Exploring a plant-soil-mycorrhiza feedback with Rhododendron maximum in a temperate hardwood forest
MetadataShow full item record
Rhododendron maximum is altering plant diversity and composition in southern Appalachian forests, but the mechanisms by which it does so are not fully understood. R. maximum may alter the nitrogen (N) cycle and create a N-based plant-soil-mycorrhiza feedback. Standing stocks of soil organic matter and inputs of leaf and root litter were greater in forest microsites with R. maximum than those without. Tannin extracts from R. maximum litter had a relatively high capacity to precipitate protein compared to extracts from tree litter. Across the growing season, soil inorganic N availability was generally lower in R. maximum soils. Our data suggest that R. maximum litter alters N cycling through the formation of recalcitrant protein-tannin complexes. 15We examined the soil fate of reciprocally-placed N enriched protein-tannin complexes. Based 15upon recovery of N from soil N pools and microbial biomass, protein-tannin complexes derived from R. maximum leaf litter were more recalcitrant than those from hardwood trees. Ericoid 15mycorrhizal roots of R. maximum were more enriched in N compared to ecto-and arbuscular mycorrhizal roots, particularly with R. maximum derived protein-tannin complexes. These results suggest that R. maximum has greater access to the N complexed by its own litter tannins compared to other forest plants and trees. We characterized the composition of the ericoid mycorrhizal root fungal community of R. maximum using both a culture-based and cloning-based approach (direct DNA extraction and amplification of the ITS region) and observed 71 putative fungal taxa. Fungi include ericoid symbionts Rhizoscyphus ericae and Oidiodendron maius, several potential symbionts in the Helotiales, Chaetothryiales and Sebacinales, several ectomycorrhizal taxa, saprotrophs and a plant pathogen. This fungal community may aid in the breakdown of complex organic substrates by producing extracellular enzymes such as laccase. We characterized a portion of the laccase gene sequence from genomic DNA of cultured root fungi. Fungal taxa possessed one to four unique laccase gene sequences and sequence polymorphisms were not related to fungal taxonomy. A N-feedback between R. maximum, soils and mycorrhizal fungi may contribute to both the expansion of this shrub and the concomitant suppression of other plant species in southern Appalachian forests.