Arbuscular Mycorrhizal Fungi May Account for a Phosphorus-Facilitation Strategy

Mycorrhizal phosphorus (P)-scavenging strategies are inefficient in severely P-impoverished environments; yet many mycorrhizal species occur here. How these species acquire P and how their acquisition coordinates with root and leaf traits remain unknown. We measured rhizosheath acid-phosphatases, leaf P-resorption efficiency and root and leaf economic traits and defence metabolites, such as phenolics and flavonoids, of Hibbertia hypericoides (Dilleniaceae) and H. racemosa in monoculture with arbuscular mycorrhizal fungal inoculation and in a mixture with P-mobilising Banksia attenuata (Proteaceae) with or without arbuscular mycorrhizal fungal inoculation. Phosphorus acquisition of both Hibbertia species was facilitated by B. attenuata, but through different interactions: H. hypericoides, non-mycorrhizal despite having been inoculated, used its roots, whereas H. racemosa, forming arbuscular mycorrhizal associations following inoculation, relied on mycorrhizal interactions with B. attenuata. This was linked to positions along the 'collaboration' dimension of root economic space and coordinated with root defence metabolites. Hibbertia species diverged along the leaf economic spectrum, with higher scores (more conservative strategy) negatively correlated with leaf P-resorption efficiency. We conclude that arbuscular mycorrhizal fungi play a key role in the P-acquisition strategy of H. racemosa. Root and leaf traits and rhizosheath acid-phosphatase activity efficiency interacted and contributed to the growth of both Hibbertia species.