Diversity of plant-parasitic nematodes and their relationships with some soil physico-chemical characteristics in improved fallows in western Kenya

A survey was conducted in the food crop production area of western Kenya to characterise the nematode communities in different cropping systems and to monitor the effect of improved fallows on the abundance, diversity and community structure of plant-parasitic nematode assemblages. Soil samples were taken from short-term natural fallows (< 3 years), old natural fallows (> 20 years), maize/beans cultivated fields and improved fallows with Crotalaria grahamiana, Sesbania sesban and Tephrosia vogelii. The Shannon diversity index and evenness were used to assess nematode diversity in the different cropping systems. The relationships between plant-parasitic nematodes and the soil physico-chemical properties were investigated with principal component analysis and co-inertia analysis. Planting trees/shrubs as improved fallows in croplands dramatically increased the populations of nematodes in the soil. However, the diversity of plant-parasitic nematodes was reduced. The distribution of plant-parasitic nematodes was correlated with that of some soil physical and chemical properties. Scutellonema spp. were dominant in the improved fallows where the highest values of magnesium and potassium were recorded. The plant-parasitic Dorylaimida (Xiphinema spp. and Paratrichodorus minor) were more abundant in the maize/beans plots and this abundance seemed related to high soil bulk density. Meloidogyne spp. and Rotylenchulus borealis had a strong positive correlation with soil organic matter and clay, and were abundant in the long-term natural fallows. The correlations between soil physico-chemical properties and nematodes indicate that, apart from the direct influence of the host plant, the soil characteristics play an important role in the abundance, distribution and structure of nematode communities. This validates the potential of nematodes as bio-indicator organisms of soil status. (C) 2001 Elsevier Science B.V. All rights reserved.