Agriculture Reference
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Natural radiocarbon isotope dating has proved to be a useful tool for investigat-
ing the residence times of carbon in soils. The half life of (5730 years) provides a tool
for aging the old carbon in soils. In contrast, the recent injection of into the atmo-
sphere by nuclear tests in the 1960s ('bomb radiocarbon') labelled all photosynthesised
material and provides a tracer of younger organic carbon. Figure II.6 shows a radiocarbon
profile of a temperate-climate, forested alfisol (Elzein and Balesdent, 1995). Positive
values in the upper layers denote the incorporation of bomb radiocarbon in the soil. The
L layer had a radiocarbon content equal to atmospheric in 1993. In the F layer,
a five-year mean carbon residence time was calculated. Higher values in the upper 5 cm
of the A horizon correspond to the peak of bomb-radiocarbon in the 1960s. The authors
simulated the carbon and radiocarbon profiles (lines) with a model that couples the
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