Thin Layer δ13C and D14C Monitoring of “Lessive” Soil Profiles

The natural 14C and 13C content of soil organic matter and their dependence on depth for two Alfisols are presented. This soil type which covers a large area of the earth's surface is characterized by clay migration processes (“Lessivé”). The samples were taken as successive horizontal layers of 2cm depth from an area of ca lm2 size as deep as the C content allows 14C analysis. The minima of the D14C distribution decrease with depth, while the maxima increase in the upper, leached horizon (A1) due to bomb 14C and decrease in the lower, clay illuviated (Bt). δ13C indicates proceeding decomposition in A1 and protection of carbon, probably due to the formation of clay humus complexes in Bt. δ13C values were also used for age correction of the 14C data due to isotopic fractionation. The D14C and δ13C depth distributions are characterized by sharp peaks at the boundaries of the horizons, probably caused by the influence of textural changes on the transport of C with percolating water.