Partitioning biotic and abiotic components of soil CO2 fluxes using subsurface CO2 dynamics and stable carbon isotopes, Taylor Valley, Antarctica

Abstract

To date, surface CO₂ fluxes in soils of the McMurdo Dry Valleys in Antarctica have been assumed to represent heterotrophic respiration. Important aspects of soil ecosystem function have been inferred on the basis of this assumption. This study used high-resolution measurements of CO₂ concentration, and, for the first time, the stable C isotopic composition of surface CO₂ fluxes and subsurface CO₂ profiles to: 1) link surface and subsurface CO₂ dynamics, 2) test whether abiotic mechanisms of CO₂ production and consumption can explain diel variability in surface CO₂ flux rates, and 3) partition biotic and abiotic components of surface CO₂ fluxes at two sites with contrasting organic C levels in soils of Taylor Valley. In the 2008/09 austral summer, surface CO2 flux rates and subsurface CO₂ concentration and δ13CCO2 profiles were measured at the warmest and coolest times of the day at two sites with contrasting soil organic C contents. Site A, located near a small lake, can be considered a “biological hotspot”, with the lake providing a contemporary organic C source via algal growth and subsequent wind-dispersal onto nearby soils. Site B had no contemporary lacustrine-derived organic C supply, and soil organic C levels were typical of the low levels in Dry Valley soils. A physical model, describing the temperature-driven dissolution and exsolution of CO₂ according to Henry’s Law, and incorporating empirical data from field measurements of soil moisture content, pH, and temperature changes, showed that an abiotic mechanism of CO₂ uptake and release could explain changes in subsoil CO₂ concentrations. However, the twice-daily sampling regime did not resolve diel changes or rates of change in surface CO₂ fluxes and subsurface CO₂ concentration and δ13CCO2 profiles. During the 2009/10 austral summer, a 48-h time-series of surface CO2 fluxes and subsurface CO₂ concentration and δ13CCO2 profiles were measured simultaneously at Sites A and B, at 4-hourly intervals. The increased sampling resolution revealed dynamic changes in subsurface soil CO₂ concentration and δ13CCO2 profiles as soil temperatures varied throughout the diel cycle. At Site B, highly depleted surface CO₂ fluxes were explained by dynamic fractionation effects, and could not be attributed to biological respiration. Short-lived (4 h) periods of steady-state efflux and influx showed CO₂ production (exsolution) and consumption (dissolution) of relatively enriched (−5.2‰) and depleted CO₂ (−11.4‰), respectively. This is consistent with the kinetic fractionation expected as a result of preferential exsolution of 13CO2 and dissolution of 12CO2. At Site A, static chambers were unreliable for determining surface CO₂ flux rates. Nonetheless, variation in subsurface CO₂ concentration and δ¹³CCO2 profiles was consistent with heterotrophic respiration dominating soil CO₂ production. Unexpectedly, the highest rates of CO₂ production occurred during cooler periods of the day, possibly as a result of cooling-induced condensation providing biota with water sufficient to stimulate their activity. An abiotic contribution to Dry Valley soil CO₂ fluxes must be considered in future studies of Dry Valley ecosystem activity and C cycling. Measures of surface CO2 flux at single points in time do not necessarily capture a solely biological signal.