A MODEL OF ONE-DIMENSIONAL STEADY-STATE CARBON-DIOXIDE DIFFUSION FROMSOIL

In analytical model is developed for one-dimensional, steady-state dif fusion of carbon dioxide (CO2) from soil with, vertical decrease of th e source term described by a power function and a constant diffusion c oefficient. The surface flux density of CO2 from the soil (f(m)) is de rived from integration of the source term with depth. The model was te sted using 2 years of monthly measurements of the soil CO2 concentrati on profile in a sand containing a Pinus radiata D. Don tree. Modelled surface flux density (f(m)) at the base of the tree was consistently g reater than surface flux density (f(o)) measured 0.35 m away with an a verage ratio of f(m) to f(o) of 2.5 (R-2 = 0.83). This was explained b y decreasing root length density (L-v) with radial distance from the t ree stem. An exponential function for decrease of L-v 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 arou nd a growing tree root system were derived. Length scales for both the decrease in root length density and CO2 flux with radial distance wer e similar. An expression to estimate the radial distance from the tree stem, that gives an average plot value of CO2 surface flux density wa s also derived. (C)1998 Elsevier Science B.V. All rights reserved.