並列タイトル等モンゴル北部森林-草原境界域における植物と土壌の窒素同位体比を用いた窒素サイクルの研究
一般注記Nitrogen isotope ratio (δ15N) of terrestrial plants and soils are recognized as indicators of N cycle, since isotopic fractionations during the processes in N cycle cause variations in these δ15N values. Recent accumulation of δ15N data for wide spatial areas has enabled to investigate the characteristics of δ15N in large spatial scale from region toglobe. These studies have revealed that the ecosystem state factors such as climate, soil age, topography, parent material, and cultivation contribute spatial variation of δ15N. However, the contribution of vegetation type, one of the ecosystem state factors, has not been evaluated yet. To interpret the spatial patterns of δ15N into N cycle, the range ofδ15N variation with associated N process need to be evaluated for each factor. This studyaimed to clarify the variations in plant and soil δ15N and associated processes which arise from vegetation type. Six observation areas were set in the forest-grassland boundary in northern Mongolia, and sampling sites were set along forest to grassland boundary within an area. Each site was classified into forest site or boundary site, and needles of siberian larch (Larix sibirica Ledeb.) and soils were investigated. The different type of accumulation of organic layer was observed in this region between forest and boundary sites: “moder type” in forest site and “mull type” boundary site. Needle and soil δ15N, and the difference in δ15N between needle and soil (Δδ15N) showed clear spatial trends along forest to grassland boundary in an area, and those trends were also consistent in all areas: needle and soil δ15N were low in forest sites and high at boundary sites, whereas Δδ15N was large in forest sites and small at boundary sites. The range of Δδ15N observed in this region was 7‰ (-8.4‰ to -1.8‰), which occupied half of the estimated range in globe. The N cycle processes suggested bynitrogen isotope mass balance and observed environmental parameters matched the typical characteristics of moder type in forest sites and and mull type at boundary sites. Small Δδ15N observed at grassland boundary was explained by rapid recycling of Nbetween larch and soil without significant microbial immobilization, whereas largeΔδ15N observed in forest was explained by significant microbial immobilization which retained 15N-riched part of available N in soil, and additional N supply from organic layer to larch. This study proposes the new relationship between the type of the accumulation of organic layer and the variation in the difference between plant and soil δ15N (Δδ15N) through the divergence of available N between uptake by larch and immobilization by microbes along forest to grassland boundary.
(主査) 教授 杉本 敦子, 特任教授 吉川 久幸, 准教授 山本 正伸, 准教授 山下 洋平, 教授 木庭 啓介 （京都大学生態学研究センター）
環境科学院（地球圏科学専攻）
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