Modeling wave-like dynamics of oligotrophic and copiotrophic bacteria along wheat roots in response to nutrient input from a growing root tip

Dynamics of oligotrophic bacteria (OB) have not been modeled in soil nor along roots. We extended a spatial-temporal model “BACWAVE”, describing wave-like dynamics of copiotrophic bacteria (CB) isolated on C-rich media to include dynamics of OB isolated on C-poor media and broad-range bacteria (BRB) that can grow on both types of media. BACWAVE simulates wave-like distributions of bacteria along wheat roots from their temporal dynamics with growth and death rates depending on readily utilizable substrate (RUS) concentrations, consisting of a pulse of substrate from a passing root tip and constant substrate background flux (BGF) from decaying root cortex, dead bacterial cells and soil organic matter. For this paper, colony forming units (CFUs) of bacteria isolated on poor and rich media from wheat roots (24 samples along each root) in a low C soil (0.7% C) were transformed from spatial CFUs to temporal biomass, taking root growth rate, bacterial cell sizes and cultivability into account. Contrary to the relative growth rates of CB and BRB, which increase with substrate according to Monod equations, the relative growth rate of OB is related to substrate concentration according to a modified Monod equation with a quadratic inhibition term. Relative death rates differ between trophic groups in that CB and BRB death rates decrease monotonically with substrate concentration, while the OB death rate first decreases and then increases with substrate concentration. Model parameters and initial values of state variables were estimated with an iterative optimization method based on experimental data, and fell in the range of values reported in the literature. Initial microbial biomass and kinetic parameters were lower for OB than for BRB and CB. The modified model was tested with an independent data set of bacteria along wheat roots in a soil with higher C content (1% C). BGF and initial microbial populations were higher than for the calibration data set, but other model parameters were the same for both data sets. A satisfactory fit was obtained between experimental and modeled CFUs on rich and poor media.