(二)、 Soil air diffusion .a difference in the concentration of a particular gas in one zone compared with any other zone in the soil can result in the movement of that gas by a process called diffusion The process of diffusion can be represented by a flow equation in which the driving force is the gas concentration gradient (Fick’sLaw) gd==D x .where qd is the diffusive flux(mass diffusing across a unit area per unit time), d is the gas diffusion coefficient, and dc/dx is the concentration gradient of the gas Soil respiration（二）、Soil air diffusion • The process of diffusion can be represented by a flow equation in which the driving force is the gas concentration gradient (Fick’s Law). dx dc qd = −D •where qd is the diffusive flux (mass diffusing across a unit area per unit time), D is the gas diffusion coefficient, and dc/dx is the concentration gradient of the gas. •A difference in the concentration of a particular gas in one zone compared with any other zone in the soil can result in the movement of that gas by a process called diffusion. Soil respiration: