In soil, the relationships between root grazing herbivores, microbial pathogens and microbial decomposers heavily influence plant primary production. In glasshouse experiments using native dune grasses, ectoparasitic nematodes and local microbial inoculum, we tested the idea that plants, microorganisms and plant parasites are in a delicate balance between maximizing nutrient mineralization, inhibiting microbial pathogens and maintaining plant biomass. We did this by addressing the following questions: 1.) Does root herbivory increase nutrient cycling through the microbial loop, thereby increasing bacterial and protozoan biomass?  2.) How do these changes affect plant primary production? We answered these questions in an empirical study using coastal dune plants potted in a glasshouse.  The experiment consisted of five treatments: low nematode density, high nematode density, carbon amendment (to simulate root leakage), carbon and nitrogen amendment (to simulate root leakage and mineralization by consumers) and untreated control. We used phospholipid fatty acid analysis to determine microbial loop response, and we measured plant response as above and below ground biomass.

Results/Conclusions

Microbial enemies of nematodes can control population size of root grazing nematodes, thus protecting the plant from overgrazing.  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.  However microbial nematode control is dependent on plant and nematode species, and the mechanisms responsible for this variation have not been established.  In fact, nematode grazing activity may stimulate microbial growth through mineralization of nutrients and increased availability of root carbon. In our research, we are resolving the mechanisms of interaction between root herbivores, microorganisms (the microbial loop including protozoa and bacteria) and the plant host. Specifically, we focus on the microbial loop for its critical role in nutrient mineralization. In the past, both positive and negative feedbacks between microorganisms and herbivorous nematodes have been established.  Our research describes an unexplored interaction network in soil food webs where herbivorous nematodes and the microbial loop affect nutrient availability and plant productivity.