It follows from the earlier discussion that dating of the 'ecosystem age' of the CO2 flux is most interesting. Conventional 14C dating based on the radioactive decay of the isotope with a half-life of c. 5770 years offers no major possibilities in contexts where half of the soil respiration is root respiration of recent photosynthates (see before) and the other half is due to decay of recently deposited litter. However, nuclear weapons testing 1954—1962 provided a significant transient input of 14C to the atmosphere (e.g., Walton et al, 1970) andjenkinson and Rayner (1977) demonstrated that this could enable tracing of that C through the plant-soil system. The 14C levels are slowly approaching the so-called 'modern' pre-bomb 14C levels, and the method will become less and less useful as differences in 14C abundance between years diminish. However, today it is still a most powerful tool.
The so-called 'bomb-C' age of current foliage is identical to that of ambient atmospheric CO2, and as suggested above, most of the root respiration should have the same age. The heterotrophic soil community feeds on aboveground and root litters, and products of their decomposition. The age of the C deposited on to the floor of a boreal forest as fresh aboveground litter was estimated to c. 5 years (Harrison et al., 2000). There is, on the other hand, a considerable controversy about fine root turnover rates and hence also about the age of roots contributing as root litter (see Hogberg et al, 2002 for references). Many researchers argue that the fine root system of temperate and boreal trees has a turnover rate of roughly once a year or even much faster, while the few !4C measurements made of the fine roots suggest ages of 3-18 years (Gaudinski et al, 2000, 2001). As pointed out by Gaud-inski et al (2001) the latter could, however, reflect the age of the fine roots that survive for a long period, and may not be the average lifetime for fine roots. A recent study of fungi suggested that the C in ectomycorrhizal species had an age since plant photosynthesis of 0-2 years, while saprotrophic fungi contained C that was at least 10 years old (Hobbie et al, 2002). As a comparison, Gaudinski et al (2000) estimated that the average residence time of C in the plant-soil system was 8 years, while the average age of the C in soil respiration was 4 years. The latter of course reflects the large contribution of root respiration of recent photosynthates.
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