78 / 2024-08-11 14:09:14

Spatiotemporal variations of dissolved nitrous oxide concentrations in urban landscape waters in subtropical region

Abstract: Urban landscape waters are susceptible to nitrogen (N) pollution, which may result in supersaturation of dissolved nitrous oxide (N₂O), making them a potential source of atmospheric N₂O. However, the spatiotemporal variations and underlying causes of dissolved N₂O concentrations in urban landscape waters remain unclear. Herein, the spatiotemporal variations of dissolved N₂O concentrations and its controlling factors in surface water of an interconnected river-lake system of urban landscape waters in a subtropical sub-watershed were studied. The average dissolved N₂O concentration was 0.81 μg N L^–1, with an annual average N₂O saturation of 303.51%, indicating the urban landscape waters could act as a source of potential N₂O emission to atmosphere. The observation between 7:00-11:00 am could represent the daily average dissolved N₂O concentration, regardless of season variations. Seasonal mean concentrations of dissolved N₂O, N, phosphorus (P), chlorophyll-a (Chl-a) and dissolved oxygen (DO) in the water were highest in winter and lowest in summer. The lake (TL) in the central area of the sub-watershed of landscape water showed significantly lower concentrations of dissolved N₂O, N and P, but higher water temperature, pH, concentrations of Chl-a and DO than its upstream (TUR) and downstream rivers (TDR). The dissolved N₂O concentrations were positively correlated with concentrations of N and P, but negatively correlated with pH and concentrations of Chl-a, and DO. Meanwhile, Chl-a concentrations were positively correlated with concentrations of N, P and DO throughout the sub-watershed of urban landscape waters. Our results indicated that there were intensified competitions for N between the N₂O producers and algae particularly in summer or in the TL area. It was suggested that the seasonal and spatial distribution characteristics of environmental and nutritional conditions and algal growth were important driving factors for the seasonal and spatial variations of dissolved N₂O concentrations. Therefore, the variations of dissolved N₂O concentrations were mainly attributed to the comprehensive cascade effects of dynamics in the local climate and concentrations of nutrients and algae. This study highlighted that the comprehensive cascade effects of nutrients levels and algal growth may significantly increase the complexity of spatiotemporal variations of dissolved N₂O concentrations in the urban landscape waters in subtropical regions. This research can be conducive to provide a theoretical basis for controlling N pollution and reducing the risk of potential N₂O emission in the eutrophic urban landscape waters.