Reprinted frorn tlte Soil Sciance Societ! of America .loumal Volume 6-3, no. 5, Sept._Oct. 1999 677 South Segoe Rd., Madison, WI 53?I I USA changes in the rate of soil respiration could have large effects on the atmospheric CO2 concentration. Current studies demonstrate that increased rates of soil respira- tion may result from increases in atmospheric CO2 (Johnsonet a1.,7994; Vose et al., 1995;Hungate et al., 1997; Ball and Drake, 1998) and/or soil temperature (Jenkinson et al., 1991; Kirschbaum, 1995;Winkler et al., 1996; Christensen et al., 1997). Soil respiration is derived from both root respiration and the respiration of soil organic matter (SOM) by heterotrophs. For this study, we define root respiration as the sum of live root respiration, mycorrhizal respira- tion, and microbial respiration of labile carbon derived from live roots. Sometimes known collectively as rhizo- sphere respiration, these processesare tightly coupled with plant photosynthesis (Robinson and Scrimgeour, 1995; Horwath et al., 1994). Increases in root respiration may indicate increasedcarbon inputs to the soil through greater photosynthesis, specific root activity, or root biomass (Rouhier et al., 1996; Hungate et al., 1997). Increases in respiration of SOM by soil heterotrophs, in contrast, reduce the potential for carbon storage in the soil. If we are to predict feedbacks between global change and soil processes, we must first understand the relativecontributions of root respirationand respiration by soil heterotrophsto total soil respiration. The relative contribution of roots to soil respiration is difficult to determine. as reflected bv the wide ranse of publishedestimates for soils undei trees and tree seedlings-from 5% (Philipson et al., 1975) to 90"/" (Rouhier et al., 1996;Thierron and Laudelout, 1996). Much of the variability in these estimates might origi- nate from the variety of measurement techniques used, eachwith a unique set of limitations.Estimates that rely Abbreviations: FACE. free air carbon dioxide enrichment: SOM. soil orsanic matter. Separation of Root Respiration from Total Soil Respiration Using Carbon-l3 Labeling during Free-Air Carbon Dioxide Enrichment (FACE) Jeffrey A. Andrews,* Kevin G. Harrison, Roser Matamala, and William H. Schlesinger ABSTRACT Soil respiration constitutes a major component ofthe global carbon cycle and is likely to be altered by climatic change. However, there is an incomplete understanding of the extent to which various processes contribute to total soil respiration, especially the contributions of root and rhizosphere respiration. Here, using a stable carbon isotope tracer, we separate the relative contributions of root and soil heterotrophic respiration to total soil respiration in situ. The Free-Air Carbon diox- ide Enrichment (FACE) facility in the Duke University Forest (NC) fumigates plots of an undisturbed loblolly pine (Pinus taeda L.) forest with CO, that is strongly depleted in rrC. This labeled CO, is found in the soil pore space through live root and mycorrhizal respiration and soil heterotroph respiration of labile root exudates. By measuring the depletion of 8C02 in the soil system, we found that the rhizosphere contribution to soil CO2 reflected the distribution of line roots in the soil and that late in the growing season roots contributed 55o/o of total soil respiration at the surface. This estimate may represent an upper limit on the contribution of roots to soil respiration because high atmospheric CO2 often increases in root density and/or root activity in the soil. Qorls contain the largestactiveterrestrialcarbon pool LJ on Earth, and through soil respiration,contribute an annual flux of CO2 to the atmosphere that is 10 times greater than that from fossil fuel combustion (Schle- singer,1997). Because of the sizeof this flux, even small J.A. Andrews, Dep. of Ecology & Evolutionary Biology-MS 170, Rice Univ.. 6100 Main Street, Houston, TX 77005-18921 K.G. Har- rison. Dep. of Geology and Geophysics, Boston College, Chestnut Hill, MA 02167; R. Matamala, Dep. of Botany, Duke Univ., Box 90340, Durham, NC 27708; and W.H. Schlesinger, Dep. of Botany and Division of Earth and Ocean Sciences.Nicholas School of the Environment, Duke Univ., Box 90340,Durham, NC 27708.This work was completed while the senior author was at the Dep. of Botany, Duke Univ., Durham, NC. Received 4 March 1998. +Corresponding author (jandrews@ruf .rice.edu). Published in Soil Sci. Soc. Am. J.63:1429-1435 fl999\.