Recent work on the ecology of mycorrhizal fungi has largely focused on characterizing soil conditions in ectomycorrhizal (ECM)-dominated ecosystems and arbuscular mycorrhizal (AM)-dominated ecosystems to understand how these groups of fungi impact biogeochemical cycles. In reality, forest ecosystems often contain both ECM and AM species, likely leading to interactions between these two fungal groups. Possible interactions in ecosystems with both AM and ECM fungi include competition for resources and complementarity in nutrient acquisition. These functions may differently influence soil carbon pools and fluxes. The objective of this study was to understand how soil carbon efflux in forests with mixtures of ECM and AM fungi differs from forests dominated by either AM or ECM-associated trees.
Work was conducted at the Hubbard Brook Experimental Forest in Woodstock, NH, where twelve 10x10 meter plots were established in June 2016. Plots contained a range of mycorrhizal associations, from 96% AM-associated to 100% ECM-associated trees by basal area. Instantaneous soil respiration was measured monthly with a greenhouse gas analyzer from July to October 2016. Litterfall mass and species composition were recorded from a collection basket adjacent to each respiration collar.
Plots were classified as ECM or AM-dominated if the total basal area associating with each fungal type exceeded 60%. Total litterfall mass did not vary among ECM-dominated plots, AM-dominated plots, and mixed plots (p=0.23). Further, average soil respiration was unrelated to ratios of litter mass from AM vs ECM trees (p=0.57), indicating that differences in litter chemistry between AM and ECM trees do not drive patterns of soil respiration at Hubbard Brook. Preliminary data suggests plots with mixtures of ECM and AM trees do not respire more than plots dominated by either ECM or AM species (p=0.11), but plot-level diversity may be positively related to soil respiration (p=0.04). These results indicate that niche complementarity between AM and ECM fungi in mixed forests may be less important than functional complementarity among tree species. These data add to the growing body of research about mycorrhizal associations as predictors of stand-level soil carbon dynamics, and future work will explore potential mechanisms behind this pattern in mixed forests.