[1] Canopy and soil CO2 concentration and isotopic measurements were conducted in a slash pine forest during a progressive drought in the southeastern United States (1) to determine and compare variations in delta(13)C of foliage (deltaCf), soil (deltaCs), and ecosystem-respired CO2 (deltaCr) and (2) to evaluate the usefulness of a two end-member oxygen isotope ratio (delta(18)O of CO2) approach to partition nighttime ecosystem respiration into soil and plant components at different heights within the canopy. The deltaCf was enriched by 2.2+/-0.3 (standard error) parts per thousand during the extreme drought in May relative to September when precipitation was above normal. The enrichment in deltaCf exceeded changes in deltaCr and deltaCs for the same time period (1.6+/-0.5 and 1.0+/-0.4parts per thousand, respectively). Lower variations in the C-13 of soil-respired CO2 relative to the variations of the autotrophic component can buffer changes in deltaCr, which integrates the C-13 signature of canopy and soil respired CO2. Application of a two end-member model to canopy delta(18)O-CO2 indicated that soil CO2 contribution to nighttime CO2 buildup within the canopy decreased from 100% at the soil surface to 0% within the canopy in September. In May, during the extreme drought period, soil respiration rate was 2.7 times lower than the rate in September despite similar soil temperatures (23degrees in May versus 19degreesC in September). Soil and aboveground respiration equally contributed to the nocturnal CO2 buildup within the canopy in May. Our model was limited in that it produced an upper limit value for soil respiration and neglected respiration by woody tissue.
