A model of one-dimensional steady-state carbon dioxide diffusion from soil

An analytical model is developed for one-dimensional, steady-state diffusion of carbon dioxide (CO2) from soil with, vertical decrease of the source term described by a power function and a constant diffusion coefficient. The surface flux density of CO2 from the soil (fm) is derived from integration of the source term with depth. The model was tested using 2 years of monthly measurements of the soil CO2 concentration profile in a sand containing a Pinus radiata D. Don tree. Modelled surface flux density (fm) at the base of the tree was consistently greater than surface flux density (f0) measured 0.35 m away with an average ratio of fm to f0 of 2.5 (R2=0.83). This was explained by decreasing root length density (Lv) with radial distance from the tree stem. An exponential function for decrease of Lv and the surface flux density of CO2 with increasing radial distance from the tree stem and an analytical expression of the total CO2 flux from the soil around a growing tree root system were derived. Length scales for both the decrease in root length density and CO2 flux with radial distance were similar. An expression to estimate the radial distance from the tree stem, that gives an average plot value of CO2 surface flux density was also derived.