Decomposition of plant matter is the primary pathway by which carbon and nutrients enter forest soil. While saprotrophic fungal species are known to dominate this process, evidence suggests that both arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi may also influence litter decomposition. Both AM and ECM fungi may reduce litter decomposition by outcompeting free-living soil decomposers for limiting nutrients, but both fungal groups may also accelerate decomposition by either a) exuding extracellular enzymes in the forest floor (ECM) or b) priming other microbial decomposers (AM). To test the influence of both AM and ECM fungi on litter decomposition, we established a series of paired litter decomposition plots along a mycorrhizal gradient ranging from AM-dominated to ECM-dominated soil at the Hubbard Brook Experimental Forest in Woodstock, NH. Paired plots consisted of one undisturbed control plot adjacent to a plot which was trenched to 30 cm depth, excluding the majority of fine roots and mycorrhizal hyphae. We measured litter mass loss, extracellular enzyme activity, and soil N availability in each plot. We also tested whether the effect of mycorrhizal fungi on litter decomposition may depend on substrate chemistry by incubating four species of leaf litter collected from the surrounding forest.
Results/Conclusions
Average litter mass loss varied with species, ranging from 52% mass remaining (white ash; Fraxinus americana) to 72% mass remaining (American beech; Fagus grandifolia) after 9 months of incubation. For all litter species, decomposition rate was significantly higher in subplots with access to roots and mycorrhizal hyphae compared to their adjacent trenched subplots (p=.015). Patterns in litter decomposition rate (k) along the mycorrhizal gradient also varied with species: for sugar maple (Acer saccharum) and yellow birch (Betula allegheniensis), litter decayed faster in ECM-dominated plots, while white ash litter decomposed faster in AM-dominated plots. American beech litter decomposition was not influenced by the mycorrhizal gradient. While there was no pattern in soil nitrate or ammonium concentrations across the mycorrhizal gradient, after 6 months of incubation, nitrogen-acquiring enzyme activity on beech leaves was three times higher on leaves collected from ECM-dominated plots compared to AM-dominated plots (p=.03). These results highlight the importance of soil microbial communities as biological controls on litter decomposition in northern forests.