Using measurements of soil CO2 efflux and concentrations to infer the depth distribution of CO2 production in a forest soil

Carbon dioxide (CO2) from soil respiration is the focus of many ecosystem-atmosphere studies. However, due to the difficulty in obtaining measurements, there is a relative lack of information on the behavior of CO2 in the soil. This paper describes an accurate method that can be used in the field to measure CO2 concentration in small (10-cm) samples of soil air using an infrared gas analyzer. Measurements on samples drawn by syringe from different depths are compared to those calculated using a simple steady-state model derived from the conservation equation for soil CO2 and Fick’s law of diffusion. The study site is a second growth coastal temperate Douglas-fir forest plantation (53 yr old) located on the eastern slope of Vancouver Island, Canada. Measurements of CO2 concentrations were obtained from two locations at six soil depths between 0.02 and 1 m at various times of the year in 2000 and 2001. The vertical profiles of CO2 concentrations generally had similar shapes throughout the year although there was considerably more short-term variability in the shallow layers. Below-ground CO2 concentrations were higher during summer and decreased during the winter. Non-zero CO2 concentration gradients between the 0.5- and 1-m depths suggested some CO2 production below the 1-m depth. By matching modelled CO2 concentrations with measured values and using measured CO2 efflux, we used the model to determine the CO2 production profile. Calculations of CO2 production profiles indicated that more than 85% of the CO2 efflux from this forest soil originates at depths shallower than 30 cm. Key words: Soil carbon dioxide, CO2, soil respiration, steady-state model