Carbon Isotope Dynamics in Some Tropical Soils

Abstract

We determined δ 13 C and D 14 C in some agricultural soil profiles of the tropics: 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 13 C and 14 C content of organic carbon. Regular plowing or puddling leads to uniform isotope concentrations in the topsoil. Decomposition of organic matter raises the δ 13 C value, and vertical translocation raises the δ 13 C value with depth. The change of cultivation from pulses (C3-type metabolism of photosynthesis) to sorghum (C4) results in a decrease of δ 13 C with depth in the topsoil. Where the clay content in the subsoil is high, δ 13 C remains constant, due to fixation of organic carbon to clay minerals, and D 14 C decreases with depth. Below the clay-enriched zone, δ 13 C declines and D 14 C 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 D 14 C.