Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 4 3 2007 10.5194/bg-4-385-2007 http://www.biogeosciences.net/4/385/2007/ http://www.biogeosciences.net/4/385/2007/bg-4-385-2007.html http://www.biogeosciences.net/4/385/2007/bg-4-385-2007.pdf 385 394 2007-06-26 Long-term steady state 13C labelling to investigate soil carbon turnover in grasslands K. Klumpp kklump@clermont.inra.fr J. F. Soussana R. Falcimagne INRA, UR874 Grassland Ecosystem Research, Clermont-Ferrand, France We have set up a facility allowing steady state 13CO2 labeling of short stature vegetation (12 m2) for several years. 13C labelling is obtained by scrubbing the CO2 from outdoors air with a self-regenerating molecular sieve and by replacing it with 13C depleted (−34.7±0.03&permil;) fossil-fuel derived CO2 The facility, which comprises 16 replicate mesocosms, allows to trace the fate of photosynthetic carbon in plant-soil systems in natural light and at outdoors temperature. This method was applied to the study of soil organic carbon turnover in temperate grasslands. We tested the hypothesis that a low disturbance by grazing and cutting of the grassland increases the mean residence time of carbon in coarse (>0.2 mm) soil organic fractions. Grassland monoliths (0.5×0.5×0.4 m) were sampled from high and low disturbance treatments in a long-term (14 yrs) grazing experiment and were placed during two years in the mesocosms. During daytime, the canopy enclosure in each mesocosm was supplied in an open flow with air at mean CO2 concentration of 425 µmol mol−1 and δ13C of −21.5±0.27&permil;. Fully labelled mature grass leaves reached a δ13C of −40.8 (±0.93) and −42.2&permil; (±0.60) in the low and high disturbance treatments, respectively, indicating a mean 13C labelling intensity of 12.7&permil; compared to unlabelled control grass leaves. 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