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 (δ¹³C) of CO₂ 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 respiration over 2 months in a subalpine forest in Colorado, USA (the Niwot Ridge AmeriFlux forest). Observations included automated measurements of CO₂ and δ¹³C of CO₂ in the soil efflux, the soil gas profile, and forest air. There was strong diel variability in soil efflux but no diel change in the δ¹³C of the soil efflux (δ_R) or the CO₂ produced by biological activity in the soil (δ_J). Following rain, soil efflux increased significantly, but δ_R and δ_J did not change. Temporal variation in the δ¹³C of the soil efflux was unrelated to measured environmental variables, and we failed to find an explanation for this unexpected result. Measurements of the δ¹³C of the soil efflux with chambers agreed closely with independent observations of the isotopic composition of soil CO₂ production derived from soil gas well measurements. Deeper in the soil profile and at the soil surface, results confirmed established theory regarding diffusive soil gas transport and isotopic fractionation. Deviation from best-fit diffusion model results at the shallower depths illuminated a pump-induced ventilation 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 δ¹³C of the soil efflux relative to δ¹³C of production derived from soil profile measurements was likely caused by transient pressure-induced transport with small horizontal length scales.