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

Abstract. Recent studies have examined temporal fluctuations in the amount and carbon isotope content (δ 13 C) of CO 2 produced by the respiration of roots and soil organisms. These changes have been correlated with diel cycles of environmental forcing (e.g., sunlight and soil temperature) and with synoptic-scale atmospheric motion (e.g., rain events and pressure-induced ventilation). We used an extensive suite of measurements to examine soil respirationover 2 months in a subalpine forestin Colorado, USA (the Niwot RidgeAmeriFlux forest). Observations included automated measurements of CO 2 and δ 13 C of CO 2 in the soil efflux, the soil gasprofile, and forest air. There was strong diel variability in soil effluxbut no diel change in the δ 13 C of the soil efflux(δ R ) or the CO 2 produced by biological activity in the soil (δ J ). Following rain, soil effluxincreased significantly, but δ R and δ J did not change. Temporal variation in the δ 13 C of the soil effluxwas unrelated to measured environmental variables, and we failed to find an explanation for this unexpected result. Measurements of the δ 13 C of the soil effluxwith chambers agreed closely with independent observations of the isotopic composition of soil CO 2 production derived from soil gaswell measurements. Deeper in the soil profile and at the soil surface, results confirmed established theory regarding diffusive soil gastransport and isotopic fractionation. Deviation from best-fit diffusion model results at the shallower depths illuminated a pump-inducedventilation artifact that should be anticipated and avoided in future studies. There was no evidence of natural pressure-induced ventilation of the deep soil. However, higher variability in δ 13 C of the soil effluxrelative to δ 13 C of production derived from soil profile measurements was likely caused by transient pressure-induced transport with small horizontal length scales.