Observing 13C labelling kinetics in CO2 respired by a temperate grassland ecosystem
2009
Gamnitzer, Ulrike | Schäufele, Rudi | Schnyder, Hans
• 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 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 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 (≈ autotrophic respiration) and the other was provided by decomposition of structural plant biomass (≈ heterotrophic respiration).
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