Heavy grazing disrupts positive effects of arbuscular mycorrhizae symbiosis on community productivity and stability under low and high phosphorus conditions

Aims We quantified the potential influence of arbuscular mycorrhizal fungi (AMF) on primary productivity and community stability and tested whether the effects of AMF depend on soil phosphorus (P) availability and grazing intensity in a semiarid grassland. Methods We manipulated the abundance of AMF and soil P availability (P addition and no P addition) under four levels of grazing intensity. The aboveground biomass of plant species, functional groups and community, as well as community temporal stability were examined under these conditions. Results Under no to moderate grazing, benomyl decreased the aboveground biomass in C.sub.4 grasses and forbs, but increased the aboveground biomass in C.sub.3 grasses under low P conditions; thus no obvious changes in community primary productivity were observed. These results indicated that the mycorrhizal dependency of C.sub.4 grasses and forbs is higher than that of C.sub.3 grasses under low P conditions. However, community primary productivity was greatly enhanced by benomyl due to the increased biomasses of C.sub.3 grasses and forbs under high P conditions. Under heavy grazing conditions, benomyl increased community primary productivity under both low and high P conditions. AMF presence increased the community temporal stability by promoting the compensatory effects between plant species under no to moderate grazing conditions; whereas AMF decreased the community stability by forming a parasitic symbiosis with their plant hosts under heavy grazing conditions. Conclusions Our findings demonstrate that the effects of benomyl application to suppress AMF on primary productivity and community stability are strongly mediated by soil P availability and grazing intensity, which improves our understanding for the roles of AMF-plant symbiosis on ecosystem functioning in grazed systems.