Environmental drivers of grazing effects on arbuscular mycorrhizal fungi in grasslands

Arbuscular mycorrhizal fungi (AMF), common symbiotic root-associated soil biota, play a key role in maintaining ecosystem function and stability. However, how AMF are affected by livestock grazing in grassland ecosystem is variable, and this uncertainty in mycorrhizal responses to grazing is mainly due to the context-dependent nature of the AMF symbiosis association. The effects of grazing on AMF are through grazing-induced changes in the environment experienced by the mycorrhizal fungi and this includes both plant and soil-related factors. Here, we introduce a conceptual framework that highlights potential drivers and mechanistic pathways through which long-term grazing impacts AMF. We focus on known drivers of AMF abundance and diversity that are expected to be influenced by grazing including plant above-ground biomass and diversity, nutrient availability (soil available nitrogen and phosphorus) and edaphic properties (soil organic carbon, soil pH, soil bulk density and soil moisture). Given that the identity and relative importance of each of these drivers may differ across different ecosystems, the application of our framework of potential environmental drivers allows us to generalize, organize and help formulate hypotheses for testing the impact of various grazing management approaches on AMF. We also argue the herbivory-AMF relationship can be moderated by climate, seasonality and topography. We reviewed the current state of knowledge of livestock grazing impacts on AMF and discussed some of the reasons behind conflicting results among empirical studies. We also provide suggestions for future research aimed at improving experimental design to unravel underlying mechanisms determining the impacts of grazing on AMF.