The effects of tree-mycorrhizal type on soil organic matter properties from neighborhood to watershed scales

Abstract

A large portion of terrestrial carbon (C) is stored as soil organic matter (SOM) so the balance between C inputs to SOM and soil C loss through respiration has important implications for global climate change. Tree-mycorrhizal association is a promising predictor of SOM dynamics, with the effect of ectomycorrhizal (ECM) trees and associated 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. To determine the spatial variability of ECM effects on SOM pools within a forest stand, we used likelihood modelling techniques to compare non-spatial models, which average out ECM effects at the stand scale, against spatial models, which account for the size and spatial distribution of ECM-associated trees within a stand (i.e., the tree neighborhood). To investigate how watershed-scale variability in soil fertility and pH mediate ECM effects, we quantified forest floor leaf litter and soil properties in forest stands along mycorrhizal gradients within four adjacent watersheds that varied in parent material and rainfall. We found that ECM effects on SOM pools scaled with stand-level ECM dominance rather than varying based on individual trees at the neighborhood scale. Although forest floor leaf litter and mineral C:N ratios consistently increased with ECM dominance across watersheds, ECM effects on O horizon formation and the composition of SOM at depth differed among watersheds. Specifically, watersheds with lower soil pH and fertility exhibited greater changes along mycorrhizal gradients in O horizon depth, leaf litter δ15N and in the difference between δ15N of leaf litter and mineral soil fractions. Overall, this study shows the potential for intrinsic soil properties to mediate the effects of ECM trees and associated fungi on SOM formation and persistence in the tropics.