森林立地学会誌 森林立地46(2), 2004, 69〜75
Jpn.J.For.Environment
46(2), 69-75 2004
洗滌培養法を用いた反応速度論的解析による表層土壌の現地窒素無機化量の推定
平井敬三＊＊，＊＊＊
＊＊森林総合研究所四国支所・＊＊＊現所属：独立行政法人森林総合研究所立地環境研究領域
スダジイ二次林における斜面位置の違いや伐採にともなう窒素無機化特性を検討するため，非破壊土壌試料による洗滌培養法を用いた反応速度論的解析から検討した。反応速度論的解析で得た各種パラメータと地温観測データを利用して，これら土壌の年間無機化量を推定した。また，同時に破壊土壌試料によるびん培養法を行い，表層土壌の窒素無機化におよぼす土壌攪乱の影響を検討した。可分解性の窒素量（N0）は斜面上部の土壌で多かったが，尾根では無機加速度を示す速度定数（k）が大きく，見かけの活性化エネルギー（Ea）は小さく無機化されやすかった。深さ5cmまでの単位面積あたりの年間窒素無機化量は13.5〜38.5kg ha-1yr-1であり，尾根の無機化量が大きかった。伐採地では速度定数が小さく，活性化エネルギーが同じ尾根にある林内の非伐採地の土壌に比べて大きくなったことから，無機化量（N）は小さくなった。さらに無機化プールに対する年間窒素無機化量の割合を示す無機化率（N/N0）が非伐採の林内尾根や斜面上部より小さく，回転速度が遅くなった。一方，硝化率が非伐採地に比べて高く，硝化活性が伐採により高まった。土壌の攪乱を伴う破壊土壌を用いたびん培養法よる無機化量は非破壊土壌から求めた洗滌培養法の2.5〜3倍あり，無機化率は洗滌培養法で1.0〜3.1%，びん培養法で5.4〜7.5％とびん培養法で高かった。
Keizo Hirai : Estimation of annual nitrogen mineralization amounts in the topsoil from kinetics analysis using the leaching incubation method
We investigated the characteristics of nitrogen mineralization in the
topsoil at different topographic positions and the effects of clear cutting in
a Castanopsis cuspidate secondary forest by subjecting undisturbed soil
samples to kinetic analysis using the leaching incubation method. We also
estimated annual amounts of nitrogen mineralization in the soil at each site
from parameters of kinetic analysis and recorded soil temperatures of these
soils. We also compared leaching method using the undisturbed soil and bottle
incubation method using the disturbed soil to evaluate the effect of soil
disturbance on the characteristics of nitrogen mineralization. As a result,
easily decomposable nitrogen (N0) was found to be highest in
the soil at the soil at the site of upper slope position, but the rate constant
of mineralization (k) and apparent activation energy (Ea) were highest
in the soil at the ridge site. Estimated amounts of nitrogen mineralization
were ranged from 13.5 to 38.5kg ha-1yr-1 5cm-1
at the sites with the highest amounts observed at the ridge site. In the soil
at the site of a clear cutting where in ridge site, as rate constant of
mineralization decreased and apparent activation energy was increased compared
at the ridge site in the forest where was not cut. Then, turnover rate which
indicates ratio of annual mineralization amounts (N) to mineralizable
pool (N0) was lowest among sites. But nitrification was
increasing at the site of a clear cutting. Values of nitrogen mineralization
were three times higher with the bottle incubation method than with the
leaching method. Mineralization rates of the leaching method at about 1.0-3.1%,
were also 2.5-3 times higher than those of the bottle method which were about
5.4-7.5%.
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