What Regulates Soil CO 2 Concentrations? A Modeling Approach to CO 2 Diffusion in Deep Soil Profiles

About 10% of the CO₂ in the above-ground atmosphere is annually recycled back to the atmosphere from the soil. Soil respiration (the sum of root and microbial respiration) increases soil CO₂ concentrations by 10- to 100-fold or more than that in the above-ground atmosphere, and such elevated concentrations have a variety of significant effects on soil systems. Because increased soil respiration is expected to result from future global climates, due both to elevated atmospheric CO₂ and temperature, we developed a multibox model without steady-state assumptions to investigate the factors that regulate soil CO₂ concentrations. Results suggest that below-ground CO₂ concentrations are predominantly controlled by tortuosity of pores, depth distributions of root and microbial activity, and total production of soil CO₂. Considering the dependence of soil respiration rates on climate, our results are significant in suggesting that future climate change will increase soil CO₂ concentrations in direct proportion to increases of below-ground respiration.