Carbon assimilation and turnover in grassland vegetation using an in situ (CO2)-C-13 pulse labelling system

A mobile laboratory was developed to administer a controlled flow of C-13 l abelled CO2 at ambient concentrations (similar to 350 ppm) in the field. Th e stable isotope delivery (SID) system consists of an isotope-mixing unit w ith how control to a series of 12 independent labelling chambers. In-line C PU controlled infrared gas analysers allow automated measurement of chamber CO2 concentrations and gas flow management. A preliminary experiment was e stablished on an upland pasture located at the NERC Soil Biodiversity exper imental site, Sourhope, UK, in August 1999, The objective of this investiga tion was to determine the magnitude of pulse-derived C incorporation into a typical upland plant community. To achieve this, the SID system was deploy ed to pulse-label vegetation with CO2 enriched with C-13 (50 atom %) at amb ient concentrations (similar to 350 ppm) on two consecutive days in August 1999, Samples of headspace CO2, shoot and root were taken on four occasions over a period of 28 days after C-13 labelling. These materials were then p repared for C-13/C-12 ratio determination by continuous-flow/combustion/iso tope ratio mass spectrometry (CF-C-IRMS). Results showed that pulse derived CO2-C was assimilated at a rate of 128 +/- 32 mu g g shoot-C hour(-1). Dyn amic samplings showed that pulse-derived C-13 concentrations in the labelle d plant tissues declined by 77.4 +/- 6% after 48 hours. The rapid decline i n C-13 concentrations in plant matter was the result of C loss from the pla nt in the form of respired CO2 and root exudates, and dilution by subsequen t unlabelled C assimilates, This novel system offers considerable potential for in situ tracer investigations. Copyright 2000 John Wiley & Sons, Ltd.