Soil carbon release along a gradient of physical disturbance in a harvested northern hardwood forest

Changes in soil respiration associated with forest harvest could increase net loss of CO2 to the atmosphere relative to pre-harvest values. By excavating quantitative soil pits across a gradient of physical disturbance in a harvested northern hardwood forest, this study examines C release from mineral soil. Mineral soil samples were analyzed for pH, percent organic matter (%OM), C and N concentration, δ13C, and total C per unit area. Results show a relationship between degree of disturbance and C concentration in soil 10–30 cm beneath the O-horizon. Highly disturbed sites show C depletion, with horizons from disturbed sites containing 25% less total C than the least disturbed sites. δ13C signatures of soil profiles at these sites show vertical mixing of plant-derived material into deeper mineral horizons. Mixing, as a result of physical disturbance, could have led to the observed C depletion by physical or chemical destabilization, or through the promotion of microbial respiration in deep mineral soil. Regardless of the mechanism, these results suggest elevated CO2 emissions from soil following harvest, and, thus, have implications for the validity of wood biomass as a carbon neutral energy source.

Research highlights▶ Forest soils that were physically disturbed during harvest contain lower amounts of total carbon at 10–30 cm depth in the mineral soil than less disturbed sites. ▶ δ13C trends of highly disturbed soil profiles suggest that isotopically depleted forest-floor derived material has been transported into deeper mineral soil horizons, perhaps by increased dissolved organic carbon (DOC) flux. ▶ Increased microbial activity, as indicated by δ13C, in highly disturbed soils likely has contributed to carbon depletion in certain mineral soil horizons. ▶ Effects of high-disturbance forest harvesting likely extend into deep mineral soil carbon pools, which may result in greater impacts on atmospheric CO2 concentration than is currently estimated.