Many plants form associations with arbuscular mycorrhizal fungi (AMF). In this mutualistic interaction, the plant provides carbon to the AMF in exchange for mineral nutrition. This transfer of carbon to AMF represents a direct input of plant-derived carbon to the soil organic carbon (SOC) pool. Trophic interactions have the potential to disrupt this relationship, as fauna feed on plant and fungal tissues. In two experiments, we sought to understand how aboveground herbivory and belowground fungivory altered plant productivity and AMF growth and activity in a switchgrass (Panicum virgatum) system. For the first experiment, we grew switchgrass inoculated with local soil microbes including AMF, and we then manipulated the presence/absence of an aboveground herbivore, fall armyworm (Spodoptera frugiperda). Plants were exposed to the herbivore for two weeks. For the second experiment, we grew switchgrass inoculated with local soil microbes including AMF, and we then manipulated the presence/absence of the belowground fungivorous nematode Aphelenchus avenae. Plants were exposed to the fungivore for three months. For both experiments, we assessed above- and belowground plant productivity, AMF activity in switchgrass roots and in soil, and overall soil microbial activity.
Results/Conclusions
In the first experiment, we found the aboveground herbivory resulted in a 44% decrease in leaf area, 67% decrease in aboveground plant biomass, and a 42% decrease in belowground plant biomass. Control plants had AMF root colonization levels of 45%, while plants exposed to herbivory had colonization rates of 58%. This indicates that plants exposed to herbivore stress may be relying more on their AMF mutualists. Soil microbial activity, as measured via extracellular enzyme activity, was strongly reduced for plants in the herbivory treatment with reductions ranging from 44-78%. In the second experiment, we found that belowground fungivory resulted in a 21% decrease in belowground plant biomass, while having no significant effect on aboveground plant biomass. Analysis of AMF and soil microbial responses to fungivory are ongoing, but preliminary data suggest that fungivory has no significant effect on AMF activity, with root colonization rates around 55% for plants with and without fungivores. Collectively, our results indicate that tri-trophic interactions may have effects on mycorrhizal and soil microbial activity, and these effects appear to be stronger from herbivory relative to fungivory. This has implications for how much C is allocated belowground and ultimately processed by the soil microbial community.