Effect of land use on the quantity of organic matter in different soils from the Lesser Antilles

The effect of different types of land use (banana or food crops vs pastured fallow or artificial meadow) on the quantity and the quality of soil carbon content was investigated on three types of soil of the Lesser Antilles (Andept, Ultisol, Vertisol). Organic carbon content of various particle size fractions of the 0- 10 cm layer of the Vertisol was determined by a physical soil fractionation method. Soil organic matter content depended upon land uses and soil types. For the Andept, banana crops led to a soil carbon status (0-40 cm) that was higher than that of pastured fallow (13.3 and 12.6 kg C.m2 respectively). In contrast, for the Ultisol and the Vertisol, organic matter content was lower under food crops than under pastured fallows or pastures, with a more pronounced variation for the vertisol. For this soil, the decrease of organic carbon content in the 0-10 cm layer was mainly due to a decrease in the 0-5 μm and >50 μm fractions identified as microbial metabolites and root exudates, and plant residues respectively. ----- L'effet compare des cultures bananieres ou marai'cheres et des jacheres paturees ou des prairies sur le stock de matiere organique est etudie pour trois types de sols (andosol, sol ferrallitique, vertisol) des Petites Antilles. La caracterisation par fractionnement granulometrique de la matiere organique de I'horizon 0-10 cm du vertisol est utilisee pour preciser la nature des variations du stock. Le stock de matiere organique depend du systeme de culture et du type de sol. Pour I'andosol, le statut organique est plus eleve sous bananes que sous jachere paturee. Au contraire, pour le sol ferrallitique et le vertisol, le stock de matiere organique de ces sols est plus faible sous cultures maraicheres que sous jachere paturee et prairie. L' effet depressif des cultures maraicheres sur le statut organique est le plus important sur le vertisol. Dans ce sol, la diminution du stock organique de I'horizon 0-10 cm est principalement due a la diminution des composes organiques d'origine microbienne, des exudats racinaires et des debris vegetaux