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 (δ15N) 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 δ15N of DON in comparison with the δ15N 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 δ15N of NH4+, NO3−, DON, and total N were analyzed, and the volume-weighted monthly mean N deposition and δ15N 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 δ15N of DON (0.3 and 0.2‰ in 2007 and 2008, respectively) was higher than that of NH4+ (−3.0 and −6.8‰, respectively) but similar to that of NO3− (1.5 and −0.5‰, respectively). The seasonal pattern of molar ratio of NH4+/NO3− in combination with δ15N 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 δ15N 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.

► We report N deposition rate and isotope ratio of NH4+, NO3−, and dissolved organic N (DON).
► The contribution of DON to total N deposition was similar to that of NH4+ and two times higher than that of NO3−.
► The δ15N values of DON were higher than those of NH4+ but similar to those of NO3−.
► The quantitative and isotopic relationships of DON with NH4+ suggest that the source of DON is coupled with NH4+ sources.