The potential of humic material in sombric-like horizons of two brazilian soil profiles as an efficient carbon sink within the global C cycle

Abstract

Sombric horizons, which are dark subsurface horizons with high carbon contents, are considered as a potential C sink. We selected two profiles with dark horizons occurring between a lighter A and B horizons from Brazil. To study the nature and stability of their organic matter, we performed a characterization of its chemical composition by applying solid-state 13C NMR spectroscopy on the soils samples. The chemical resistance of the humic substances was tested using oxidation with acid dichromate and permanganate. Further, the isotopic composition (δ 13C) was determined. The organic fraction which survived oxidation with permanganate but was lost after oxidation with acid dichromate represents the biggest C pool of the profiles. Compared to the upper horizon, the sombric-like horizons contained higher amounts of total carbon, although this increase did not enhance the stable fraction. It revealed a higher aromatic C content concomitantly with a decrease of C-alkyl C. These results confirm that in our soils, depth and aromatic C content cannot be related to biochemical recalcitrance. Solid-state 13C NMR spectroscopy of the remains after chemical oxidation indicates the presence of charred residues in our sombric-like horizons. Based on δ 13C values (around -19‰) which are close to that obtained from tropical grass, this charred material is likely to derive from former grassland fires.