Measuring and modelling the isotopic composition of soil respiration: insights from a grassland tracer experiment
1Lehrstuhl für Grünlandlehre, Technische Universität München, Alte Akademie 12, 85350 Freising-Weihenstephan, Germany
2Dept. of Biology, University of Utah, 257 South 1400 East, Salt Lake City, UT 84112, USA
*now at: School of Natural Sciences, University of California, Merced, 5200 North Lake Road, Merced, CA 95343, USA
Abstract. The carbon isotopic composition (δ13C) of CO2 efflux (δefflux) in ecosystems is generally interpreted to represent the actual isotopic composition of respiration (δresp). 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 (δatm) or δresp causes isotopic disequilibria in the soil-atmosphere system. Such disequilibria generate divergence of δefflux from δresp (termed disequilibrium effect).
Here, we use a soil CO2 transport model and data from a 13CO2/12CO2 tracer experiment to quantify the disequilibrium between δefflux and δresp. The model accounted for diffusion of CO2 in soil air, advection of soil air, dissolution of CO2 in soil water, belowground and aboveground respiration of both 12CO2 and 13CO2 isotopologues. The tracer data were obtained in a grassland ecosystem exposed to a δatm of −46.9‰ during daytime for 2 weeks. Nighttime δefflux 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.
The δefflux measurements of the laboratory-based and steady-state systems were consistent, and likely reflected δresp (see Gamnitzer et al., 2009). Conversely, the δefflux 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 δatm 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 δresp 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 δresp or diurnal changes in the mole fraction and δ13C of atmospheric CO2. Dissolution effects are most important under alkaline conditions.