverine nitrogen cycle and N2O production. Source regions of numerous rivers in the world lie in high-elevation zones, but the abundance, community, and activity of ammonia oxidizers in rivers in high-elevation regions have rarely been investigated. This study revealed distinctive distribution patterns and community structures for ammonia oxidizers in five high-elevation rivers of the Qinghai-Tibet Plateau, and the individual and combined effects of low temperature, low nutrients, and strong solar radiation on ammonia oxidizers were elucidated. The findings of this study are helpful to broaden our knowledge on the biogeography and distribution pattern of ammonia oxidizers in river systems. Moreover, this study provides some implications to predict the performance of ammonia oxidizers in high-elevation rivers and its variations under global climate warming.

Streams and rivers are important sources of nitrous oxide (N 2 O), a powerful greenhouse gas. Estimating global riverine N 2 O emissions is critical for the assessment of anthropogenic N 2 O emission inventories. The indirect N 2 O emission factor (EF 5r ) model, one of the bottom-up approaches, adopts a fixed EF 5r value to estimate riverine N 2 O emissions based on IPCC methodology. However, the estimates have considerable uncertainty due to the large spatiotemporal variations in EF 5r values. Factors regulating EF 5r are poorly understood at the global scale. Here, we combine 4-year in situ observations across rivers of different land use types in China, with a global metaanalysis over six continents, to explore the spatiotemporal variations and controls on EF 5r values. Our results show that the EF 5r values in China and other regions with high N loads are lower than those for regions with lower N loads. Although the global mean EF 5r value is comparable to the IPCC default value, the global EF 5r values are highly skewed with large variations, indicating that adopting region-specific EF 5r values rather than revising the fixed default value is more appropriate for the estimation of regional and global riverine N 2 O emissions. The ratio of dissolved organic carbon to nitrate (DOC/NO 3 − ) and NO 3 − concentration are identified as the dominant predictors of region-specific EF 5r values at both regional and global scales because stoichiometry and nutrients strictly regulate denitrification and N 2 O production efficiency in rivers. A multiple linear regression model using DOC/NO 3 − and NO 3 − is proposed to predict region-specific EF 5r values. The good fit of the model associated with easily obtained water quality variables allows its widespread application. This study fills a key knowledge gap in predicting region-specific EF 5r values at the global scale and provides a pathway to estimate global riverine N 2 O emissions more accurately based on IPCC methodology. K E Y W O R D S dissolved organic carbon, EF 5r , greenhouse gas, nitrogen cycle, nitrous oxide, river, urban rivers, water quality | 7271 WANG et al.

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