Lipid abundance and composition of a humic Oxisol as a function of land use

Abstract

Soil organic matter of a humic Oxisol has been characterized by analyzing litter and soil lipid composition under vegetal covers (coffee, pasture and fragment of native forest). Soil and litter samples were collected from each site at two depths (0-10 and 60-100 cm). Lipid composition was determined by gas chromatography-mass spectrometry (CG-MS). In the solvent extracts of the soil lipids, even-numbered nalkanoic acids in the range of C 16 -C 32 indicate a soil organic matter input from higher plant waxes. The major compound classes in both the soil and the litter included saturated n-alkanoic acids, with average values in the total spectrum of 58 and 53%, respectively Clear differences in lipid composition were found between the topsoil and subsurface. Long chain alkylic compounds (alkanoic acids and alkanes) accumulated preferably at depths of 60-100 cm. On the surface, the soil from natural vegetation had a total lipid value higher than those cultivated soils, and showed a predominance of unsaturated alkanoic acids at 60-100 cm. Preservation of alkylic moieties with depth was observed under forest and pasture, indicating that these systems of land use are prone to lipid accumulation. Lipids, especially the long-chain (> C 20 ) ones, may be important with regard to C stabilization and humification processes in subsoil of humic Oxisols, highlighting the importance of lipids compounds in the bulk structure of organic matter these soils. Key words: soil and litter lipid, soil management, tropical soil, grass, coffee Abundância e composição de lipídios em Latossolo Húmico em função do uso do solo RESUMO: A matéria orgânica de um Latossolo Húmico foi caracterizada por meio da análise da composição lipídica do solo e da serapilheira sob coberturas vegetais (café, pastagem e fragmento de mata nativa). Em cada área foram coletadas amostras de solo em duas profundidades (0-10 e 60-100 cm), assim como amostras da serapilheira. A composição lipídica foi determinada por cromatografia gasosa-espectroscopia de massa. No extrato lipídico do solo a ocorrência de ácidos alcanóicos com número par de carbonos (C 16 -C 32 ) indicou presença de cera de plantas superiores na composição da matéria orgânica do solo. Tanto no solo quanto na serapilheira a principal classe de compostos incluiu n-alcanóicos saturados, com valores médios de 58 e 53% do total do cromatograma, respectivamente. Clara diferença foi encontrada entre os extratos lipídicos obtidos na superfície e subsuperfície do solo. Compostos alquílicos de cadeia longa (ácidos alcanóicos e alcanos) acumularamse preferencialmente entre 60-100 cm. Superficialmente, o solo sob vegetação natural apresentou maiores teores de lipídios totais que os solos cultivados, mostrando também predominância de ácidos alcanóicos insaturados entre 60-100 cm. Nas áreas sob mata e pastagem houve, em profundidade, preservação de alcanos de cadeia curta; evidenciando maior conservação dos mesmos nestes sistemas de uso da terra. Os lipídios, especialmente os de cadeia longa (>20), podem ...

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