Response of riverine N2O to anthropogenic intensity and seasonal hydrological drivers in a high-altitude basin.

Abstract

High-altitude alpine rivers represent a critical source of uncertainty in global nitrous oxide (N₂O) budgets; however, a lack of systematic field observations limits the understanding of greenhouse gas feedback mechanisms in these fragile ecosystems. This study investigated the spatiotemporal patterns and key driving mechanisms of riverine N₂O emissions in the upper Yellow River through systematic monitoring across diverse landscape units (permafrost, wetland, seasonally frozen ground, reservoir, and urban) during high-flow and low-flow seasons. The results indicate that the upper Yellow River acts as a net source of atmospheric N₂O, with an annual emission of 0.085 Gg N₂O-N yr^-1. Urban and reservoir reaches contributed 32.9% and 23.9% of the total emissions, respectively. Dissolved N₂O concentrations exhibited significant seasonal heterogeneity (Low-flow mean: 15.25 ± 4.63 nmol L^-1; High-flow mean: 8.90 ± 2.64 nmol L^-1). Notably, ebullition, a largely overlooked pathway, accounted for 34.2% and 22.6% of total fluxes in high-flow and low-flow seasons, respectively. Mechanistic analysis suggests a seasonal shift in riverine ecosystem function: transitioning from physical transport dominance in the high-flow season to internal biogeochemical processing dominance in the low-flow season. High-flow dynamics were primarily associated with physical hydrology, revealing a distinct non-linear response to population density that highlights the limits of dilution capacity. Conversely, the low-flow season was substrate-limited, exhibiting a direct linkage between anthropogenic nitrogen loading and N₂O levels. These findings improve the understanding of riverine N₂O dynamics and provide a scientific basis for more accurate regional emission inventories and targeted management in high-altitude basins.