One promising alternative to help mitigate the impact of global change on world ecosystems is to promote the increased storage of atmospheric carbon dioxide in components of terrestrial ecosystems (Houghton et al, 2001; McCarthy et al, 2001). Photosynthesis continuously extracts CO2 from the atmosphere and forms plant biomass, which is again mineralized by microorganisms to re-form atmospheric CO2. Some of the plant carbon formed accumulates as plant biomass in terrestrial ecosystems, and some is transformed into microbial biomass or new molecules are synthesized from it to form soil organic matter (SOM). Excluding the ocean components, the atmosphere, aboveground biomass, and soil organic matter form three of the major pools of the global carbon cycle (Fig. 3.1). Each of these pools differs in the amount of carbon stored and its stability or lifetime. For example, the aboveground biomass and the atmospheric carbon pools store 720 GtC and 620 GtC, respectively (1 giga ton of carbon = 1015 g of carbon). Together these two pools contain less carbon than soil organic matter, which holds 1580 GtC. Soil organic matter (SOM) is known to have high 14C ages, with some carbon dating back to the last glaciation 14000 years ago (Wang et al, 1996). Both the amount and age of the carbon in the SOM pool supports the suggestion that this pool could be a target for increasing carbon sequestration from the atmosphere. Unfortunately, comparatively little is known about the mechanisms and the dynamics of carbon storage in soils (Schimel et al, 2001). Even less is known about how soil carbon storage might be influenced by autotrophic and heterotrophic organisms (Catovsky et al, 2002). Most of our understanding of soil carbon derives from simple input-output models that consider soil carbon only at an aggregated level of bulk carbon. Most soil carbon models developed on this aggregated carbon level make use of three pools with different longevities or time scales—yearly, decadal, and



millennial—to describe the dynamics of soil carbon (Jenkinson et al., 1987; Parton et al., 1987). However, these models neglect important knowledge derived from molecular studies of soil organic matter, such as the existence of different chemical forms of carbon, i.e., carbohydrates or lignin, having different stability against decomposition (Gleixner etal., 2001a). The models also ignore the existence of chemical gradients in soil profiles (Hedges and Oades, 1997) and the role of dissolved carbon for the distribution of carbon within soil depth profiles (Neff and Asner, 2001). Moreover, the fundamental importance of soil macro- and microorganisms for the recycling of carbon in soils has not even been considered (Scheu, 2001). This chapter will summarize our current understanding of the dynamics of soil carbon, including a focus on the new insights gained from compound specific investigations. Studies of stable plant biomarkers will be described here, such as leaf waxes, to trace the flow of plant litter, and the application of labile microbial biomarkers, such as phospholipid fatty acids from microbial cell walls, to trace the carbon sources of soil microorganisms. Finally, natural stable isotope labeling experiments using C3 and C4 plants will give insight into the stability and turnover of soil organic matter.

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