Characteristics of gas diffusion coefficient and CO_2 flux at different slope site and different depth in forest soil

Abstract

Seasonal changes of CO_2 flux at the different slope sites and different depths were investigated in order to clarify CO_2 flux of forest soil. Experimental plots were established at a lower site (Japanese cedar, elevation 730m), a middle site (Japanese cypress, 760m) and an upper site (Japanese cypress, 800m) on a sequence of slope in the University Forest of Tokyo Univ. of Agric. and Technol., Gunma Prefecture. The stand age was 42-years-old and the soil parent material was phyllite of Paleozoic and Mesozoic strata. Gas diffusion coefficients in soil (D) were measured at each soil depth (0-5, 5-15, 15-25, 25-35, 35-50cm) at three slope positions using the improved gas diffusion instruments developed by Osozawa and Kubota (1987). The seasonal change of CO_2 flux from the soil surface at each depth was calculated using the CO_2 concentration and the CO_2 diffusion coefficient. The relative gas diffusion coefficient in the soil air (D/D_0) increased with increasing air porosity at the all soil depths in every slope position. Though, the linear relationships between the D/D_0 and the air porosity was seen at 0-5cm, it approximated exponentially below 5cm. In the same depth, there were no difference in D/D_0 among slopes. At the lower site in summer, the CO_2 flux from soil surface was 3 to 4 times as large as the flux in winter, and the flux at the other sites was showed several ten times. The maximum CO_2 flux from the surface soil was about 6×10^<-5>g・m^<-2>・s^<-1> at all sites. The CO_2 turnover time in 50cm of soil was about 10 hours in summer. There was an exponential relationship between soil temperature and CO_2 flux from the soil surface. Annual amounts of CO_2 flux from the surface soil measured by gas diffusion method was estimated as 13.4t・ha^<-1>・y^<-1> at the lower site, 6.5t・ha^<-1>・y^<-1> at the middle site and 9.9t・ha^<-1>・y^<-1> at the upper site.