A MATHEMATICAL-MODEL FOR DISPERSAL OF BACTERIAL INOCULANTS COLONIZINGTHE WHEAT RHIZOSPHERE

A mathematical model has been constructed to describe bacterial growth and movement in the rhizosphere. In the model, bacteria are introduce d into the soil on inoculated seeds and growth occurs, after seed germ ination, on material produced as root exudates. Movement of substrates away from the rhizosphere into the bulk soil is by diffusion and micr obial movement is mediated by carriage on the root surface. The relati onship between specific growth rate and substrate concentration is des cribed by Monod kinetics and death occurs at a constant specific rate. An important component of the model is treatment of the effects of ma tric potential on the distribution and activity of bacteria in differe nt microhabitats. Simulation of the model quantifies the distribution of both bacteria and substrate with depth and time in the rhizosphere and demonstrates significant differences between substrate concentrati ons at high and low matric potentials. Sensitivity analysis of model p redictions indicates the parameters which govern microbial growth to b e more important determinants of microbial movement than plant-associa ted parameters. Predictions of the model compared well with experiment al data on microbial movement in the rhizosphere of wheat plants grown in microcosms, and inoculated with luminescence-marked Pseudomonas fl uorescens, and provide the basis for quantitative risk assessment foll owing environmental release of genetically-engineered microorganisms.