653 / 2024-09-18 20:14:57

Biogeochemical drivers of N2O dynamics along a river-estuary continuum: the critical role of N2O reduction in lowering emissions

greenhouse gas,nitrogen cycle,aquatic system,salinity,biogeochemistry

Session 3 - The nitrogen cycle towards a sustainable ocean: from microbes to global biogeochemistry

Despite extensive research on nitrous oxide (N₂O) emissions in river and estuary ecosystems, the dynamics and drivers of N₂O along the river-estuary continuum remain underexplored, even though these regions are closely linked in terms of N₂O emissions. This study investigated the spatiotemporal variations of N₂O and its key drivers along the Pearl River-estuary continuum over three seasons (spring, summer, and winter) using isotopocule measurements and microbial analyses. N₂O saturation (89.9%-1238.6%) exhibited a hump-shaped pattern along the continuum, closely aligning with the distribution of the nighttime light index, indicating significant anthropogenic influences on N₂O dynamics. Nitrate and dissolved oxygen consistently affected N₂O saturation levels from the river to the estuary, while salinity exerted opposing effects—enhancing N₂O saturation in the river but reducing it in the estuary. In the river, the nitrogen-to-phosphorus ratio, associated with higher phosphorus levels and potentially stronger exogenous N₂O inputs, dominated N₂O dynamics. In contrast, the carbon-to-nitrogen ratio was more influential in the estuary. Water flow and salinity conditions also significantly shaped N₂O dynamics, resulting in divergent trends of N₂O saturation and flux along the continuum. Denitrification was the primary pathway of N₂O production (93.9% - 98.6%) throughout the continuum, with nitrification contributing more in the estuary. N₂O reduction consistently decreased N₂O saturation levels, and the extent of N₂O reduction increased with higher salinity levels from 1‰ to 35‰, where greater ratios of nosZII:nosZI were observed. This study reveals shifting N₂O transformation pathways and drivers along the river-estuary continuum, which have significant implications for N₂O estimates and the development of mitigation strategies in aquatic systems.