Seasonal Variations of Nitrogen Isotopic Ratios of Ammonium and Nitrate in Precipitations Collected in the Yahiko – Kakuda Mountains area in Niigata Prefecture, Japan

Precipitation was collected at the mountain top (620m, a.s.l.) and mountain foot (47m, a.s.l.) of the Yahiko-Kakuda Mountains area, which is located in the western part of the Niigata Plane in central Japan. Major ion constituents and nitrogen isotopic ratios of ammonium (•¹⁵N•NH₄⁺) and nitrate (•¹⁵N•NO₃) were measured in order to investigate the sources and behavior of NH₄⁺ and NO₃ in precipitations. The concentrations of sea-salt constituents considerably increased in the cold season from November to March, and for the majority of the ion species, the concentrations at the mountain foot were higher than those at the mountain top. The precipitation weighed monthly and seasonal means of •¹⁵N• NH₄⁺ values in the warm season were lower than those in the cold season at both the mountain top and mountain foot. The•¹⁵N•NH₄ values at the mountain foot were lower than those of mountain top in both seasons. This seasonal variation of ¹⁵N•NH₄⁺ could be caused by the incorporation of ammonia gas (NH₃) with lower •¹⁵N values, emitted from agricultural activities. On the other hand, the monthly and seasonal means of •¹⁵N•NO₃^• values showed noticeable seasonal difference with higher in the cold season at both the mountain top and mountain foot, however, elevation difference was not observed in the warm season. The obtained •¹⁵N•NH₄⁺ and •¹⁵N•NO₃• values in this work were regarded as intermediate in comparison with the reported values. No significant correlations were observed either between NH₄^• concentrations and•¹⁵N•NH4⁺ values or between NO₃^• concentrations and •¹⁵N•NO₃^• values.