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

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 (CO₂) emission from subsoils through direct observations is limited. This study aimed to quantify CO₂ emission from subsoils and determine factors that control CO₂ 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 CO₂ emission from the subsoils was found to be responsible for 6 %–23 % of total CO₂ emission from the upper 60-cm mineral soil across all sites. Radiocarbon signatures of CO₂ released from the subsoils indicated the decomposition of decades-old SOM in the subsoils. The correlations between CO₂ emission rate and soil factors across both soil types suggested that the CO₂ 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.