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Recent studies have examined temporal fluctuations in the amount and carbonisotope content (δ13C) of CO2 produced by the respiration ofroots and soil organisms. These changes have been correlated with diel cyclesof environmental forcing (e.g., sunlight and soil temperature) and withsynoptic-scale atmospheric motion (e.g., rain events and pressure-inducedventilation). We used an extensive suite of measurements to examine soilrespiration over 2 months in a subalpine forest in Colorado, USA (the NiwotRidge AmeriFlux forest). Observations included automated measurements ofCO2 and δ13C of CO2 in the soil efflux, the soil gasprofile, and forest air. There was strong diel variability in soil efflux butno diel change in the δ13C of the soil efflux (δR)or the CO2 produced by biological activity in the soil(δJ). Following rain, soil efflux increased significantly,but δR and δJ did not change. Temporalvariation in the δ13C of the soil efflux was unrelated to measuredenvironmental variables, and we failed to find an explanation for thisunexpected result. Measurements of the δ13C of the soil effluxwith chambers agreed closely with independent observations of the isotopiccomposition of soil CO2 production derived from soil gas wellmeasurements. Deeper in the soil profile and at the soil surface, resultsconfirmed established theory regarding diffusive soil gas transport andisotopic fractionation. Deviation from best-fit diffusion model results atthe shallower depths illuminated a pump-induced ventilation artifact thatshould be anticipated and avoided in future studies. There was no evidence ofnatural pressure-induced ventilation of the deep soil. However, highervariability in δ13C of the soil efflux relative to δ13Cof production derived from soil profile measurements was likely caused bytransient pressure-induced transport with small horizontal length scales

Environmental forcing does not induce diel or synoptic variation in the carbon isotope content of forest soil respiration