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

The carbon isotopic composition (δ13C) of CO2 efflux (δCefflux) from soil is generally interpreted to represent the actual isotopic composition of the respiratory source (δCRs). However, soils contain a large CO2 pool in airfilled 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 δ13C of atmospheric CO2 (δCatm) or δCRs causes isotopic disequilibria in the soilatmosphere system. Such disequilibria generate divergence of δCefflux from δCRs (termed “disequilibrium effect”). Here, we use a soil CO2 transport model and data from a CO2/CO2 tracer experiment to quantify the disequilibrium between δCefflux and δCRs 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 and CO2 isotopologues. The tracer data were obtained in a grassland ecosystem exposed to a δCatm of −46.9 ‰ during daytime for 2 weeks. Nighttime δCefflux 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 δCefflux measurements of the laboratory-based and steady-state systems were consistent, and likely reflected δCRs. Conversely, the δCefflux 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 δCatm in the chamber headspace at the onset of the Correspondence to: U. Gamnitzer ([email protected]) 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 waterfilled soil pores was 18 times that of soil air. These mechanisms are potentially relevant for many studies of δCRs 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 δCRs or diurnal changes in the mole fraction and δ13C of atmospheric CO2. Dissolution effects are most important under alkaline conditions.