Microbial population and community dynamics on plant roots and their feedbacks on plant communities

Abstract

The composition of the soil microbial community can be altered dramatically due to association with individual plant species, and these effects on the microbial community can have important feedbacks on plant ecology. Negative plant-soil feedback plays primary roles in maintaining plant community diversity, whereas positive plant-soil feedback may cause community conversion. Host-specific differentiation of the microbial community results from the trade-offs associated with overcoming plant defense and the specific benefits associated with plant rewards. Accumulation of host-specific pathogens likely generates negative feedback on the plant, while changes in the density of microbial mutualists likely generate positive feedback. However, the competitive dynamics among microbes depends on the multidimensional costs of virulence and mutualism, the fine-scale spatial structure within plant roots, and active plant allocation and localized defense. Because of this, incorporating a full view of microbial dynamics is essential to explaining the dynamics of plant-soil feedbacks and therefore plant community ecology.

Figure 1

Soil microbial feedback involves two steps. First, the composition of the microbial community differentiates on the plants because of host-specific microbial growth rates. In this illustration, the light green microbe has higher growth rates on the broad-leaved grass and the dark green microbe has higher growth rates on the narrow-leaved grass. The relative benefit to the microbes is represented by the thickness of the arrows in the fitness diagram. The second step involves differential effects of the microbes on the plant species. (Left) For host-specific pathogens, the light and dark green microbes have strongest negative effects on the broad- and narrow-leaved plants, respectively, with relative virulence represented by the thickness of the red clubs. As a result, the plants that were initially most abundant have the lowest growth rates. The net consequence of this negative feedback on plant communities is illustrated at the bottom left, with both species maintained in the community over time. (Right) However, for host-specific mutualists, the light and dark green microbes have strongest positive effects on the broad- and narrow-leaved plants, respectively. As a result, the plants that were initially most common grow best. The net result of this positive feedback on the community is a loss of diversity on a local scale with a potential for the community to reach alternate stable states.

Figure 2

In this conceptual representation of soil community feedback, the presence of Plant A causes a change in the composition of the soil community, represented by S_A. This change in the soil community can directly alter the population growth rate of Species A (represented by α_A) and it can alter the growth rate (α_B) of competing plant Species B. Similarly, the presence of Plant B can cause a change in the composition of the soil community (S_B), which can directly feed back (β_B) on the population growth rate of Plant B or indirectly feed back on the growth rate of Plant B through changes in the growth rate (β_A) of competing Plant A. The exponential model predicts that the net effect of soil community dynamics on plant species coexistence is determined by the sign and magnitude of an interaction coefficient I_S = (α_A + β_B) − (α_B + β_A), which represents the net pairwise feedback. Adapted with permission from References and .