Relationships between plant-parasitic nematode community, fallow duration and soil factors in the Sudano-Sahelian area of Senegal

Abstract

Before studying the mechanisms by which changes in vegetation influence the nematode community following abandonment of a field, it is necessary to first assess the impact of environmental factors such as soil or climate on these organisms. The study was undertaken on sites of increasing fallow duration located in the same area in the Sudano-Sahelian zone of Senegal: a forest and 10 fallow sites ranging from 1 to 18 years old. Soil samples were collected over 3 years on 17 occasions, along 21 m fixed transects, located in representative vegetation zones. Plant-parasitic nematodes were extracted, identified and enumerated from soil samples. The statistical analysis showed that the sites could be split in three groups according to the plant-parasitic nematode communities. One group, corresponding to young fallows, was characterised by large populations of Scutellonema cavenessi and Tylenchorhynchus gladiolatus. A second group included most of the older fallows and was characterised by a more diversified nematode community dominated by Helicotylenchus dihystera. The forest hosted a particular community partly similar to both of the other groups. Soil physical and chemical analysis split the sites into two groups, the young fallow sites plus the forest, and the older sites. The study of the annual changes revealed contradictory tendencies, such as a very small increase in T. gladiolatus and in the soil clay content, which suggest that the sampling technique had probably slightly influenced the results. If this did occur it did not change the effect of fallow duration on the nematode community. The process of decline of certain nematode populations seemed to be extremely slow and not detectable over three successive years. Nematode species were apparently able to reproduce for a long time on poor host plants, even if one plant cohort disappeared abruptly, as was the case when the field was not replanted. This study demonstrated that the soil had a greater influence on the nematode community structure than the annual climatic variations. Consequently, the soil effect will have to be mathematically