FACE Experiments Quantify Tree Root Turnover and Carbon Sequestration Potential of Forest Soil

Summary

Stable isotopes of carbon were used as tracers at two FACE (Free-Air-CO₂-Enrichment) experiments to determine rates of root turnover and the transformation of carbon in tree roots to soil organic matter in a pine and a deciduous hardwood forest, two common forest types in North America. The new and unique information comes from FACE experiments at Duke University and the Oak Ridge National Laboratory, and is published in a November 21, 2003, Science article (“Impacts of fine root turnover on forest Net Primary Production and soil carbon sequestration potential”) authored by Roser Matamala (Argonne National Laboratory) and colleagues. The research illustrates variability in root turnover among different forest types. The results show that growth at elevated CO₂ did not accelerate root turnover in either the deciduous hardwood forest or the pine forest. Turnover of small tree roots was found to range from 1.2 to 9 years, and small roots of the hardwood trees had the most rapid turnover. Prior to these findings, small roots of trees were thought to live no longer than a year. In addition, results of the research showed that transformation of carbon in roots to soil organic matter occurs more rapidly and in greater quantity with hardwood than pine species. The research is important when considering “natural” carbon processes for sequestering excess carbon from energy sources. The longer turnover times, for example, suggest that root production and turnover in forests have been overestimated and that sequestration of anthropogenic atmospheric carbon in forest soils may be lower than currently estimated. The understanding of mechanisms also suggests improved carbon management approaches for slowing the rise of atmospheric CO₂, and partially mitigating greenhouse gas induced climate change.