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

Phytotoxicity and Benzoxazinone Concentration in Field Grown Cereal Rye (Secale cereale L.)

Winter rye (Secale cereale L.) is used as a cover crop because of the weed suppression potential of its mulch. To gain insight into the more effective use of rye as a cover crop we assessed changes in benzoxazinone (BX) levels in rye shoot tissue over the growing season. Four rye varieties were planted in the fall and samples harvested at intervals the following spring. Two different measures of phytotoxic compound content were taken. Seed germination bioassays were used as an estimate of total phytotoxic potential. Dilutions of shoot extracts were tested using two indicator species to compare the relative toxicity of tissue. In addition, BX (DIBOA, DIBOA-glycoside, and BOA) levels were directly determined using gas chromatography. Results showed that rye tissue harvested in March was the most toxic to indicator species, with toxicity decreasing thereafter. Likewise the BX concentration in rye shoot tissue increased early in the season and then decreased over time. Thus, phytotoxicity measured by bioassay and BX levels measured by GC have a similar but not identical temporal profile. The observed decrease in phytotoxic potential and plant BX levels in rye later in the season appears to correlate with the transition from vegetative to reproductive growth

Seed Dormancy in Hairy Vetch (Vicia villosa Roth) Is Influenced by Genotype and Environment

Seed dormancy complicates the agricultural use of many legume species. Understanding the genetic and environmental drivers of seed dormancy is necessary for advancing crop improvement for legumes, such as Vicia villosa. In this study, we quantify the magnitude of genetic and environmental effects on physical dormancy among 1488 maternal V. villosa plants from 18 diverse environments. Furthermore, we explore the relationship between physical dormancy and environmental conditions during seed development. Additive genetic variance (h2) accounted for 40% of the variance, while the growing environment explained 28% of the variance in physical dormancy. Maternal lines showed complete variance in physical dormancy, as one line was 100% dormant, and 56 lines were 0% dormant. Distributions of physical dormancy varied widely among seed production environments, with some site-years strongly skewed toward physically dormant seed, while other site-years exhibited little dormant seed. Twenty-three weather variables were associated with environmental and error effects of physical dormancy. High mean and minimum relative humidity, low mean and maximum temperature, and high precipitation weakly grouped with low physical dormancy. Weather variables calculated from fixed time windows approximating seed maturity to seed harvest at each site-year tended to be less predictive than biological seed drying windows calculated based on seed maturity of each maternal line. Overall, individual and cumulative effects of weather variables were poor predictors of physical dormancy. Moderate heritability indicates that breeding programs can select against physical dormancy and improve V. villosa for agricultural use. Marker-based approaches would maximize selection for physical dormancy by reducing the influence of unpredictable environmental effects