Most vascular plants form symbioses with mycorrhizal fungi that associate with roots and provide nutrients to their hosts in exchange for carbohydrates, as well as serve a range of other functions. Although mycorrhizal fungi have been studied extensively in upland ecosystems, we know much less about their ecology at aquatic-terrestrial interfaces along rivers. The objectives of this study were 1) to determine how abundances of arbuscular mycorrhizal (AM) fungi change through time as floodplain surfaces develop and 2) to describe vertical distributions of AM fungi from the forest floor to the water table along the freely flowing Tagliamento River in northeastern Italy. We sampled surface sediments (0-10 cm) from three stages of floodplain island development (depositional surfaces, pioneer islands, and established islands). We also sampled sediments vertically (0-150 cm) from the ground surface to the intersection with the water table of an established island. From these samples we characterized abundance of AM fungal propagules (infected root fragments, spores, and hyphae), as well as potential for AM colonization with a bioassay conducted under greenhouse conditions.

Results/Conclusions

Roots available to host AM fungi were not present on depositional surfaces; however, assays of mycorrhizal inoculum potential and direct measures indicated some fungal propagules were available on these young surfaces. AM fungi colonized roots from both pioneer and established islands. Mycorrhizal inoculum potential was indistinguishable between pioneer and established islands, but higher than on depositional surfaces. In addition, AM fungi were present at all depths sampled (including at the intersection with water table), providing some of the first documentation of these symbionts at such depths in a riparian setting. Mycorrhizal fungi potentially link aquatic and terrestrial habitats through direct connections with plants, soil, and ground water and may influence nutrient transfers among these subsystems.