Organic matter characteristics and distribution in Ferralsol profiles of a climosequence in southern Brazil

We studied the impact of climate, soil texture and iron mineralogy on the content and composition of organic matter in Ferralsols along a climosequence in southern Brazil. We characterized the organic matter by solid-state cross-polarization magic angle spinning (CPMAS) C-13 nuclear magnetic resonance (NMR) spectroscopy. The organic matter content in the Ferralsols increased from the lowest to the highest sites (440-950 m altitude) as a result of the increase in humidity and the decrease in temperature. This influence was more pronounced in the heavy clayey Ferralsols, suggesting that the accumulation of organic matter was enhanced by organo-mineral interactions. Iron oxides contributed to the stabilization of the organic matter in these soils, which was dominated by O-alkyl C structures followed by aromatic and alkyl C groups. The aromatic C contribution to the total organic C increases with depth at the expense of O-alkyl C, regardless of the site altitude and climate. Although the impact of the environment on the amount of organic matter and its chemical composition is confirmed, our results clearly show that this is so only for the surface horizons. With increasing soil depth the climatic influence decreased, showing that other factors become more important for the accumulation of organic matter.