Under pressure from above: Overgrazing decreases mycorrhizal colonization of both preferred and unpreferred grasses in the Patagonian steppe

Arbuscular mycorrhizal fungi (AMF) are related to plant community dynamics and ecosystem functioning. Overgrazing can negatively affect plant performance, and consequently unbalance the association with AMF. We studied the grazing effect on AMF colonization for preferred (Bromus pictus and Poa ligularis) and unpreferred grasses (Pappostipa speciosa and Pappostipa humilis) by sheep in the Patagonian steppe. For each species, AMF colonization in ungrazed, moderate and intense grazing sites was quantified. In ungrazed areas, B. pictus showed the highest extent of AMF colonization. Mycorrhizal colonization was higher during the active season, and largely reduced by intense grazing conditions. The decrease of AMF colonization was maximal for the most preferred species, but also significant for the unpreferred species. Our results suggest that overgrazing could reduce mycorrhizal benefits for the plant by reduction of AMF colonization, which can be a good indicator of ecosystem functioning, eventually revealing an increasing degree of environmental degradation.