Influence of land use on the characteristics of humic substances in some tropical soils of Nigeria

In highly weathered tropical conditions, soil organic matter is important for soil quality and productivity. We evaluated the effects of deforestation and subsequent arable cropping on the qualitative and quantitative transformation of the humic pool of the soil at three locations in Nigeria. Cultivation reduced the humic pool in the order: acetone-soluble hydrophobic fraction (HE) > humic acid (HA) > humin (HU) > fulvic acid (FA), but not to the same degree at all three sites. The C and N contents, as well as the C/N ratios of humic extracts, were large and not substantially influenced by land use. The delta C-13 values of the humic extracts were invariably more negative in forested soils thereby showing a dilution of delta C-13 signature with cultivation from C3 to C4 plants. The delta C-13 values of apolar HE fractions were generally more negative, indicating a reduced sensitivity compared with other humic fractions to turnover of crop residues. The contents of hydrophobic constituents (alkyl and aromatic C), as revealed by cross-polarization magic angle spinning (CPMAS) C-13-NMR spectroscopy, in HA, FA and HU were generally < 50%, with the exception of larger hydrophobicity in HU in the forested soil at Nsukka and HA in that at Umudike. The HE fraction contained significantly more apolar constituents, and consequently had a larger intrinsic hydrophobicity than the other humic fractions. The larger reduction of apolar humic constituents than of the less hydrophobic humic fractions, when these soils were deforested for cultivation, indicates that at those sites the stability of accumulated organic matter is to be ascribed mainly to the selective preservation of hydrophobic compounds.