Simple Formulation of the Soil Water Effect on Residue Decomposition

Soil water content, h, is a major factor affecting residue decomposition, but simple formulation of this factor is often lacking. We observed that theta significantly (P < 0.001) affected the residue decomposition constant, k(d). When theta varied from 0.09 g g(-1) to 0.23 g g(-1), k(d) ranged from 0.009 to 0.013 d(-1) and from 0.009 to 0.022 d(-1) for residues with carbon to nitrogen ratio (C/N) > 30 and C/N < 25, respectively. A theta factor was formulated in terms of the field capacity theta(FC) and the air-dry theta(d) in the form f(w) = (theta - theta(d))/(theta(FC) - theta(d)), and this was used to modify the potential k(d) as theta varied. Coupling fw with a first-order residue decomposition equation resulted in the prediction of the decomposition of four residue types in the greenhouse (R(2) = 0.94; relative root mean square error, RRMSE, = 0.06) and in the field (R(2) = 0.93; RRMSE = 0.11).