Role of Mycorrhizal Symbioses in Phosphorus Cycling

Roots of most terrestrial plants are colonized by symbiotic mycorrhizal fungi. These fungi confer various benefits to their hosts under phosphorus (P) limitation and other stress conditions. The most widespread are the arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) types. Many mycorrhizal fungi can efficiently take up P from the soil solution and transport it to the plants. This plant-directed P flux is fueled by a net flux of carbon in the opposite direction. Evidence exists that some ECM and other mycorrhizal fungi also exude large amounts of lytic enzymes and organic acids, which in turn release recalcitrant organic and mineral P into the soil. These processes can bypass organic P mineralization by free-living decomposers, effectively short-circuiting soil–plant P cycling. In addition, specific prokaryotes associate with mycorrhizal fungal hyphae, potentially enhancing access to recalcitrant P forms. This is particularly important for the AM fungi that seem to have little direct access to those P forms. The consequences of mycorrhizal diversity for soil–plant P cycling and the impact of human activities on it are briefly outlined.