Nitrogen isotope ratios of dissolved organic nitrogen in wet precipitation in a metropolis surrounded by agricultural areas in southern Korea

Despite the importance of dissolved organic N (DON) in wet precipitation, the N isotope ratio (delta N-15) of DON has rarely been reported when compared to those of inorganic N (NH4+ and NO3-). This study was conducted to investigate seasonal variation of delta N-15 of DON in comparison with the delta N-15 of NH4+ and NO3- during wet precipitation in a metropolis area that is surrounded by agricultural areas in southern Korea. For two years (2007 and 2008), all effective precipitation events (>5 mm) were collected and the delta N-15 of NH4+, NO3-, DON, and total N were analyzed, and the volume-weighted monthly mean N deposition and delta N-15 were calculated. The contribution of DON to total N deposition (41.0 and 37.3% in 2007 and 2008, respectively) was compatible to that of NH4+ and two times higher than that of NO3-. The annual mean delta N-15 of DON (0.3 and 0.2 parts per thousand. in 2007 and 2008, respectively) was higher than that of NH4+ (-3.0 and -6.8 parts per thousand, respectively) but similar to that of NO3- (1.5 and 0.5 parts per thousand, respectively). The seasonal pattern of molar ratio of NH4+/NO3- in combination with delta N-15 of NH4+ and NO3- indicated that there were multiple diffuse sources of N deposition presented and seasonal shifting of main sources of N deposition occurred in the study site. These sources included fertilized agricultural fields in the spring and early summer, livestock excrements in the middle summer and fall, and coal-fired boilers in the winter. The intimate relationship between DON and NH4+ in terms of % portion in total N and their delta N-15 suggested that the source of DON was closely coupled with that of NH4+; e.g. fertilizer and livestock excretion. A direct measurement of N species emitted from fertilized agricultural field and livestock manure is necessary to confirm the source of DON. (C) 2012 Elsevier B.V. All rights reserved.