Dynamics of soil organic matter turnover and soil respired CO 2 in a temperate grassland labelled with 13 C D. E. T HEIS a , M. J AEGGI a , D. AESCHLIMANN b , H. B LUM b , E. F ROSSARD b & R. T. W. S IEGWOLF a a Paul Scherrer Institute, Laboratory of Atmospheric Chemistry, 5232 Villigen, and b Institute of Plant Science, Swiss Federal Institute of Technology, 8092 Zu ¨ rich, Switzerland Summary The fate of carbon (C) in grassland soils is of particular interest since the vast majority in grassland ecosystems is stored below ground and respiratory C-release from soils is a major component of the global C balance. The use of 13 C-depleted CO 2 in a 10-year free-air carbon dioxide enrichment (FACE) experi- ment, gave a unique opportunity to study the turnover of the C sequestered during this experiment. Soil organic matter (SOM), soil air and plant material were analysed for d 13 C and C contents in the last year of the FACE experiment (2002) and in the two following growing seasons. After 10 years of exposure to CO 2 enrichment at 600 ppmv, no significant differences in SOM C content could be detected between fumigated and non-fumigated plots. A 13 C depletion of 3.4& was found in SOM (0–12 cm) of the fumigated soils in comparison with the control soils and a rapid decrease of this difference was observed after the end of fumigation. Within 2 years, 49% of the C in this SOM (0–12 cm) was exchanged with fresh C, with the limitation that this exchange cannot be further dissected into respira- tory decay of old C and freshly sequestered new C. By analysing the mechanistic effects of a drought on the plant-soil system it was shown that rhizosphere respiration is the dominant factor in soil respira- tion. Consideration of ecophysiological factors that drive plant activity is therefore important when soil respiration is to be investigated or modelled. Introduction Temperate grasslands cover about 20% of the land area of Europe (Soussana et al., 2004). In these ecosystems up to 98% of the total C can be found below ground (Hungate et al., 1997). Soil organic matter (SOM) contains twice the amount of C found in the atmosphere (Post et al., 1982). Within the context of the expected future changes of climatic conditions and rising atmospheric [CO 2 ] it is still uncertain if the soil C pools will be a future source or sink. Carbon newly introduced into soils is predominantly found in coarse fractions (Balesdent et al., 1987; Van Kessel et al., 2000b; Xie et al., 2005). Older, non-hydrolysable soil fractions, which consist mainly of stable humus, are soil C pools with a very slow turnover rate and are therefore conservative with regard to new C input (Pelz et al., 2005). Increased soil C input, for instance after a change in land use, is mainly seques- tered into labile C pools. If soil C input subsequently decrea- ses, the previously accumulated C is released readily (Soussana et al., 2004). Balesdent et al. (1987) calculated the rate of introduction of new C into a soil where newly grown C 4 plants caused a grad- ual change in 13 C isotopic composition of SOM in a field that previously was in isotopic equilibrium with C 3 plants. In our study, the use of 13 C-depleted CO 2 of fossil origin in a 10-year ‘free-air carbon dioxide enrichment’ (FACE) experiment, gave rise to a strong 13 C label within the soil (see Jones & Donnelly, 2004). The 13 C disequilibrium of SOM and plants after the end of the CO 2 enrichment allowed us to calculate the C turnover by applying the Balesdent model. The decrease of the 13 C label in soil organic matter and soil air after fumigation can also be modelled with exponential decay functions which give an alternative estimate of the turnover time of C. A large proportion of C that enters the soil is returned to the atmosphere by soil respiration (Jones & Donnelly, 2004), which is therefore considered a key factor for soil C turnover. Soil respiration has two major sources: (i) heterotrophic microbial respiration of SOM, and (ii) rhizosphere respiration. In this paper, we define rhizosphere respiration as the sum of respira- tion by living roots, their associated mycorrhizal fungi and het- erotrophic respiratory transformation of root exudates (after Ekblad & Ho¨gberg, 2001). The proportions of the individual Correspondence: R. Siegwolf. E-mail: rolf.siegwolf@psi.ch Received 12 May 2005; revised version accepted 24 May 2007 1364 # 2007 The Authors Journal compilation # 2007 British Society of Soil Science European Journal of Soil Science, December 2007, 58, 1364–1372 doi: 10.1111/j.1365-2389.2007.00941.x