Humic Substances of Spodic Horizons in the Coastal Plain of São Paulo State

The most common soils that occurred in environments generically called restinga are podzols that are characterized by the presence of spodic horizon (Bh or Bhm). There are few scientific studies related to the genesis of these soils in tropical regions, and there are few detailed studies assessing the chemical characteristics and composition of organic matter (OM) present in these soils, as well their relations with time. The cities Cananéia and Bertioga were selected for this research due to the presence of different sedimentary units and remaining vegetation. The characterization of humic acids (HA) was realized using spectroscopic techniques like Fourier transform infrared spectroscopy (FTIR) and fluorescence. Samples were extracted from different podzol horizons in order to achieve more details about the OM present in these soils and relate the results to the stability of carbon and the mean residence time (MRT) of OM. The main hypothesis tested was that the profiles where the OM has less MRT would have the highest contents of more recalcitrant compounds. In this context, the main results related to the objectives were as follows: The older profiles were those with the highest concentrations of more recalcitrant compounds, especially in the subsurface horizons (Bh and Bhm). This inference may indicate that with transcurrent time, OM tends to undergo changes in its composition and become recalcitrant.