Relationship between soil CO2 concentrations and forest-floor CO2 effluxes

Soil CO₂ efflux is the result of CO₂ production in soil and its transport to the surface. To better understand the biotic and abiotic factors that control soil CO₂ efflux, we compared seasonal and diurnal variations in the simultaneously measured CO₂ efflux and soil CO₂ concentration profile (10, 20 and 50 cm depths) in a 54-year-old Douglas fir forest on the east coast of Vancouver Island, Canada. Half-hourly measurements of soil CO₂ efflux were made with an automated non-steady state chamber. Soil CO₂ concentrations were measured with small solid-state infrared CO₂ sensors. Annually, soil CO₂ efflux was well described by an exponential function of soil temperature at the 2 cm depth, with a reference efflux at 10 ^oC, of 2.6 μmol m^-2 s^-1 and a Q₁₀ of 3.7. Soil CO₂ concentration increased with soil temperature, likely due to rising CO₂ production, and with soil water content due to decreased diffusivity. It also increased with depth reaching almost 10 mmol mol^-1 at the 50 cm depth, indicating CO₂ production associated with very low diffusivity at deeper depths.

Seasonal as well as diurnal variations in soil CO₂ efflux and concentrations at the 10, 20 and 50 cm depths were highly correlated with soil temperature at the 2 cm depth. Time series analysis also showed that the variations in efflux, CO₂ concentration at the 50 cm depth, and temperature at the 2 cm depth were in phase. Soil CO₂, in this rapidly draining soil, was in a near steady state condition most of the time. For example, hourly changes in CO₂ storage in the 0-20 cm soil layer were small, generally less than 0.1 μmol m^-2 s^-1. CO₂ efflux was best described by a linear function of CO₂ concentration at the 50 cm depth with a slope that decreased with soil water content (r² = 0.78 - 0.90). These results are consistent with calculations made using a multi-layer numerical model of heat, water and CO₂ transport in soil. However, more information is needed for different soil types and environmental conditions and on spatial variation of deep soil CO₂ concentrations.