Exploring a plant-soil-mycorrhiza feedback with Rhododendron maximum in a temperate hardwood forest

Ericaceous plant litter polyphenols influence the N cycle of terrestrial ecosystems through the formation of recalcitrant protein-tannin complexes. The accumulation of protein-tannin N in soils along with the ability of the ericoid mycorrhizal fungi of ericaceous plants to access protein-tannin N is a hypothesized plant-soil-mycorrhiza feedback. We examined this hypothesis in southern Appalachian forest microsites with and without the understory shrub Rhododendron maximum. Forest microsites with R. maximum have greater standing stocks of soil N and lower soil N availability compared to forest microsites without the shrub. We examined the soil fate of reciprocally placed ¹⁵N enriched protein-tannin complexes. Based upon recovery of ¹⁵N in soil N pools, microbial biomass and roots, protein-tannin complexes derived from R. maximum leaf litter were more recalcitrant than those derived from hardwood leaf litter. Ericoid mycorrhizal roots of R. maximum were more enriched in ¹⁵N compared to arbuscular and ectomycorrhizal roots of forest trees, particularly with R. maximum derived complexes. These results suggest that the N contained in recalcitrant R. maximum protein-tannin complexes are more available to R. maximum compared to forest trees. Mechanisms by which R. maximum ericoid mycorrhizal roots acquire protein-tannin N may include extracellular enzyme production by root fungi. This root fungal community is diverse and includes ericoid symbionts, ectomycorrhizal symbionts, saprotrophs and several unknown fungi. A plant-soil-mycorrhizal feedback, combined with an N conserving habit, may contribute to both the expansion of R. maximum and the concomitant suppression of other plant species in southern Appalachian forests.