Analysis of Short-Term Behavior of Atmospheric Ammonium in Forest Soil Using ^<15>N Tracer

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  • 窒素安定同位体を用いた大気由来NH_4^+の森林土壌中における初期動態の解析
  • 窒素安定同位体を用いた大気由来NH4〔+〕の森林土壌中における初期動態の解析
  • チッソ アンテイ ドウイタイ オ モチイタ タイキ ユライ NH4 ノ シンリン ドジョウ チュウ ニ オケル ショキ ドウタイ ノ カイセキ

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Abstract

The stable isotope ^<15>N was applied as ^<15>NH_4Cl to the forest floor of a 50-year-old Japanese red pine (Pinus densiflora) forest in Gunma Prefecture, to investigate the behavior of atmospheric ammonium in the soil. The ^<15>N pulse was monitored for 70 d by measuring δ^<15>N values of understory vegetation, organic soil, mineral soil and soil solution within a rooting zone (40 cm depth). The gross rates of nitrogen mineralization, immobilization, and nitrification in the soils were also measured using the ^<15>N dilution technique, to determine the relationship between the ^<15>N behavior and nitrogen turnover through microbial biomass. After 30 d, understory vegetation assimilated only 5% of the ^<15>N added. The remaining ^<15>N was retained in the organic horizon (56%) and mineral soil (44%). Even after 70 d, the organic horizon retained 37% of the ^<15>N added. The majority (95%) of the total nitrogen that leached from the organic horizon to the mineral soil was nitrate. During 30 d after ^<15>NH_4^+ addition, δ^<15>N values of nitrate increased markedly in the surface mineral soil (10 cm depth). After that, the peak of δ^<15>N values was observed in the deeper soil (40 cm depth). These results indicate that atmospheric ammonium leaches to the mineral soil after being nitrified in the organic horizon. The ammonium production and immobilization rates (145 mg N kg^<-1> d^<-1>) in the organic soil were 7-fold higher than the gross nitrification rate (20 mg N kg^<-1> d^<-1>). This suggests that microbial immobilization plays an important role in the retention of ^<15>N in the organic horizon. The rates of nitrogen transformations observed in this study were approximately 10-fold higher than those in Dutch forests, where atmospheric inputs of nitrogen have been almost equal to those in the site of this study. The rapid nitrogen turnover at this site may be attributed to higher air temperature and precipitation, and to lower soil acidity.

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