Separation of root respiration from total soil respiration using carbon-13labeling during Free-Air Carbon Dioxide Enrichment (FACE)

Soil respiration constitutes a major component of the global carbon cycle a nd is likely to be altered by climatic change. However, there is an incompl ete understanding of the extent to which various processes contribute to to tal soil respiration, especially the contributions of root and rhizosphere respiration. Here, using a stable carbon isotope tracer, we separate thf re lative contributions of root and soil heterotrophic respiration to total so il respiration in situ. The Free-Air Carbon dioxide Enrichment (FACE) facil ity in the Duke University Forest (NC) fumigates plots of an undisturbed lo blolly pine (Pinus taeda L.) forest with CO2 that is strongly depleted in C -13. This labeled CO2 is found in the soil pore space through live root and mycorrhizal respiration and soil heterotroph respiration of labile root ex udates. By measuring the depletion of (CO2)-C-13 in the soil system, we fou nd that the rhizosphere contribution to soil CO2 reflected the distribution of fine roots in the soil and that late in the growing season roots contri buted 55% of total soil respiration at the surface, This estimate may repre sent an upper limit on the contribution of roots to soil respiration becaus e high atmospheric CO2 often increases in root density and/or root activity in the soil.