Litter, mean residence time

In soils, lead concentrates in organic-rich surface horizons (NRCC 1973). In one instance, only 17 mg of soluble Pb/kg was found in soils 3 days after the addition of 2784 mg of lead (as lead nitrate)/kg (NRCC 1973). The estimated residence time of lead in soils is about 20 years complete turnover in topsoil is expected every few decades (Nriagu 1978a). In forest litter, however, the mean residence time of lead is lengthy estimates range from 220 years (Turner et al. 1985) to more than 500 years (Friedland and Johnson 1985). 

Coarse particles that are mainly fresh litter have mean residence times between 0.5 and 20 years, whereas the carbon in the clay fraction has mean residence times of about 60-80 years (Balesdent, 1996). Recently, the direct determination of molecular turnover rates using pyrolysis products was applied to SOM after vegetation change (Gleixner et al, 1999). This technique indicated mean residence times between 9 and 220 years for individual pyrolysis products for the first time. Most intriguing was the fact that... 

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