Mycorrhizal fungi provide plants with nutrients in return for photosynthate, providing the key functional link between terrestrial aboveground and belowground processes. There are systematic differences in soil carbon and nitrogen cycling in temperate forests dominated by arbuscular mycorrhizal (AM) vs. ectomycorrhizal (EcM) associations, but it is unclear if these are driven by differences in litter quality between plant species or mycorrhizal nutrient acquisition strategies. We measured effects of mycorrhizal associations on the distribution of soil C and N pools among particulate and mineral fractions. We used EcM fungi abundance as a proxy for fungal effects and canopy tree family dominance as proxy for litter quality effects. We hypothesized that EcM abundance will more strongly affect soil nitrogen while the dominance of tree families with slow-decomposing litter will more strongly affect soil carbon. We collected mineral soil (0-10 cm) across forested gradients of AM to EcM dominance in four sites across the eastern US. Within each plot, we identified all trees and measured their diameter at breast height and used ITS sequencing to identify soil fungi. The soils were fractionated into particulate and mineral-associated pools by density and also subjected to short-term incubations to test carbon availability and temperature sensitivity.
Results/Conclusions
All sites had gradients of the basal area of EcM-associated trees, but the dominant families of trees differed among sites. For example, the New Hampshire sites were dominated by trees in the Pinaceae, while the Illinois sites were dominated by trees in the Juglandaceae. The proportion of mineral-associated carbon decreased significantly as the basal area of Pinaceae and the percentage of EcM fungi increased. However, the effect of the dominant tree families was larger than the effect of the EcM fungi. The amount of nitrogen associated with minerals was only affected by the tree community and not the fungal community. Dominant plant families were also a more important driver of the availability and temperature sensitivity of soil C than the fungal community. Our results indicate that the large-scale patterns seen in soil C and N with changes in the dominance of EcM-associated trees are likely driven by the trees themselves and not their fungal symbionts, but targeted experiments are needed to definitively establish causation.