Modeling microbial communities in soil food webs

Background/Question/Methods

A growing body of food web theory studies the paradoxical idea that herbivory can have positive feedbacks on nutrient flow to plants. In more detail, the relationships between root grazing herbivores, microbial pathogens and microbial decomposers may heavily influence plant primary production.  By developing a theoretical model combined with glasshouse experiments using dune grasses, ectoparasitic nematodes and dune microbial inoculum, we tested the idea that plants, microorganisms and plant herbivores are in a delicate balance between maximizing nutrient mineralization, inhibiting microbial pathogens and maintaining plant biomass.  Individual plant species have their own local microbial flora, and this flora interacts with grazing nematodes to affect their population size as well as plant growth.  Plants tend to leak carbon through their roots.  This is ultimately beneficial to the plant by supporting a robust microbial flora.  Further carbon is released into the soil when herbivores graze on roots.  In fact, nematode grazing activity may stimulate microbial growth through an increased availability of root carbon.  The increase in microbial growth and metabolism likely results in more mineralized nutrients available to plants.  We hypothesize that there is an intermediate level of grazing intensity at which positive feedbacks to plant growth are optimized.  We have designed a series of differential equations to address this hypothesis.  In our model we manipulate grazing intensity through both the biomass of herbivores in the rhizosphere as well as the efficiency with which the herbivores utilize root carbon.  We parameterize the model and the equations with values from the literature and our own glasshouse experiment.

 Results/Conclusions

The results of our model show that herbivore efficiency can have direct impacts on the microbial metabolism and feedbacks to plant growth.  Specifically, when herbivores are less efficient with their consumption and more carbon is leaked to the soil, microbial activity increases, nutrients are mineralized and plant growth increases.  Our research describes an unexplored interaction network in soil food webs where herbivorous nematodes and microbial activity positively affect nutrient availability and plant productivity.