Forest floor CO2 flux estimated from soil CO2 and radon concentrations

Having a quantitative understanding of the carbon cycle in forests is of great importance for predicting global warming issues. Carbon dioxide production in soil is the largest CO2 source in forests, and exhibits large temporal and spatial variations. Continuous observation of soil CO2 flux at many sites over a forest is therefore necessary to obtain representative soil CO2 fluxes for the forest. In this study, a gradient method to measure soil CO2 flux indirectly from soil radon and CO2 measurements was theoretically modified to conveniently measure the soil CO2 flux from soil radon and CO2 concentrations measured at one soil depth. To experimentally test the modified method, a field observation was conducted continuously in a forest over a 31-day period. changes in the soil water content near the soil surface were small throughout the observation, a constant effective diffusivity for CO2 was assumed for the soil CO2 flux estimation. The soil CO2 flux was then calculated as the product of the effective diffusivity and the gradient of the soil CO2 concentration, each calculated from soil radon and CO2 concentrations. The estimated flux ranged from 1.9 to 5.8 μmol m−2 s−1, and, correlating well with the reference value, measured with a conventional ventilated-chamber method. We therefore conclude that the modified gradient method based on the measurement of soil CO2 and radon concentration at one depth is reliable, at least under conditions where the change in the soil water content is small.