Laboratory incubation experiments to examine the effects of increased water fluctuation on CO2 release from volcanic ash soils in a Japanese temperate forest

Our understandings of soil CO₂ release under the increased water fluctuation are still insufficient to infer the feedback of terrestrial carbon cycling in future worlds. In the present study, we are conducting laboratory incubation experiments to examine effects of increased soil water fluctuation on soil CO₂ release. Here, we use volcanic ash soils collected from a temperate deciduous broadleaved forest in Japan. Volcanic ash soil is the major soil type in Japan and is expected to have great ability to stabilize carbon as soil organic matter. As the first step of the study, we incubated volcanic ash soils at 20 °C for 120 days under two different water conditions (a continuously constant water and a fluctuated water conditions). During the incubation, fluctuated water condition was achieved with five dry-wet cycles. Then, the CO₂ release rate was measured periodically. Abundance of the stable carbon isotope in CO₂ respired from the soils (δ¹³C-CO₂) was also measured to capture changes in the origin of decomposed soil organic matter owing to the fluctuated water condition. The CO₂ release rate under the fluctuated water condition was up to 49% higher than the values predicted from a parabolic relationship between the CO₂ release and water content during incubation under the continuously constant water condition, suggesting an accelerated decomposition of soil organic matter under the fluctuated water condition. The δ¹³C-CO₂ values under the fluctuated water condition was enriched by 0.3-2.3‰ compared to those under the continuously constant water conditions, indicating that the decomposition of well-metabolized and/or old soil organic matter was enhanced by the large fluctuation of soil moisture condition. Thus, the vulnerability of soil organic carbon against the dry-wet cycles is inferred even for soils with high carbon storage capabilities. The experiments are still on-going particularly for examining the effect of increased water fluctuation on the temperature sensitivity of soil CO₂ release. Then, results and further insights from on-going experiments will be presented at the meeting.