Tree mortality from insect infestation enhances carbon stabilization in southern Appalachian forest soils
Forest insect and pathogen outbreaks may exacerbate anthropogenic climate change if they accelerate soil carbon loss to the atmosphere. We quantified soil respiration and carbon content for nearly a decade after girdling or natural infestation of hemlock (Tsuga canadensis L. Carr., a codominant species in southern Appalachian forests) by hemlock woolly adelgid (Adelges tsugae) to improve understanding of soil carbon response to disturbance from forest insect and pathogens. From 2005 to 2013, net soil respiration was similar among hemlock mortality (~50% basal area reduction) and reference hardwood plots, but both girdled and hemlock woolly adelgid-infested plots showed greater activities of β-glucosidase (a cellulose-hydrolyzing extracellular enzyme), decreased O-horizon, and decreased fine root biomass. During this period, mineral soil carbon accumulated at a higher rate in hemlock mortality plots than in reference plots in both surface (0–10 cm) and subsurface (10–30 cm) soils, driven by increases in the mineral-associated fraction of the soil organic matter. In contrast, particulate organic matter (POM) carbon accrued slowly in surface soils and declined in the subsurface of girdled plots. δ13C values of the POM fraction demonstrate increased microbial processing of surface soil organic matter over time, suggesting enhanced decomposition of organic matter in this pool. These findings indicate that hemlock mortality in this system has led to enhanced soil carbon stabilization through the transformation and translocation of carbon from detrital and POM pools to the mineral-associated organic matter pool. Accelerated responses in the girdled versus naturally infested treatments highlight limitations associated with using girdling to simulate natural mortality.