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.Since 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.