Simple formulation of the soil water effect on residue decomposition

Soil water content, θ, is a major factor affecting residue decomposition, but simple formulation of this factor is often lacking. We observed that θ significantly (P < 0.001) affected the residue decomposition constant, k_d· When θ varied from 0.09gg^-1 to 0.23gg^-1, k_d ranged from 0.009 to 0.013d^-1 and from 0.009 to 0.022d^-1 for residues with carbon to nitrogen ratio (C/N) > 30 and C/N < 25, respectively. A θ factor was formulated in terms of the field capacity θ_FC and the air-dry θ _d in the form f_w = (θ - θ_d) / (θ_FC - θ_d), and this was used to modify the potential k_d as θ varied. Coupling f_w with a first-order residue decomposition equation resulted in the prediction of the decomposition of four residue types in the greenhouse (R² = 0.94; relative root mean square error, RRMSE, = 0.06) and in the field (R² = 0.93; RRMSE = 0.11).