Relationships between abiotic and biotic soil properties during fallow periods in the sudanian zone of Senegal

Relationships between soil characteristics, various forms of soil organic m atter, microbial biomass and the structure of phytoparasitic nematode popul ations were investigated in six fallow fields aged from 1 to 26 years in th e West African Savanna (WAS) belt in southern Senegal. Soil sampling was pe rformed along two transects in each field. Herbaceous biomass and soil phys ical, chemical and biological characteristics were studied with principal c omponent analysis (PCA) and the relationships between the parameters were e xtracted with co-inertia analysis. Soil properties (mainly calcium, magnesium and total carbon contents, and c ation exchange capacity) slightly improved in the upper soil layer (0-5 cm) during the succession of vegetation. In constrast, in the 0-10 cm soil lay er, microbial biomass and total soil organic carbon content showed no clear pattern of change over time, while highest charcoal stocks were found in o lder fallows where bush fires are frequent. In the 0-40 cm layer, living ro ot biomass increased and herbaceous biomass decreased through the chronoseq uence. Evidence is presented here for particular relationships between some of the carbon components and the structure of the nematode community. Prat ylenchus and Ditylenchus species were associated with the grass vegetation of the youngest fallows. In contrast Helicotylenchus and Scutellonema were present in old fallows. The multiplication of the latter appeared closely r elated to the presence of woody fine roots, whereas, that of the former see med to be favoured by the presence of the coarsest roots of trees. Xiphinema had a higher density in soils with higher bulk density. Microbial biomass was not affected by fallow duration and was not correlated with th e abundance of non-phytoparasitic nematodes. These results suggested that t he management of crop pests such as nematodes in the soils of the WAS could be exerted through stump protection and tree plantation (improved fallow, agroforestry) during the crop-fallow cycle. (C) 2000 Elsevier Science B.V. All rights reserved.