Species-specific trade-offs between regrowth and mycorrhizas in the face of defoliation and phosphorus addition

Abstract

Arbuscular mycorrhizal fungi (AMF) enhance nutrient provision in exchange for carbon. However, a shift from nutrient to carbon limitation, induced by repeated or intense defoliation, can represent a trade-off between plant regrowth and the maintenance of mycorrhiza. The combined effects of AMF, phosphorus and defoliation on growth of A gropyron elongatum (C 3 grass, low mycorrhizal responsiveness) and B rachiaria brizantha (C 4 grass, high mycorrhizal responsiveness) were explored. Each species was subjected to a manipulative experiment with AMF inoculation (non-inoculated, inoculated), soluble P supply (low, high) and defoliation (non-defoliated, 60% defoliated). In A. elongatum , at low P supply, mycorrhizal plants showed increased growth rates following defoliation without substantial changes in AMF colonization. At high P supply instead, we found a clear trade-off between regrowth and the maintenance of mycorrhiza evidenced by growth depression (biomass and tillers) and lower AMF activity (reduction of arbuscules). In contrast, in B. brizantha , defoliation effects on plant regrowth were independent from AMF at any P supply (no trade-off). This indicates that cost-benefit relationship in defoliated plants is highly context-dependent and may vary with mycorrhizal responsiveness of species. This variation of responses can play a decisive role on plant recovery in pastures and natural grasslands subjected to herbivory. • First comparative study of AMF, defoliation and P supply in C 3 and C 4 grasses. • In the C 3 grass, AMF promoted recovery from defoliation at low P supply. • Mycorrhiza and defoliation trade off in the C 3 grass at high P supply. • In the C 4 grass, mycorrhizae were beneficial at all environmental contexts. • AMF effects on the recovery from defoliation depend on P supply and plant identity.