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

Abstract Despite the importance of dissolved organic N (DON) in wet precipitation, the N isotope ratio (δ 15 N) of DON has rarely been reported when compared to those of inorganic N (NH 4 + and NO 3 − ). This study was conducted to investigate seasonal variation of δ 15 N of DON in comparison with the δ 15 N of NH 4 + and NO 3 − 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 δ 15 N of NH 4 + , NO 3 − , DON, and total N were analyzed, and the volume-weighted monthly mean N deposition and δ 15 N 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 NH 4 + and two times higher than that of NO 3 − . The annual mean δ 15 N of DON (0.3 and 0.2‰ in 2007 and 2008, respectively) was higher than that of NH 4 + (−3.0 and −6.8‰, respectively) but similar to that of NO 3 − (1.5 and −0.5‰, respectively). The seasonal pattern of molar ratio of NH 4 + /NO 3 − in combination with δ 15 N of NH 4 + and NO 3 − 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 NH 4 + in terms of % portion in total N and their δ 15 N suggested that the source of DON was closely coupled with that of NH 4 + ; 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.