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 (δ ¹⁵N) 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 ¹⁵N of DON in comparison with the δ ¹⁵N 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 (>5mm) were collected and the δ ¹⁵N of NH ₄ ⁺, NO ₃ ^-, DON, and total N were analyzed, and the volume-weighted monthly mean N deposition and δ ¹⁵N 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 ¹⁵N 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 ¹⁵N 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 ¹⁵N 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.