Mycorrhizal type effects on leaf litter decomposition depend on litter quality and environmental context

Slower leaf litter decomposition rates for trees associated with ectomycorrhizal (ECM) fungi compared to arbuscular mycorrhizal (AM) fungi may lead to the development of conservative nitrogen cycling and accumulation of soil organic matter in surface soils of ECM-dominated forests. Slower decomposition is hypothesized to occur via two often-confounded mechanisms: production of lower quality litter by ECM-associated trees compared to AM-associated trees and competition for nitrogen between ECM and saprotrophic decomposers in ECM-dominated stands. To disentangle the effects of litter quality and stand mycorrhizal type on decomposition, we measured litter mass loss rates of two AM species higher and two AM species similar in chemical quality to two ECM species. Leaf litter was decomposed for two years in neighboring ECM- and AM-dominated stands of a wet tropical montane forest. Litter phosphorus (P) was the strongest predictor of decomposition rates across all species, with no effect of litter mycorrhizal type on decomposition rates. Only one species, which exhibited intermediate litter chemical quality and decomposition rate, decomposed significantly faster in AM- compared to ECM-dominated stands. Leaf litter decomposition rates cannot be predicted directly from litter mycorrhizal type or stand mycorrhizal type because litter chemical quality and environmental conditions mediate the manifestation of slower decomposition in ECM stands.