Estimation of the Increase Potential of Global Soil CO₂ Efflux by Surface Air and Soil Warming Based on the Mathematical Model Formula for Diurnal and Seasonal Variations in Soil Temperature

抄録

Increasing global temperatures due to high atmospheric carbon dioxide (CO₂) concentrations may cause an imbalance in the carbon circulation on earth. It has been reported that CO₂ emissions from natural soils in all terrestrial areas on Earth account for about 25% of atmospheric CO₂ emissions in the global carbon cycle. A tendency is well known that the higher the soil temperature with the higher the metabolic activity of microorganisms in the soil. Furthermore, since the global soil CO₂ emissions are huge amount that approximately seven times the total atmospheric CO₂ emissions from human origin, it is necessary to investigate the increasing rate of soil CO₂ emission rate per unit area and unit time on a global scale (global soil CO₂ efflux) that is affected by soil temperature rise due to global warming expected in the coming decades.

In this paper, we modeled soil CO₂ efflux vs. diurnal and seasonal variation in soil temperature at a measurement point using a formula that is an exponential function of soil temperature and then evaluated the effect of increasing average soil temperature due to surface air temperature rise by global warming. We have proposed a method to analogically evaluate the potential of increasing the rate of soil CO₂ efflux against a change of 1°C in soil temperature based on the model equation. The power index b (°C⁻¹) just indicates the potential of increasing the rate of soil CO₂ efflux to soil temperature rise in any land area with different soil temperatures on earth. Finally, the values of b were obtained from the published literature with field measurement data at 71 locations in 10 countries including our data measured at the Ito campus, Kyushu University. The average rate of potential increase in soil CO₂ efflux is 0.10°C⁻¹ regardless of vegetation, annual rainfall (precipitation), soil type, and soil components (carbon and nitrogen content). In conclusion, assuming the average soil temperature rises by 1°C and ignoring any effects reducing soil CO₂ reduction due to global warming, the potential increase in annual global soil CO₂ efflux is roughly estimated at 22 billion tons-CO₂/year.