ALBERT

Description: Measuring and modelling the isotopic composition of soil respiration: insights from a grassland tracer experiment Biogeosciences, 8, 1333-1350, 2011 Author(s): U. Gamnitzer, A. B. Moyes, D. R. Bowling, and H. Schnyder The carbon isotopic composition (δ 13 C) of CO 2 efflux (δ 13 C efflux ) from soil is generally interpreted to represent the actual isotopic composition of the respiratory source (δ 13 C Rs ). However, soils contain a large CO 2 pool in air-filled pores. This pool receives CO 2 from belowground respiration and exchanges CO 2 with the atmosphere (via diffusion and advection) and the soil liquid phase (via dissolution). Natural or artificial modification of δ 13 C of atmospheric CO 2 (δ 13 C atm ) or δ 13 C Rs causes isotopic disequilibria in the soil-atmosphere system. Such disequilibria generate divergence of δ 13 C efflux from δ 13 C Rs (termed "disequilibrium effect"). Here, we use a soil CO 2 transport model and data from a 13 CO 2 / 12 CO 2 tracer experiment to quantify the disequilibrium between δ 13 C efflux and δ 13 C Rs in ecosystem respiration. The model accounted for diffusion of CO 2 in soil air, advection of soil air, dissolution of CO 2 in soil water, and belowground and aboveground respiration of both 12 CO 2 and 13 CO 2 isotopologues. The tracer data were obtained in a grassland ecosystem exposed to a δ 13 C atm of −46.9 ‰ during daytime for 2 weeks. Nighttime δ 13 C efflux from the ecosystem was estimated with three independent methods: a laboratory-based cuvette system, in-situ steady-state open chambers, and in-situ closed chambers. Earlier work has shown that the δ 13 C efflux measurements of the laboratory-based and steady-state systems were consistent, and likely reflected δ 13 C Rs . Conversely, the δ 13 C efflux measured using the closed chamber technique differed from these by −11.2 ‰. Most of this disequilibrium effect (9.5 ‰) was predicted by the CO 2 transport model. Isotopic disequilibria in the soil-chamber system were introduced by changing δ 13 C atm in the chamber headspace at the onset of the measurements. When dissolution was excluded, the simulated disequilibrium effect was only 3.6 ‰. Dissolution delayed the isotopic equilibration between soil CO 2 and the atmosphere, as the storage capacity for labelled CO 2 in water-filled soil pores was 18 times that of soil air. These mechanisms are potentially relevant for many studies of δ 13 C Rs in soils and ecosystems, including FACE experiments and chamber studies in natural conditions. Isotopic disequilibria in the soil-atmosphere system may result from temporal variation in δ 13 C Rs or diurnal changes in the mole fraction and δ 13 C of atmospheric CO 2 . Dissolution effects are most important under alkaline conditions.