Plant growth component of a simple rye growth model

Cover cropping practices are being researched to reduce artificial subsurface drainage nitrate-nitrogen (nitrate-N) losses from agricultural lands in the upper Mississippi watershed. A soil-plant-atmosphere simulation model, RyeGro, was developed to quantify the probabilities that a winter rye cover crop will reduce artificial subsurface drainage nitrate-N losses given climatic variability in the region. This article describes the plant growth submodel of RyeGro, Grosub, which estimates biomass production with a radiation use efficiency-based approach for converting intercepted photosynthetically active radiation to biomass. Estimates of nitrogen (N) uptake are based on an empirical plant N concentration curve. The model was calibrated with data from a three-year field study conducted on a Normania clay loam (fine-loamy, mixed, mesic Aquic Haplustoll) soil at Lamberton, Minnesota. The model was validated with data measured from a field trial in St. Paul, Minnesota. The cumulative rye aboveground biomass predictions for the calibration years differed by -0.45, 0.09, and 0.16 Mg ha ^-1 (-17%, 9%, and 32%), and the plant N uptake predictions differed by -10.5, 8.0, and 4.0 kg N ha ^-1 (-16%, 30%, and 21%) from the observed values. The predictions of biomass production and N uptake for the validation year varied by -1.4 Mg ha ^-1 and 16 kg N ha ^-1 (-27% and 24%) from the values observed in the field study, respectively. A local sensitivity analysis of eight input parameters indicated that model output is most sensitive to the maximum leaf area index and radiation use efficiency parameters. Grosub demonstrated the capability to predict seasonal aboveground biomass production of fall-planted rye in southwestern Minnesota within an accuracy of ±30% in years when production exceeds 1 Mg ha ^-1 by mid-May, and to predict seasonal rye N uptake within ±25% of observed values.