Effect of dry-wet cycles on carbon dioxide release from two different volcanic ash soils in a Japanese temperate forest

日本の温帯林で採取した火山灰土壌の二酸化炭素放出に乾燥-湿潤サイクルが及ぼす影響

永野 博彦   ; 安藤 麻里子  ; 小嵐 淳   

Nagano, Hirohiko; Atarashi-Andoh, Mariko; Koarashi, Jun

温暖化によって増大すると予測されている土壌水分の変動が火山灰土壌の二酸化炭素(CO)放出に及ぼす影響を評価することを目的に本研究を行った。茨城県内の温帯林で採取した理化学性の異なる2種類の火山灰土壌(土壌AおよびB)を乾燥-湿潤サイクル条件および一定水分条件それぞれで培養し、CO放出速度を比較した。乾燥-湿潤サイクルにおけるCO放出速度は水分一定条件での放出速度よりも最大49%大きかった。また乾燥-湿潤サイクルによるCO放出の増大は、典型的な火山灰土壌である黒ボク土により近い土壌Bでより大きかった。放出されたCOの炭素安定同位体比を測定したところ、乾燥-湿潤サイクルによって年代の古いもしくは微生物代謝の進んだ土壌有機物の分解が促進されたことが示唆された。以上より、国内に広く分布する火山灰土壌の中でも土壌有機物安定化の能力が特に高いとされてきた黒ボク土であっても、他の土壌と同様に乾燥-湿潤サイクルに対しては脆弱であることが示唆された。また、将来起こりうる土壌水分の変動増大は、土壌のCO放出を促進する重大なポテンシャルを有することも示された。

In the present study, two volcanic ash soils (soil A and B) from a temperate broad-leaved forest in eastern Japan were incubated under repeated dry-wet cycles and continuously constant moisture conditions. The primary aims were to quantify the potential for enhancement of carbon dioxide (CO) release owing to increased water fluctuation and to examine differences in the responses of volcanic ash soils with different physicochemical properties. Soil B, rather than soil A, was a typical Andosol. During incubation at 20 C for 120 days with five dry-wet cycles, the CO release rate was measured periodically. Abundance of the stable carbon isotope in CO (C-CO) was measured to capture changes in the origin of decomposed soil organic matter (SOM) owing to the dry-wet cycles. The CO release rate under the dry-wet cycles was up to 49% higher than the values predicted from a parabolic relationship between CO release and water content during incubation under the continuously constant moisture condition. The magnitude of CO release enhancement was 2.7-fold higher in soil B relative to that in soil A. The C-CO value in the dry-wet cycles was enriched by 0.3-2.3% compared to that during incubation under the continuously constant moisture conditions, suggesting that the decomposition of old and/or well-metabolized SOM was enhanced by the dry-wet cycles. Thus, the present study suggests that Andosols, which have been believed to have a strong SOM stabilization ability, are vulnerable to dry-wet cycles. Then, increased water fluctuation in a future warmer world would have significant potential to stimulate CO release from soils.