Modeling water potential of cover crop residues on the soil surface

Cover crops are usually planted between cash crops to protect the soil, take up residual soil nitrate, and release nitrogen (N) to subsequent crops. Following cover crop termination, residues may remain on the soil surface, where their decomposition is largely dependent on residue temperature and water potential (ψresidue). While it is possible to continuously measure residue temperature, continuous measurements of ψresidue are impractical. Thus, a practical model to estimate ψresidue would be useful for models of residue decomposition. To obtain data for a model of ψresidue for cereal rye (Secale cereale L.) and crimson clover (Trifolium incarnatum L.), we conducted studies that evaluated the effects of (1) residue stage of decomposition on water release curve; (2) relative humidity (RH) on ψresidue; (3) soil moisture on residue gravimetric water content (θg);(4) precipitation on θg; and (5) diurnal changes in RH and temperature on ψresidue. Results showed that water release curves for cereal rye and crimson clover changed as decomposition progressed, and that parameters for these curves could be estimated from residue lignin content. Both types of residues rewetted at a similar rate when exposed to high RH, but when rewetted by rainfall, cereal rye required a lower amount of rainfall than crimson clover to reach maximum water content. Results of these studies were used to develop, calibrate, and validate a model of ψresidue that requires inputs of initial water and lignin contents of the residue as well as hourly values of relative humidity, air temperature, and precipitation. Comparison of observed vs. simulated data indicated the model simulates reasonably well the observed diurnal patterns of surface ψresidue with R2=0.84. Because this model requires a small set of parameters and input variables, its use may be more practical than that of more detailed models.