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

Abstract. The carbon isotopic composition (δ¹³C) of CO₂ efflux (δ¹³C_efflux) from soil is generally interpreted to represent the actual isotopic composition of the respiratory source (δ¹³C_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 δ¹³C of atmospheric CO₂ (δ¹³C_atm) or δ¹³C_Rs causes isotopic disequilibria in the soil-atmosphere system. Such disequilibria generate divergence of δ¹³C_efflux from δ¹³C_Rs (termed "disequilibrium effect").

Here, we use a soil CO₂ transport model and data from a ¹³CO₂/¹²CO₂ tracer experiment to quantify the disequilibrium between δ¹³C_efflux and δ¹³C_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 ¹²CO₂ and ¹³CO₂ isotopologues. The tracer data were obtained in a grassland ecosystem exposed to a δ¹³C_atm of −46.9 ‰ during daytime for 2 weeks. Nighttime δ¹³C_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 δ¹³C_efflux measurements of the laboratory-based and steady-state systems were consistent, and likely reflected δ¹³C_Rs. Conversely, the δ¹³C_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 δ¹³C_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 δ¹³C_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 δ¹³C_Rs or diurnal changes in the mole fraction and δ¹³C of atmospheric CO₂. Dissolution effects are most important under alkaline conditions.