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

摘要

Precipitation was collected from May 15, 2001 to November 18, 2002, at the mountain top (620 m a.s.l.) and mountain foot (47 m a.s.l.) of the Yahiko–Kakuda Mountains area, which is located in the western part of the Niigata Plain in central Japan. Major ion constituents and nitrogen isotopic ratios of ammonium (δ15N-NH₄ +) and nitrate (δ15N-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 δ15N-NH₄ + values in the warm season were lower than those in the cold season at both the mountain top and mountain foot. The δ15N-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 δ15N values, emitted from agricultural activities. On the other hand, the monthly and seasonal means of δ15N-NO₃ − values showed noticeable seasonal difference with higher in the cold season at both the mountain top and mountain foot; however, the elevation difference was not observed either in the warm or cold season. The obtained δ15N-NH₄ + and δ15N-NO₃ − values were regarded as intermediate in comparison with the reported values. No significant correlations were observed either between NH₄ + concentrations and δ15N-NH4+ values or between NO₃ − concentrations and δ15N-NO₃ − values. These results suggest that different factors may affect the nitrogen isotopic variations and concentration variations of NH₄ + and NO₃ − in precipitations collected at the two sites.