Measuring and modelling the isotopic composition of soil respiration: insights from a grassland tracer experiment

The carbon isotopic composition (delta C-13) of CO2 efflux (delta C-13(efflux)) from soil is generally interpreted to represent the actual isotopic composition of the respiratory source (delta C-13(Rs)). However, soils contain a large CO2 pool in air-filled pores. This pool receives CO2 from belowground respiration and exchanges CO2 with the atmosphere (via diffusion and advection) and the soil liquid phase (via dissolution). Natural or artificial modification of delta C-13 of atmospheric CO2 (delta C-13(atm)) or delta C-13(Rs) causes isotopic disequilibria in the soil-atmosphere system. Such disequilibria generate divergence of delta C-13(efflux) from delta C-13(Rs) (termed "disequilibrium effect"). Here, we use a soil CO2 transport model and data from a (CO2)-C-13/(CO2)-C-12 tracer experiment to quantify the disequilibrium between delta C-13(efflux) and delta C-13(Rs) in ecosystem respiration. The model accounted for diffusion of CO2 in soil air, advection of soil air, dissolution of CO2 in soil water, and belowground and aboveground respiration of both (CO2)-C-12 and (CO2)-C-13 isotopologues. The tracer data were obtained in a grassland ecosystem exposed to a delta C-13(atm) of -46.9% during daytime for 2 weeks. Nighttime delta C-13(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 delta C-13(efflux) measurements of the laboratory-based and steady-state systems were consistent, and likely reflected delta C-13(Rs). Conversely, the delta C-13(efflux) measured using the closed chamber technique differed from these by -11.2 %. Most of this disequilibrium effect (9.5 %) was predicted by the CO2 transport model. Isotopic disequilibria in the soil-chamber system were introduced by changing delta C-13(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 CO2 and the atmosphere, as the storage capacity for labelled CO2 in water-filled soil pores was 18 times that of soil air. These mechanisms are potentially relevant for many studies of delta C-13(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 delta C-13(Rs) or diurnal changes in the mole fraction and delta C-13 of atmospheric CO2. Dissolution effects are most important under alkaline conditions.