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Increasingly, the characteristics of nitrogen (N) loading have been recognized to be critical for the maintenance and restoration of water quality in agricultural watersheds, in response to the spread of water eutrophication. This paper estimates N loading and investigates its influencing factors in ten small watersheds variously dominated by forest and agricultural land use types in the subtropics of China, over an observation period of 23–29 months. The results indicate that the average concentrations of total nitrogen (TN), NH₄⁺–N, and NO₃⁻–N were 0.83, 0.07, and 0.46 mg N L⁻¹in the forest watersheds and 1.49–5.16, 0.21–3.23, and 0.99–1.30 mg N L⁻¹in the agricultural watersheds, respectively. Such concentrations exceed the national criteria for nutrient pollution in surface waters considerably, suggesting severe stream pollution in the studied agricultural watersheds. The average annual TN loadings (ANL) were estimated to be 1,640.8 kg N km⁻² year⁻¹in the agricultural watersheds, 63.3–86.1 % of which was composed of dissolved inorganic N (DIN; comprising NO₃⁻–N and NH₄⁺–N). The watershed with intensive livestock production (i.e., the maximum livestock density of 2.66 animal units (AU) ha⁻¹) exhibited the highest ANL (2,928.7 kg N km⁻² year⁻¹) related to N loss with effluent discharge. The results of correlation and principle component analysis suggest that livestock production was the dominant influencing factor for the TN and NH₄⁺–N loadings and that the percentages of cropland in watersheds can significantly increase the NO₃⁻–N loading in agricultural watersheds. Therefore, to restore and maintain water quality, animal production regulations and more careful planning of land use are necessary in the agricultural watersheds of subtropical China.
