The Innovation Team of Non-point Source Pollution Control of the Institute of Agricultural Resources and Regional Planning (IARRP) of the Chinese Academy of Agricultural Sciences (CAAS) investigated the hydrological transport pathways of dissolved organic nitrogen (DON) and their seasonal changes in an agricultural watershed based on long-term hydrology and water quality monitoring in a typical subwatershed of Erhai Lake basins. The research results were published in the international academic journal "Journal of Hydrology".

Nitrogen in rivers is mainly composed of inorganic nitrogen and organic nitrogen. Dissolved inorganic nitrogen (DIN) has been emphasized for its impact on eutrophication in surface waters and targeted as the main N forms that be controlled in the watershed. However, recent studies have shown that DON  can also stimulate the growth of phytoplankton, producing nearly 10 times more chlorophyll-a than DIN. Previous studies have underestimated the role of DON in promoting eutrophication. Meanwhile, DON can react with chlorine, a commonly used disinfectant, to generate a large number of carcinogenic nitrogenous disinfection by-products (N-DBPs) in the process of drinking water treatment. At present, most water environment management and restoration measures mainly target DIN, resulting in the detrimental effects of DON concentration on drinking water safety being ignored. Therefore, it is of great significance to understand the temporal dynamic characteristics and transport pathways of DON concentrations and loads in rivers to control water eutrophication and protect drinking water.

Based on the team's long-term water quality and quantity monitoring data in the typical sub-watershed of Erhai Lake basin, this study analyzed the temporal trend of DON concentration and load in the watershed, and attempted to quantify the contribution of surface runoff and base flow to DON loads in the watershed for the first time. Using the load estimation model (LOADEST) and non-parametric change point analysis (nCPA), the changes of DON load of different seasons in the Fengyu River Basin in the region from 2011 to 2016 were comprehensively studied.

The results showed that the DON load in the Fengyu River Watershed showed interannual and seasonal variations, and the rainy season was the hotspot of DON loss, accounting for 72.5% of the annual load. At the same time, the base flow is the main pathway of DON transport, accounting for 74.4% of the total DON load on average, and reaching 83.9% of the total DON load in the dry season. In addition, the streamflow threshold of n DON concentration changes ranges from 2.03 m3/s to 2.15 m3/s. When the streamflow is lower than 2.15 m3/s, the concentration of DON increases rapidly, which poses a threat to drinking water safety. This study will provide a scientific basis for DON control in the watershed and the protection of drinking water sources.

Fig. 1 The daily DON load of the Fengyu River Watershed from 2011 to 2016 based on the LOADEST model; A represents the daily load of streamflow and base flow; B and C represent the average daily load of streamflow and base flow in different months

Xu Qiyu, a doctoral student at the IARRP, is the first author of the paper, and researchers Du Xinzhong and Zhai Limei are co-corresponding authors. This research has received funding from the National Natural Science Foundation of China (Grant Nos. 42107076; U20A20114) and the Fundamental Research Funds for Central Non-profit Scientific Institution (No. 1610132022008).

Citation format and the link:

Xu, Q., Zhai, L., Liu, X., Du, X. and Liu, H., 2023. Hydrological transport pathways of dissolved organic nitrogen and their seasonal changes in an agricultural watershed. Journal of Hydrology, 617, 129054 .

Paper link: https://www.sciencedirect.com/science/article/pii/S0022169422016249