Calculation of CO2 flux from forest soil using 222Rn calibrated method

The objective of this study was to measure the CO2 fluxes from forest soils by the Rn-222 calibrated method and the static chamber method, and to discuss the differences between the methods. Determination of the diffusion coefficient of CO2 in soil, D-CO2, by the Rn-222 method allowed us to avoid determining other soil characteristics such as water content, total porosity and tortuosity, which have large uncertainties. The CO2 flux was thus calculated from the determined CO2 diffusion coefficient and the observed gradients of soil CO2 at the soil surface. CO2 flux was also measured with the static chamber method using a chamber placed on the soil surface and calculated from the concentration change in the chamber with time. This study was carried out from July to October, 1994 in two Japanese forests with different vegetation types, one coniferous and the other a mixed deciduous-conifer forest. There was considerable variability in Rn-222 fluxes from the soil surface and in CO2 concentrations in the soil air during observational periods. The Rn-222 fluxes ranged from 1382 to 3100 dpm m(-2) h(-1) at the Tsukuba site and from 433 to 3500 dpm m(-2) h(-1) at the Nikko site, D-CO2 calculated from the results of the Rn-222 method ranged from 0.048 to 0.085 cm(2)s(-1) at the Tsukuba site and from 0.006 to 0.066 cm(2)s(-1) at the Nikko site. The results of CO2 flux measurements by the Rn-222 method were in agreement with those by the static chamber method within the observed range of error. A multivariate analysis of variance of the results of the two methods revealed no significant differences between them. Vertical profiles of soil CO2 source strength were also calculated from the Rn-222 and CO2 concentration gradient data to investigate seasonal change in the soil CO2 production rate. Carbon dioxide production rates varied significantly by season, ranging from 14 (31 October) to 134 mmol m(-3) h(-1) (11 July) at 5 cm depth at the Tsukuba site and from 5.2 (27 October) to 134 mmol m(-3) h(-1) (14 July) at 5 cm depth at the Nikko site. These seasonal changes in the CO2 source strength in the surface soil corresponded with changes in CO2 fluxes from the soil. (C) 1997 Elsevier Science B.V.