Simulating Nutrient Uptake by Single or Competing and Contrasting Root Systems

Simulation of nutrient uptake by competing root systems is necessary to assist in the quantitative understanding of these processes, to predict the consequences of competition for nutrients, and to prioritize future research on the mechanisms of nutrient competition. Our objective was to apply previously published concepts to a computer model based on solute transport theory that would have the ability to simulate nutrient uptake by root systems of one or two competing plant species. Analytical solutions were used for calculating the volume of soil allocated to each root system and the concentrations of solute at the root surfaces. We included in the model (i) a depletion zone that increased with time until it reached the no-transfer boundary, (ii) an adjustment of the average concentration in the depletion zone to account for newly encountered solute as the zone increased in radius, (iii) a variable root-absorbing power (α), and (iv) a routine to correctly account for all soil in simulations with two contrasting root systems. Predicted uptake for single-species scenarios in soils of contrasting nutrient supply characteristics was verified against that predicted by the commonly used Barber-Cushman model, NUTRIENT UPTAKE (maximum discrepancy was 23%), which utilizes a numerical solution. The ratio of uptakes predicted by both models was a function of α, root radius, effective-diffusion coefficient, and buffer power in a more complex way than previously suggested in the literature. Sensitivity analysis indicated that, of all parameters, K uptake by slash pine (Pinus elliottii Engelm. var. elliottii) relative to competing grass would be most sensitive to variations in root-length density of the grass, or in soil water content, if only soil parameters were examined. A contribution of the Agric. Exp. Stn. Journal Series no. R-02521.