Wavelike Distributions of Microbial Populations along an Artificial Root Moving through Soil

In a previous paper, we described wavelike distributions of bacterial populations along roots of wheat, and hypothesized that one mechanism underlying these distributions might be growth and death cycles of microorganisms in response to a moving nutrient source, the root tip. Similar wavelike distributions in microbial biomass were obtained using a simulation model for growth and death of bacteria in relation to their substrate (BACWAVE). The model was parameterized with data from one experiment on rhizosphere bacterial populations along wheat roots, and compared against a similar but independent experiment. In experiments described in this paper, similar wavelike distributions in bacterial populations were observed in response to a single artificial exudate moving linearly through a soil that had been air-dry for almost 2 years. The period of the spatial waves was longer when the tip of the artificial exudate moved at a speed of 4.2 cm/day compared to a tip moving at 1.1 cm/day, but after transformation into the temporal domain, the periods of the waves were similar for both moving speeds. The observed distributions were simulated using the BACWAVE model with similar parameter values as derived from the experiment with wheat roots mentioned above. The results presented in this paper confirm our hypothesis that wavelike distributions of bacterial population along plant roots can arise from "exudates" released primarily from the root tip, without the need for additional exudation points.