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>100 years in the mineral soil. Thus almost 50% of the carbon supplied to the soil by litter is decomposed in less than 20 years, 30-40% remains less than 100 years and only 10-15% is transformed into substances that are difficult to decompose and which accumulate in the soil where they remain more than 100 years, or these substances are so valuable for heterotrophic organisms (essential amino acids, pyridine, and others) that they are passed on between generations. The maximum age of carbon in the soil is around 3000-6000 years (Schlesinger 1990; Schachtschabel et al. 1998; Schulze et al. 1999). Only if soil horizons are buried, i.e. cut off from microbial decomposition, greater ages are attained in fossilised horizons.

A qualitative understanding of how carbon is transformed in the soil is possible, particularly by using mathematical models. An example is the CENTURY model (Parton et al. 1988), which shows the transformation and its dependence on availability of resources - C, N, P and S (Fig. 3.3.11 A). The model shows that, at all stages in decomposition, carbon is returned to microbial biomass. The proportion of carbon ending up in recalcitrant form is less than 1% of GPP. The turnover of carbon in microbial biomass means

BL Soil litter L/N Llgnln-to-nltrogen ratio A Llgnin fraction T Soil silt and clay fraction

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