Observing 13C labelling kinetics in CO2 respired by a temperate grassland ecosystem

Abstract

Summary The kinetic characteristics of the main sources of ecosystem respiration are quite unknown, partly because of methodological constraints. Here, we present a new open-top chamber (OTC) apparatus for continuous 13C/12C labelling and measurement of ecosystem CO 2 fluxes, and report the tracer kinetics of nighttime respiration of a temperate grassland. The apparatus includes four dynamic flow-through OTCs, a unit mixing CO2-free air with 13C-depleted CO2, and a CO2 analyser and an online isotope ratio mass spectrometer. The concentration (367 ± 6.5 μmol mol-1) and carbon isotopic composition, δ13C, (-46.9 ± 0.4‰) of CO2 in the OTCs were stable during photosynthesis as a result of high air throughflux and minimal incursion through the buffered vent. Soil CO 2 efflux was not affected by pressure effects during respiration measurements. The labelling kinetics of respiratory CO2 measured in the field agreed with that of excised soil + vegetation blocks measured in a laboratory-based system. The kinetics fitted a two-source system (r2 = 0.97), with a rapidly labelled source (half-life 2.6 d) supplying 48% of respiration, and the other source (52%) releasing no tracer during 14 d of labelling. Of the two sources supplying ecosystem respiration, one was closely connected to current photosynthesis (≈ autotrophic respiration) and the other was provided by decomposition of structural plant biomass (≈ heterotrophic respiration). © 2009 New Phytologist.