Nitrogen (N) losses through runoff from cropland and atmospheric deposition contributed by agricultural NH
3 volatilization are important contributors to lake eutrophication and receive wide attention. Studies on the N runoff and atmospheric N deposition from the paddy ecosystem and how the agriculture-derived N deposition was related to NH
3 volatilization were conducted in the paddy ecosystem in the Erhai Lake Watershed in southwest China. The critical period (CP) with a relatively high total N (TN) and NH
4+-N deposition occurred in the fertilization period and continued one week after the completion of fertilizer application, and the CP period for N loss through surface runoff was one week longer than that for deposition. Especially, the mean depositions of NH
4+-N in the CP period were substantially higher than those in the subsequent period (
p < 0.01). Moreover, agriculture-derived NH
4+ contributed more than 54% of the total NH
4+-N deposition in the CP period, being positively related to NH
3 volatilization from cropland soil (
p < 0.05). The N concentrations were higher in the outlet water of ditches and runoff in May than in other months due to fertilization and irrigation. Therefore, to reduce the agricultural N losses and improve lake water quality, it is important to both reduce agricultural NH
4+-N deposition from NH
3 volatilization and intercept water flow from the paddy fields into drainage ditches during the CP.
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