SIMULATING NUTRIENT-UPTAKE BY SINGLE OR COMPLETING AND CONTRASTING ROOT SYSTEMS

Simulation of nutrient uptake by competing root systems is necessary t o assist in the quantitative understanding of these processes, to pred ict the consequences of competition for nutrients, and to prioritize f uture research on the mechanisms of nutrient competition. Our objectiv e was to apply previously published concepts to a computer model based on solute transport theory that would have the ability to simulate nu trient uptake by root systems of one or two competing plant species. A nalytical solutions were used for calculating the volume of soil alloc ated 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 adjustme nt of the average concentration in the depletion zone to account for n ewly encountered solute as the zone increased in radius, (iii) a varia ble root-absorbing power (alpha), and (iv) a routine to correctly acco unt for all soil in simulations with two contrasting root systems. Pre dicted uptake for single-species scenarios in soils of contrasting nut rient supply characteristics was verified against that predicted by th e commonly used Barber-Cushman model, NUTRIENT UPTAKE (maximum discrep ancy was 23%), which utilizes a numerical solution. The ratio of uptak es predicted by both models was a function of alpha root radius, effec tive-diffusion coefficient, and buffer power in a more complex way tha n previously suggested in the literature. Sensitivity analysis indicat ed that, of all parameters, K uptake by slash pine (Pinus elliottii En gelm. var. elliottii) relative to competing grass would be most sensit ive to variations in root-length density of the grass, or in soil wate r content, if only soil parameters were examined.