報告書番号

T03042

タイトル（和文）

大気由来窒素による土壌酸性化影響の評価

タイトル（英文）

Evaluation of the effect of nitrogen deposition on acidification of forest soils

概要 （図表や脚注は「報告書全文」に掲載しております）

欧州の森林とほぼ同量の窒素が沈着しているわが国のアカマツ林を対象に，土壌の酸中和能力と大気由来窒素の動態を調べた．林床に窒素同位体（15NH4+）を散布し，その移行割合を調べた結果，1年後も約60％が深さ40 cmまでの土壌に，20％が植物に保持され，根域外への流出量は8％にすぎなかった．また，土壌に保持された15Nのうち80～90％が有機態に変化していた．土壌中の窒素の形態変化速度を室内実験により調べた結果，NH4+ の有機化速度は硝化速度の約7倍大きく，酸性化した欧州の有機化速度よりも1桁大きかった．大気降下物や硝化反応によって負荷されているH+ は，土壌から塩基の供給により完全に中和されていた．以上より，本調査地は，酸性化した欧州の土壌よりも塩基による酸中和能力が高く，大気由来窒素を有機化し固定する能力が高いことが推察された．そのため，窒素の沈着量が現在のレベルで推移する限り，土壌や陸水が酸性化する可能性は低いと推定される．

概要 （英文）

To evaluate of the effects of nitrogen deposition on soil acidification, the capacity to retain nitrogen was investigated by appling 15N tracer（15NH4Cl）to the forest floor in a red pine (Pinus densiflora) forest ecosystem in central Japan, where atmospheric inputs of nitrogen（50 kg N ha-1 yr-1）have almost been equal to those in nitrogen-saturated Dutch forests which soils has already acidified. The gross rates of nitrogen mineralization, immobilization and nitrification in the soils were also measured using the 15N dilution technique. After 1 yr, about 60% of the 15N applied was retained in the organic (30%) and mineral (30%) soil horizons, and about 20% was assimilated by vegetation. 15NO3- leached from the rooting zone accounted for only 8%. About 80～90％ of the 15N retained in the soils existed in organic nitrogen pool. The ammonium production and immobilization rates (145 mg N kg-1 day-1) in the organic soil were 7-fold higher than the gross nitrification rate (20 mg N kg-1 day-1). The rates of nitrogen transformations observed in this study were approximately 10-fold higher than those in Dutch forests. Proton derived from atmospheric deposition and nitrogen transformations (mainly nitrification) were almost completely neutralized primarily by base-cation release from the soil. These suggests that the soil of the studied forest has higher capacity to retain nitrogen and to neutralize acid than Dutch soils have. It is concluded that soil acidification due to nitrogen deposition is unlikely to occur at present in the site.

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