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

P>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 CO2 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 mu mol mol-1) and carbon isotopic composition, delta 13C, (-46.9 +/- 0.4 parts per thousand) of CO2 in the OTCs were stable during photosynthesis as a result of high air throughflux and minimal incursion through the buffered vent. Soil CO2 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 (approximate to autotrophic respiration) and the other was provided by decomposition of structural plant biomass (approximate to heterotrophic respiration).