[A-CG46] Biogeochemical Cycles in Land Ecosystem

[ACG46-07] Rewetting of dried soils and greenhouse gas dynamics: impact evaluation and application using Japanese soils

*Hirohiko Nagano¹, Yuri Suzuki¹, Syuntaro Hiradate², Mariko Atarashi-Andoh³, Jun Koarashi³ (1.Niigata University, 2.Kyushu University, 3.Japan Atomic Energy Agency)

Keywords:Soil carbon, SOM decomposition, Extreme weather, Future earth

Changes in soil carbon dynamics, which are 5-10 times greater than global anthropogenic carbon dioxide (CO₂) emissions, can significantly impact atmospheric CO₂ concentrations and global environmental changes, but many aspects remain unclear. In this presentation, we focus on the drying and rewetting cycles (DWC) of soils, which are also a concern as an environmental change associated with global warming, and introduce our research on the effects of DWC on soil organic matter-derived CO₂ emissions and the dynamics of greenhouse gases other than CO₂, as well as the establishment of a method for estimating soil carbon dynamics using DWC. To verify the effects of DWC on soil CO₂ emissions, we conducted an incubation experiment (Nagano et al., 2019 SSPN) using two soils collected from a deciduous broadleaf forest in Kitaibaraki, Japan, and found that CO₂ emissions under DWC conditions were up to 49% higher than those in a control area where the moisture content was kept constant during incubation. Furthermore, in an incubation experiment (Suzuki et al., 2025 SOIL) using 10 soils collected from Japanese forests and grassland, the CO₂ emission under DWC conditions was 1.3 to 3.7 times higher than in the control area with constant moisture, and the variation in the rate of increase in CO₂ emission between soils was positively correlated with the amount of Al+0.5Fe (Alp+0.5Fep) extracted with pyrophosphate. The amount of Alp+0.5Fep is considered to be an indicator of the amount of reactive Al or Fe complexes with organic matter (metal-organic complexes). It has been considered responsible for the high carbon storage capacity of volcanic ash soils widely distributed in Japan. These results suggested that the carbon conservation ability of metal-organic complexes may be vulnerable to sudden changes in the moisture environment, such as DWC. In addition, in developing a method to estimate soil carbon dynamics using DWC, we are focusing on the destruction of microbial cells and organic matter release that occurs with the rewetting of dried soil and are attempting to estimate the microbial biomass from the water-extracted organic matter of air-dried soil, which is a common form of long-term soil storage. A study using 42 soils collected from the surface layer (0-10 cm depth) of a deciduous broadleaf forest catchment in Kitaibaraki (Nagano et al., 2023 FFGC) showed that it is possible to estimate the variation in soil organic matter properties between soils by analyzing the stable isotope abundance ratios (δ¹³C and δ¹⁵N) of dissolved organic matter extracted with water from air-dried soils of these soils. Furthermore, a study using 50 soils collected from a total of 10 forests and grasslands in Japan (Nagano et al., under review) revealed that the amount of dissolved organic matter extracted with water from air-dried soils of these soils showed a very strong positive correlation (R² = 0.94, p < 0.01) with the microbial biomass carbon measured by the conventional chloroform fumigation-extraction method using raw soil. These results suggest that it may be possible to estimate the soil microbial biomass and its organic matter utilization characteristics that directly drive soil carbon dynamics and greenhouse gas dynamics by appropriately analyzing air-dried soil stored in the laboratory without collecting new soil samples. Further research in the future is expected to lead to further elucidation of terrestrial carbon cycles and greenhouse gas dynamics and to the development of more sophisticated models for predicting future environmental changes.