Quantitative evaluation of carbon dioxide emissions from the subsoils of volcanic and non-volcanic ash soils in temperate forest ecosystems

火山灰土壌と非火山灰土壌が分布する温帯林における下層土壌から放出される二酸化炭素量の定量評価

阿部 有希子; 中山 理智*; 安藤 麻里子  ; 丹下 健*; 澤田 晴雄*; Liang, N.*; 小嵐 淳   

Abe, Yukiko; Nakayama, Masataka*; Atarashi-Andoh, Mariko; Tange, Takeshi*; Sawada, Haruo*; Liang, N.*; Koarashi, Jun

森林土壌に蓄積されている炭素の半分以上が下層土壌(30cm以深)に存在している。しかし、下層土壌の陸域炭素循環における寄与やそれを制御する要因については未解明な点が多い。そこで本研究では、下層土壌からのCO放出量を定量評価するとともに、CO放出に影響を与える要因を明らかにすることを目的とした。有機炭素蓄積特性の異なる2つのタイプの森林土壌(火山灰土壌と非火山灰土壌)を対象に、深さ60cmまでの土壌を採取し、培養実験により深さごとのCO放出速度を測定した。また、放出されたCOの放射性炭素同位体比を分析した。その結果、下層土壌からのCO放出は、全体(深さ0-60cm)の放出量の6-23%を担い、1950年以降に固定された有機炭素の分解に起因していることが明らかになった。下層土壌からのCO放出は、土壌微生物が利用しやすい有機炭素の量と微生物バイオマス量に規定されていることが示唆された。

Subsoils (typically below a depth of 30 cm) contain more than half of global soil carbon (C) as soil organic C (SOC). However, the extent to which subsoil SOC contributes to the global C cycle and the factors that control it are unclear because quantitative evaluation of carbon dioxide (CO) emission from subsoils through direct observations is limited. This study aimed to quantify CO emission from subsoils and determine factors that control CO emission, focusing on the decomposability of soil organic matter (SOM) and the characteristics of the mineral-SOM association in soils. Therefore, a laboratory incubation experiment was conducted using surface soils (0-10 cm and 10-25 cm depth) and subsoils (30-45 cm and 45-60 cm depth) collected from four Japanese forest sites with two different soil types (volcanic ash and non-volcanic ash soils). The CO emission from the subsoils was found to be responsible for 6%-23% of total CO emission from the upper 60-cm mineral soil across all sites. Radiocarbon signatures of CO released from the subsoils indicated the decomposition of decades-old SOM in the subsoils. The correlations between CO emission rate and soil factors across both soil types suggested that the CO emission from the subsoils is mainly controlled by the amounts of SOC easily available to soil microbes and microbial biomass C, not by the amounts of reactive minerals. Given the potential active participation of subsoils in terrestrial C cycling, most of the current soil C models that ignore subsoil C cycling are likely to underestimate the response of soil C to future climate change. The quantitative and mechanistic understanding of C cycling through a huge subsoil C pool is critical to accurately evaluating the role of soil C in the global C balance.