スギとヒノキ人工林における土壌呼吸の季節変化

Abstract

京都市山科区の安祥寺国有林内のスギ, ヒノキ人工林に, 異なる林齢, 立地条件の林分を調査区として選び土壌呼吸量の季節変化, 年間の土壌呼吸量を測定した。土壌呼吸速度は, ヒノキ壮齢林で2. 5 - 12 (CO_2, g/m_2・day), ヒノキ若齢林で2. 5 - 13, スギ若齢林で2. 5 - 14, 斜面上部のヒノキ若齢林で4 - 13とすべての調査区において明瞭な季節変化を示した。土壌呼吸速度は, 夏に高く, 冬に低くなった。土壌呼吸速度と土壌表面温度の聞に有意な相関関係が認められた。土壌含水率と土壌呼吸の間には明瞭な関係は認められず, 本調査地では温度が土壌呼吸速度変化の主な支配要因となっていた。土壌呼吸速度は温度の上昇とともに指数的に増加し, 両者の関係は, log CO_2=a+bt, (CO_2: 土壌呼吸速度, t: 土壌表面温度, a, b: 定数) により近似できた。この式を用いて, 各調査区の各月の平均温度から年間の土壌呼吸量を推定した。年間の土壌呼吸量は, 斜面下部のヒノキ壮齢林, ヒノキ若齢林で約4. 5 (C, t/ha・year), スギ若齢林で4. 7 - 4. 9となり, 同一の立地条件では林齢, 樹種による土壌呼吸量に差は認められなかった。一方, 斜面上部のヒノキ若齢林での土壌呼吸量は, 約5. 8 (C, t/ha・year) となり, 斜面下部の調査区より高い値を示した。これらの結果から, 土壌呼吸量は植栽された樹種やその林齢より林分の立地条件, 特に土壌条件により異なることが示された。

Seasonal changes in soil respiration rates were measured in seven study plots chosen in Chamaecyparis obtusa and Cryptomeria japonica plantations located on the upper and bottom parts of a forest slope in the Anshouji National Forest, Kyoto Prefecture, Japan. Soil respiration rates were measured over a two year period in the seven plots and showed clear seasonal changes with a summer peak. Seasonal patterns of soil respiration rates were similar among the seven study plots. Seasonal variations soil respiration rates were as follows; 2.5 to 12 (CO2, g/m2･day), 2.5 to 13, and 2.5 to 14 in 35 and 15 year Chamaecyparis and Cryptomeria plots located on the lower part of the slope, and 15 year Chamaecyparis plots on the upper part of the slope. Soil respiration was highly correlated with soil temperature (r=0.86 to 0.89) and rates of soil reapiration showed a logarithmic increase in the seven study plots with rising soil temperatures. Neither of the study plots showed a close relationship between soil respiration and soil moisture except during drought conditions in summer. Cumulated annual totals of carbon dioxide evolution were estimated using the relationships between soil temperature and soil respiration rates and were as follows. On the bottom part of the slope, the annual total carbon was 4.6 (t/ha year), 4.4, and 4.8 in the 15-year and 35-year old Chamaecyparis and in 15-year old Cryptomeria plot, respectively. On the upper part of the slope, annual total carbon was 5.5 (t/ha year) in 15 year Chamaecyparis plot and was higher than in the 15 year Chamaecyparis plots on the lower part of the slope. The present results suggest that the soil respiration rates reflect mainly the mineralization of soil organic matter accumulated in the soil and are independent of litter fall from trees and the productivity of plants.