To assess the impact of human activity on the nitrogen (N) cycle and evaluate the sources of N in surface water, the N budget for Laoguanhe River Watershed (LRW), a typical agricultural watershed and one of the upstream tributaries of Danjiangkou Reservoir in China, was developed by using measurement data on N fluxes and literature data on other parameters. Over the whole watershed, fertilizers, human and livestock excreta, atmospheric deposition, biotic fixation, N from crop residue used as fertilizer, and imported animal feeds contributed 65.7%, 20.7%, 6.1%, 5.6%, 1.7%, and 0.3%, respectively, to total N input (40,816.6 kg N km^-2 year^-1). N transported to water bodies, denitrification, harvesting crops, and ammonia volatilization contributed 32.0%, 25.2%, 23.0%, and 19.8% of total N output (21291.2 kg N km^-2 year^-1), respectively. The N budget for the LRW suggested that more than 50% of the N input was lost to the environment, and about 17% was discharged as riverine N, which indicated that agricultural and human activities in the watershed substantially impacted water quality, and so altered the N biogeochemistry process.