Belowground Carbon Turnover and Ecosystem Carbon Dioxide Source Attribution: A Radiocarbon-Based Approach.

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How the net uptake of atmospheric CO₂ concentration by forests will respond to future changes in climate and land-use is not well understood. Across deciduous broadleaf forests, variations in soil respiration (R_S) explain much of the spatial and inter-annual variability in net ecosystem exchange (NEE). R_S is likely to be influenced by climate and land-use change, but prediction of future changes is challenged by difficulty in modeling fluxes over space and time. One challenge is that R_S includes CO₂ released via autotrophic (root) respiration and heterotrophic (microbial) respiration, very different processes that respond to temperature and moisture differently, and are likely to respond asymmetrically to future changes in climate and ecosystem characteristics. In addition, changes in climate or land-use that favor microbial activity may change C sources used by microbes (e.g. cause priming of stable soil C).

Our goal is to determine the short-term effects of forest management on R_E and R_S fluxes in a mature northern hardwood forest and to attribute changes in the regional biosphere signal of atmospheric ¹⁴CO₂ to this disturbance using ¹⁴C as an isotopic tracer. Eddy covariance methods can be used to estimate bulk CO₂ fluxes but they cannot discriminate process nor C source of respired CO₂. It is these processes, which are parameterized in predictive models and may change with future climate and land-use change that our observations address. We are comparing observations of ¹⁴C in CO₂ fluxes and soil organic matter pools of varying turnover time made before and after selective harvest for a well studied region to determine which belowground carbon pools contribute to changes in overall ecosystem C storage and flux following management induced disturbance. Our approach and preliminary pre-treatment results will be presented.