Spatiotemporal variations and dominated environmental parameters of nitrous oxide (NO) concentrations from cascade reservoirs in southwest China.

Anthropogenic activity has caused rivers and reservoirs to become sources of nitrous oxide (NO), which is thought to play an important role in global climate change. There are thermal and DO stratification in deep-water reservoirs with long hydraulic retention time, which change NO production mechanism compared with shallow-water reservoirs. To promote our understanding of the relationship of NO production in reservoirs at different depths, spatiotemporal variations in water environmental factors and NO from cascade reservoirs of Chaishitan (CST), Longtan (LT), Yantan (YT) and Dahua (DH) reservoirs in the Zhujiang River were detected, and the LT and YT reservoirs were compared as representatives of deep-water and shallow-water reservoirs in April and July 2019. The average NO concentrations in the LT and YT reservoirs were 22.82 ± 2.21 and 21.55 ± 1.65 nmol L, respectively. During spring and summer, the WT (water temperature) and DO (dissolved oxygen) concentrations in the YT reservoir were well mixed. In contrast, the LT reservoir, as a deep-water reservoir, had thermal and DO stratifications in both the shallow and middle water, especially in the summer when the solar radiation intensity was high. During summer stratification, the DO concentration in the LT reservoir showed obvious spatial variation, ranging from 1.23 to 9.84 mg L, while the DO concentration in the YT reservoir showed very little variation, ranging from 6.45 to 7.09 mg L. Structural equation modeling results showed that NH was the main determinant of the NO concentration in the YT reservoir, and DO was the most influential factor of the NO concentration in the LT reservoir. These results suggest significant variations in the factors influencing NO concentration among reservoirs. Additionally, the mechanisms of NO production differ between deep-water and shallow-water reservoirs. This study highlights the spatio-temporal variations and influential factors contributing to NO concentration. Furthermore, it discusses the production mechanisms of NO in different types of reservoirs. These findings contribute to our understanding of NO distribution in hydropower systems and provide valuable data for the management of hydropower facilities and research on greenhouse gas emissions.