Becker-Heidmann, P., Scharpenseel, H.-W. (1989): Carbon isotope dynamics in some tropical soils. – In: Long, A., Kra, R., Srdoč, D. (Hg.): Proceedings of the 13th International Radiocarbon Conference, S. 672–679. Radiocarbon 31,3. Dept. of Geosciences, University of Arizona, Tucson, Arizona
We determined δ13C and D14C in some agricultural soil profiles of the tropics: a Ustalf from the semi-arid tropics (India), a Udult, an Aquoll, and an Aquept from the humid tropics (Philippines). We used a thin layer sampling, resulting in high resolution depth distribution patterns of natural 13C and 14C content of organic carbon. Regular plowing or puddling leads to uniform isotope concentrations in the topsoil. Decomposition of organic matter raises the δ13C value, and vertical translocation raises the δ13C value with depth. The change of cultivation from pulses (C3-type metabolism of photosynthesis) to sorghum (C4) results in a decrease of δ13C with depth in the topsoil. Where the clay content in the subsoil is high, δ13C remains constant, due to fixation of organic carbon to clay minerals, and D14C decreases with depth. Below the clay-enriched zone, δ13C declines and D14C rises again, due to a chromatographic-like effect. At some horizon boundaries, inhomogeneities in texture delay percolation locally, thus preventing sorption and causing peak values of D14C.