Source of nitrogen in wet deposition to a rice agroecosystem at Tai lake region

Nitrogen (N) in wet deposition can provide significant nutrients to algae, which potentially contributes to eutrophication in waterbodies, and to nutrient surplus of rice–wheat crops in the Tai lake region, Jiangsu Province, China. Quantifying the N compounds in wet deposition and determining their sources is important to understand how to control eutrophication in lakes and to improve recommendations for fertilizer use. In this study, the concentrations of inorganic N, molar ratios of NH4+/NO3− and the natural 15N abundance of NH4+ (δ15NH4+) in wet deposition were determined for 78 precipitation events between June 2003 and July 2005. Samples were collected at two sites in Tai lake region, a watershed rice agroecosystem currently experiencing eutrophication. The average N wet deposition amounted to 27 kg N ha−1 yr−1, with 60% in the form of NH4+. Annually, two cycles of depleted and enriched δ15NH4+ indicate the shifting of main source of NH3 volatilization between chemical fertilizer and excreta of local residents and domestic animals, surface waters, and other organic N. The peak in NH4+/NO3− ratio, coupled with depleted δ15NH4+ in mid-June, coincided with the rice-transplanting period, which is indicative of surplus fertilizer application. Enriched δ15NH4+ values in August–October and in April–May were indicative of enhanced emissions from excreta and polluted waterbodies in the warmer seasons. Findings suggest that δ15NH4+ could be used to indicate the sources of NH3 volatilization, and the NH4+/NO3− ratio to indicate the intensity of wet N deposition.