Urban rivers are essential in retaining nutrients, but little is known about nitrogen cycling in these rivers in semiarid areas. We measured chemical and isotopic compositions of ammonium (NH4+-N) and nitrate (NO3-N) to investigate spatial–temporal variation of nitrogen cycling in the Fuhe River in the North China Plain. Nitrogen pollution in the river was mainly induced by extra NH4+-N inputs which come from the discharges of urban sewage and effluents of wastewater treatment plants in upstream. NH4+-N obtained from decomposing organic matter of sediments can diffuse into the overlying water. Intense nitrification then occurs at the terrestrial–aquatic interface. Due to less vegetation in spring and autumn, loss of NH4+-N is mainly caused by nitrification. In contrast, significant NH4+-N is absorbed by plants in summer. NO3-N generated from nitrification can be denitrified during the study period. The highest NO3-N loss (about 86.3%) was observed in summer. The contribution of NO3-N loss due to denitrification is 44.6%. The remaining 55.4% is due to plant uptake. The results suggested that nitrogen cycling in the river is related to temperature and dry–wet cycles. And vegetation restoration along the river could benefit the incremental improvements to the aquatic ecosystem.

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