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

Abstract

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