Sources of nitrate‑nitrogen in urban runoff over and during rainfall events with different grades

Nitrogen discharged from urban areas greatly deteriorates the water quality of downstream surface water. In this study, sub-hourly high-frequency samples of urban runoff during six rainfall events were collected at the outfall of the stormwater network in an urban watershed in Beijing to explore nitrate export and transportation. Isotopic values of local road dust, soil, and network sediment were measured and used as the sources of nitrate to better elucidate the sources of the urban watershed. The contributions of various sources over and during three rainfall events with different grades were quantified and compared. The results showed that the contribution of sources changed dramatically over and during rainfall events. Along with the increase in the total rainfall amount and the going on of rainfall events, the wash-off effect in the atmosphere and on land surfaces played a more important role in nitrate output. Atmospheric deposition was the dominant contributor of nitrate in heavy and storm events (mean 59.3% and 64.8%, respectively). Network sediment contributed large proportions of nitrate in moderate and heavy events (mean 35.6% and 15.9%, respectively). The contribution of soil increased substantially in the storm event (mean 26.1%). Road dust and network sediment contributed greatly in the early stage of the heavy event. The contribution of fertilizer in heavy events was mainly because of the wash-off of road dust. The changing pattern of sources from atmospheric deposition to inorganic N fertilizer existed during the process of the storm event. The contribution of NO₃^- fertilizer from soil surfaces increased substantially in the later stage of the storm event. These results provide valuable references for urban nutrient management and mitigation measures implementation.