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Organic matter from surface horizons of olive grove soils under organic and conventional management, compared to natural vegetation (undisturbed), on two contrasting geological substrates (colluvial limestones and marls) in a semi-arid Mediterranean area were characterized to evaluate the influence of management and soil type.

Higher percentages of total carbon, total humic extract and humin fractions were observed in the organic management and undisturbed soil samples. The main qualitative functional and compositional changes induced by management were related to the aromaticity of humic acids (HA), which were lower in organically managed samples than conventional management. This may be due to continuous effective incorporation of alkyl compounds from spontaneous vegetation, which would explain its typical lignin patterns and higher E-4/E-6 ratios (visible spectra absorbance to 465 and 665 nm). The main qualitative changes affecting organic matter were induced by the geological substrate (soil type). Soil samples from marls had a lower humic to fulvic acids ratio, and their HA were more aliphatic (with respect to the 2920 cm(-1) infrared band) and more intense with organic management. These samples also had well-defined amide and carbohydrate infrared bands (labile compounds) and lower E-4 optical density. These results showed less efficient biodegradation of aliphatic components. On the contrary, greater decomposition of organic matter was detected in the HA fraction from colluvial limestones, which was transformed into more stable and evolved forms of humus. This may be due to overlapping of current soil management and its biogenic background, representative of the original soil in this area, where pedogenic development is more intense. Characteristics of soil organic matter in the undisturbed soil were between conventional and organic management, which shows how difficult it is to find natural, well-conserved soil to serve as a reference in the agricultural context studied. Our findings reveal that more sustainable soil management and development of management practices more in harmony with the original evolution of the soil and its relationship to the parent material are especially important in this area. (C) 2011 Elsevier B.V. All rights reserved.