Combined effects of damming and drought on nitrogen dynamics in an ephemeral river of North China

Abstract

Damming and drought have dramatically altered water cycle dynamics in rivers, which further affects nitrogen biogeochemical cycles; however, their combined effects have rarely been studied, especially for ephemeral rivers with variable water flows. Taking the Dagu River of North China as the study area, we examined the spatiotemporal variability of nitrogen in river water and its relation to damming and drought. The total nitrogen (TN), dissolved inorganic nitrogen (DIN) and NO3‾ concentrations varied significantly in space and time, but the NH4+ and NO2‾ concentrations changed little. Spatially, compared with the upper reaches, the TN, DIN and NO3‾ concentrations in the lower reaches decreased by 71%, 81% and 85%, respectively, which was concurrent with the isotopic enrichment of nitrate. Temporally, their concentrations in dry seasons were more than twice of wet seasons, and increased from 2018 (normal year) to 2019 (dry year). The cascade dams may promote the NO3‾ removal towards the lower reaches through assimilation, which combined with nitrification and strong evaporation to enhance δ18O–NO3‾ enrichment and disproportionate δ15N–NO3‾/δ18O–NO3‾ ratio. Further, the damming effects can be aggravated by droughts because of no flow, extended nitrogen residence time and increased groundwater discharge with higher nitrogen contents, which jointly elevated the nitrogen level. Hence, the combined effects of damming and drought should be fully considered for management of water environment and resources in regions with ephemeral rivers.