The origin and accumulation of soil carbon and nitrogen across mycorrhizal gradients depends on soil fertility

Georgia Seyfried, Plant Biology, University of Illinois, Champaign, IL, Charles D. Canham, Cary Institute of Ecosystem Studies, Millbrook, NY, James W. Dalling, Plant Biology, University of Illinois at Urbana-Champaign, Urbana, IL and Wendy H. Yang, DOE Center for Advanced Bioenergy and Bioproducts Innovation, University of Illinois at Urbana-Champaign, Urbana, IL

Background/Question/Methods
Tree mycorrhizal association is a promising predictor of soil organic matter dynamics, with the effect of ectomycorrhizal (ECM) fungi scaling with the percent basal area of ECM-associated trees in a forest stand. However, uncertainty remains in the generality of ECM effects across the different spatial scales at which soil properties are known to vary. Concentrated effects of trees on the soil beneath them may shape variation in SOM dynamics at the neighborhood scale while watershed-scale variation in soil fertility could mediate the mechanisms by which ECM fungi affect SOM dynamics. Our study was conducted in a lower montane tropical forest in western Panama where one ECM tree species, Oreomunnea mexicana, grows in high abundance patches among otherwise species rich forest composed of trees that associated with arbuscular mycorrhizal fungi. To determine spatial variability of ECM effects within a forest stand, we used likelihood modeling techniques to compare non-spatial models in which ECM effects average out at the stand scale to spatial models which account for the DBH and spatial distribution of ECM associated trees within a stand. To test the effects of soil fertility and pH on ECM effects, we quantified forest floor and soil properties along mycorrhizal gradients within four watersheds that varied in parent material, forming soils that ranged in fertility.
Results/Conclusions
We found that ECM effects on SOM pools scaled with ECM dominance along mycorrhizal gradients rather than varying at the scale of individual trees. The direction and magnitude of ECM effects on SOM properties varied among watersheds. In lower fertility watersheds, O horizon depth and leaf litter δ15N increased with ECM dominance while δ15N of mineral associated organic matter (MAOM) and particulate organic matter (POM) decreased with ECM dominance. In contrast, in higher fertility watersheds, O horizon depth and δ15N of leaf litter, POM, and MAOM were relatively unaffected by ECM dominance. Our results suggest greater reliance of host trees on 15N-depleted N transferred from their symbionts and greater contributions of 15N-enriched N from ECM biomass to POM and MAOM fractions in lower fertility watersheds. Overall, ECM effects on SOM pools may scale with ECM dominance within a forest, but watershed-scale differences in soil nutrient availability and pH may lead to variation in this relationship among forests.