PHOSPHORUS UPTAKE BY ROOT SYSTEMS - MATHEMATICAL-MODELING IN ECOSYS

The uptake of P by plant root systems is believed to be controlled by the concentration of soluble orthophosphate at the root surface. If a P transformation model in which this concentration is calculated were coupled to a root and mycorrhizal growth model in which this concentra tion is used to calculate P uptake, then it should be possible to simu late P uptake under different soil and climate conditions if soil prop erties relevant to the control of P concentration are known. To test t his idea, models for the transformation and transport of inorganic and organic P were coupled to ones for root growth and nutrient uptake as part of the ecosys modelling program. Seasonal estimates of soluble P concentration, root growth and P uptake from the combined models were tested with data measured from barley under fertilized and unfertiliz ed treatments in a long term P fertilizer experiment conducted on two different soils. In both soils the fertilizer treatment increased simu lated and measured soluble P concentrations from 0.1-0.2 to 0.2-0.4 g m(-3), annual P uptake from 0.6-0.7 to 1.2-1.4 g m(-2), and annual DM accumulation from 400-500 to 700-800 g m(-2), Increases in soluble P c oncentrations caused by fertilizer P were reproduced in the model from changes in the balance between the desorption and dissolution of soli d P on one hand, and the uptake of P by root and mycorrhizal systems o n the other. Increases in P uptake caused by fertilizer P were reprodu ced in the model from higher solution P concentrations, root uptake ki netics, and from functional equilibria for C and P exchange simulated among mycorrhizal, root and shoot components of the plant. There was a tendency in the model to overestimate P uptake later in the growing s eason in the unfertilized treatment which could be corrected if parame ters for root uptake kinetics were reduced after anthesis. Because the model is constructed independently of data for P uptake, and avoids t he use of site-specific parameters, it may provide a means of estimati ng uptake under different managements and climates from soils of known properties.