2.6. The calculation of CO2 flux in deep soil layers
The CO2 flux (Fs, μmol m-2s-1 ) in deep soil layers is simulated using Fick’s First Law of Diffusion.
Fs =-DsΔC(z)/Δz (1)
Where, Ds is the diffusion coefficient (m2s-1) of CO2 in the soil, C is the CO2 concentration (μmol m-3), and z is the depth of the soil layer (m).
Ds = εDa (2)
Where, Da is the diffusivity of CO2 in the free atmosphere, Da = 1.47 × 10 -5 m2s-1; ε is the relative diffusivity of CO2 in the soil.
ε is simulated commonly using empirical models as follows:
Moldrup-2000 model:ε= (φ − θ)2.5φ-1 (3)
Where, θ is the soil volumetric water content (cm3cm-3); φ is the soil porosity, φ = ρbm, ρb is the soil volume mass (g cm-3), ρm is the soil specific gravity, and ρm = (2.65 g cm-3) in the mineral soil, m is a constant, m = 3 in this study.
According to the basic characteristics of topsoil CO2flux, the proper calculation model was chose for correcting the flux calculation.