Measured forest soil C stocks and estimated turnover times along an elevation gradient

Abstract

The purpose of this research was to measure forest soil C stocks and estimate turnover times along a 1.3 km rise in elevation in the southern Appalachian Mountains. Turnover times were calculated based on estimated soil C inputs and near steady state soil C stocks in the O-horizon and the mineral soil to a 30-cm soil depth. Monte Carlo methods were used in the calculations to accommodate uncertainties in model parameterization. Measured mean stocks and calculated median turnover times of soil C ranged from 4.4 to 12.2 kg C m − 2 and 11 to 31 years, respectively. The predicted turnover times reflect a high proportion of labile soil C over the selected sampling depth. Both forest soil C stocks and the predicted turnover time of soil C increased with altitude. There were no consistent trends for stand basal area and forest soil C inputs along the elevation gradient suggesting that altitudinal changes in soil C stocks and turnover times could be attributed to different rates of soil organic matter decomposition. Soil respiration flux did not vary significantly with altitude, but soil respiration normalized for existing soil C stocks indicated decreasing rates of soil organic matter decomposition with increasing elevation. Measured soil N stocks and differences in O-horizon litter chemistry indicated increasing N availability from low-elevation to high-elevation forests. Soil C storage and turnover time increased from warmer, drier, less N-rich forests to colder, wetter, more N-rich forests. Considered together, these data support assumptions of a declining rate of soil organic matter decomposition with increasing elevation in the southern Appalachian Mountains. Representative characterization of long-term litter C inputs is one of the most challenging aspects of the present analysis.