Mathematical modeling of PGPR inoculation into the rhizosphere

One of the main problems when introducing beneficial microbes to the plant rhizosphere is that the plant growth promoting rhizobacteria (PGPR) do not survive or do not execute their specific function. The goal of our research was to evaluate microbial inoculant survival in rhizospheres, using mathematical modeling and computer-based simulations. We tested several abiotic factors effects on PGPR survival: the availability of soluble organic compounds and molecular oxygen, and the concentration of mineral nitrogen in soil. The principal biotic factors considered were the direct and indirect interactions between PGPR and resident microorganisms, protozoan predation, and bacterial parasitism. A model system of four non-linear ordinary differential equations was developed to simulate the growth of PGPR populations in the rhizosphere. Simulation results indicated that the competition for limiting resources between the introduced population and the resident microorganisms was the most important factor determining PGPR survival. The most effective PGPR inoculation was expected in organic and mineral poor soils or stressed soils, when development of the resident microflora was inhibited. Another important factor for PGPR survival was compatibility between the composition of the host plant root exudates, and ability of the PGPR to utilize those compounds.