Plant litter chemistry and mycorrhizal roots promote a nitrogen feedback in a temperate forest

Relationships between mycorrhizal plants and soil nitrogen (N) have led to the speculation that the chemistry of plant litter and the saprotrophy of mycorrhizal symbionts can function together to closely couple the N cycle between plants and soils. We hypothesized that a tannin-rich, ericoid mycorrhizal (ERM) plant promotes the retention of protein-tannin N in soil, and that this N source is accessible to saprotrophic ERM symbionts and their hosts, but remains less available to co-occurring ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) symbionts and their hosts. We tested this feedback hypothesis in a southern Appalachian forest community composed of two microsites: a hardwood microsite with ECM and AM trees in the overstorey and understorey, and an AM herb layer; and a rhododendron microsite where the understorey and herb layer are replaced by ERM rhododendron. We synthesized N-15-enriched protein-tannin complexes from leaf litter extracts representing each forest microsite and examined the fate of N-15 in soil volumes 3 months and 1 year after the complexes were placed in the field. Protein-tannin complexes derived from the rhododendron microsite led to a higher retention of N-15 in soil organic matter and a lower recovery in dissolved N pools than those from the hardwood microsite, supporting the hypothesis that rhododendron tannins create stable complexes that increase organic N retention in soils. Rhododendron complexes led to greater N-15-enrichment in ERM roots than in AM and ECM roots, supporting the hypothesis that rhododendron can better access the N complexed by its own litter tannins than can co-occurring forest trees and plants. Our results suggest that both fungal saprotrophy and a high specific root length contribute to the ability of ERM roots to acquire N from complex organic sources. Synthesis. This study provides evidence of an intricate N feedback where plant litter chemistry influences the cycle of N to maximize N acquisition by the host's mycorrhizal roots, while hindering N acquisition by mycorrhizal roots of co-occurring plants. Feedback processes such as these have the potential to drive patterns in nitrogen cycling and productivity in many terrestrial ecosystems.