Changes of humic substances characteristics from forested to cultivated soils in Ethiopia

The dynamics of the stable fractions of soil organic matter (SOM), the humic substances, has been rarely studied in tropical soils. We evaluated extraction yields, elemental composition, isotopic δ13C abundance and solid-state 13C NMR spectra of humic fractions isolated from five forested and cultivated soils in Ethiopia in order to assess the influence of deforestation and subsequent cultivation on the composition of humic fractions. In all locations, humic (HA) and fulvic (FA) acids, humin (HU) and a humic hydrophobic fraction extracted with an acetone–HCl solution (HE) were decreased as a result of deforestation and cultivation, but the magnitude of decrease was site-specific. While the elemental composition of HA, FA and HU did not vary significantly with land use, that of the hydrophobic HE generally decreased with deforestation. Cultivation varied the δ13C values of humic fractions with location and their high values suggested a history of C4 plants incorporation in soil. The 13C NMR spectra of HA, FA and HU showed that hydrophobic components (HB) were generally lower than hydrophilic ones (HI), resulting in a degree of hydrophobicity (HB/HI) being < 1.00. On average, cultivation decreased the HB/HI values of these humic fractions in all soils except for the Andisol. The HE material was much richer in hydrophobic groups showing a HB/HI ranging from 1.48 to 3.44 in the forested sites, but cultivation substantially reduced HB/HI ratios also for HE. Our data suggest that deforestation and progressive cultivation in tropical conditions decreased the content of soil humic substances and that the resulting humic pool was poorer in alkyl components and less hydrophobic than in forested conditions.