title: The effect of physical back-diffusion of 13CO2 tracer on the coupling between photosynthesis and soil CO2 efflux in grassland.
abstract: Pulse labelling experiments provide a common tool to study short-term processes in the plant-soil system and investigate below-ground carbon allocation as well as the coupling of soil CO(2) efflux to photosynthesis. During the first hours after pulse labelling, the measured isotopic signal of soil CO(2) efflux is a combination of both physical tracer diffusion into and out of the soil as well as biological tracer release via root and microbial respiration. Neglecting physical back-diffusion can lead to misinterpretation regarding time lags between photosynthesis and soil CO(2) efflux in grassland or any ecosystem type where the above-ground plant parts cannot be labelled in gas-tight chambers separated from the soil. We studied the effects of physical (13)CO(2) tracer back-diffusion in pulse labelling experiments in grassland, focusing on the isotopic signature of soil CO(2) efflux. Having accounted for back-diffusion, the estimated time lag for first tracer appearance in soil CO(2) efflux changed from 0 to 1.81±0.56 h (mean±SD) and the time lag for maximum tracer appearance from 2.67±0.39 to 9.63±3.32 h (mean±SD). Thus, time lags were considerably longer when physical tracer diffusion was considered. Using these time lags after accounting for physical back-diffusion, high nocturnal soil CO(2) efflux rates could be related to daytime rates of gross primary productivity (R(2)=0.84). Moreover, pronounced diurnal patterns in the δ(13)C of soil CO(2) efflux were found during the decline of the tracer over 3 weeks. Possible mechanisms include diurnal changes in the relative contributions of autotrophic and heterotrophic soil respiration as well as their respective δ(13)C values. Thus, after accounting for physical back-diffusion, we were able to quantify biological time lags in the coupling of photosynthesis and soil CO(2) efflux in grassland at the diurnal time scale.
authors.affiliation: a Institute of Agricultural Sciences, ETH Zurich , Zurich , Switzerland.
authors.forename: Susanne
authors.initials: S
authors.lastname: Burri
authors.forename: Patrick
authors.initials: P
authors.lastname: Sturm
authors.forename: Thomas
authors.initials: T
authors.lastname: Baur
authors.forename: Matti
authors.initials: M
authors.lastname: Barthel
authors.forename: Alexander
authors.initials: A
authors.lastname: Knohl
authors.forename: Nina
authors.initials: N
authors.lastname: Buchmann
mesh_terms: D002245:Carbon Dioxide / Q000378:metabolism*; D002247:Carbon Isotopes / Q000378:metabolism; D004058:Diffusion; D004784:Environmental Monitoring; D065948:Grassland; D010788:Photosynthesis; D018517:Plant Roots / Q000378:metabolism*; D013557:Switzerland; D013997:Time Factors
chemical_list: D002247:Carbon Isotopes; D002245:Carbon Dioxide
keywords: 13CO2; carbon allocation; carbon-13; grassland ecosystem; laser spectroscopy; plant–soil interactions; pulse labelling; soil respiration; stable isotope tracer techniques; time lag