Streams and rivers are important sources of nitrous oxide (N2O), a powerful greenhouse gas. Estimating global riverine N2O emissions is critical for the assessment of anthropogenic N2O emission inventories. The indirect N2O emission factor (EF5r) model, one of the bottom‐up approaches, adopts a fixed EF5r value to estimate riverine N2O emissions based on IPCC methodology. However, the estimates have considerable uncertainty due to the large spatiotemporal variations in EF5r values. Factors regulating EF5r 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 meta‐analysis over six continents, to explore the spatiotemporal variations and controls on EF5r values. Our results show that the EF5r values in China and other regions with high N loads are lower than those for regions with lower N loads. Although the global mean EF5r value is comparable to the IPCC default value, the global EF5r values are highly skewed with large variations, indicating that adopting region‐specific EF5r values rather than revising the fixed default value is more appropriate for the estimation of regional and global riverine N2O emissions. The ratio of dissolved organic carbon to nitrate (DOC/NO3−) and NO3− concentration are identified as the dominant predictors of region‐specific EF5r values at both regional and global scales because stoichiometry and nutrients strictly regulate denitrification and N2O production efficiency in rivers. A multiple linear regression model using DOC/NO3− and NO3− is proposed to predict region‐specific EF5r 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 EF5r values at the global scale and provides a pathway to estimate global riverine N2O emissions more accurately based on IPCC methodology.

Indirect nitrous oxide emission factors of fluvial networks can be predicted by dissolved organic carbon and nitrate from local to global scales

Shifts in the high-resolution spatial distribution of dissolved N2O and the underlying microbial community and processes in the Pearl River Estuary

The hyporheic zone (HZ), where surface water and groundwater interact in sediments beneath streams, presents unique conditions for nutrient dynamics, such as carbon and nitrogen (N) cycling. Organic carbon (OC) in aquatic systems has two distinct forms: dissolved organic carbon (DOC) and particulate organic carbon (POC). OC affects N reduction and controls the occurrence of denitrification, a primary process by which nitrate (NO3-) is removed as gas (N2 or N2O) to the atmosphere. When POC is the predominant form of OC in streams, how POC influences NO3- reduction within a HZ remains unclear. We established a new reactive transport model incorporating POC transport and filtration processes to assess how POC deposition affects NO3- removal. Sediment permeability decreases as POC is filtrated in the streambed. Nitrate influx is reduced due to POC-induced sediment clogging. The deposited POC in the shallow streambed serves as an OC source and supports higher biogeochemical reaction kinetics because OC is an important substrate promoting microbial activities. The filtrated POC pool enhances denitrification, thus causing a higher NO3- removal efficiency. In contrast, the POC pool is typically assumed to be distributed homogeneously in sediments, potentially causing a significant underestimation of stream-borne NO3- removal. A lower hydraulic conductivity, a smaller POC grain size, and a larger POC filtration efficiency and in-stream POC concentration allow for extensive POC deposition within the shallow streambed that favors nitrate removal. This study provides a better understanding of N processing and an accurate estimation of NO3- removal potential in HZs. This article is protected by copyright. All rights reserved. 

Effect of Particulate Organic Carbon Deposition on Nitrate Reduction in the Hyporheic Zone

Riverine ecosystems are a significant source of nitrous oxide (N2O) worldwide, but how they respond to human and natural changes remains unknown. In this study, we developed a compound model chain that integrates mechanism-based modeling and machine learning to understand N2O transfer patterns within land, rivers, and the atmosphere. The findings reveal a decrease in N2O emissions in the Yangtze River basin from 4.7 Gg yr−1 in 2000 to 2.8 Gg yr−1 in 2019, with riverine emissions accounting for 0.28% of anthropogenic nitrogen discharges from land. This unexpected reduction is primarily attributed to improved water quality from human-driven nitrogen control, while natural factors contributed to a 0.23 Gg yr−1 increase. Notably, urban rivers exhibited a more rapid N2O efflux (FN2O), with upstream levels nearly 3.1 times higher than rural areas. We also observed nonlinear increases in FN2O with nitrogen discharge intensity, with urban areas showing a gradual and broader range of increase compared to rural areas, which exhibited a sharper but narrower increase. These nonlinearities imply that nitrogen control measures in urban areas lead to stable reductions in N2O emissions, while rural areas require innovative nitrogen source management solutions for greater benefits. Our assessment offers fresh insights into interpreting riverine N2O emissions and the potential for driving regionally differentiated emission reductions. 

Estimating Yangtze River basin's riverine N2O emissions through hybrid modeling of land-river-atmosphere nitrogen flows.

Rivers play a significant role in the global nitrous oxide (N 

Long-Neglected Contribution of Nitrification to N2O Emissions in the Yellow River.

Rivers are important sites for nitrogen (N) cycling processes. N removal is thought to occur predominantly in the benthic zone and the role of the water column is frequently neglected. Here, we find that the water‐air and sediment‐water fluxes of N2 and N2O decreased, while the water column production increased with stream order through 4‐year observations across six river networks in China. The water column contribution increased with stream order and accounted for more than 50% in rivers above fifth order. The increase in the contact area of suspended sediment (SPS)‐water caused by higher SPS concentrations and river depths resulted in the shift as river size increased. N removal and N2O emissions would be underestimated approximately by 50% if neglecting water column processes for the six river networks. This study highlights the necessity and provides a pathway to incorporate water column processes into models to balance the global N budget.

The Dominant Role of the Water Column in Nitrogen Removal and N2O Emissions in Large Rivers

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The Dominant Role of the Water Column in Nitrogen Removal and N2O Emissions in Large Rivers