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 com munity 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 Crotalar ia 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 compo nent analysis and co-inertia analysis. Planting trees/shrubs as improved fallows in croplands dramatically increas ed the populations of nematodes in the soil. However, the diversity of plan t-parasitic nematodes was reduced. The distribution of plant-parasitic nema todes was correlated with that of some soil physical and chemical propertie s. Scutellonema spp. were dominant in the improved fallows where the highes t values of magnesium and potassium were recorded. The plant-parasitic Dory laimida (Xiphinema spp. and Paratrichodorus minor) were more abundant in th e maize/beans plots and this abundance seemed related to high soil bulk den sity. Meloidogyne spp. and Rotylenchulus borealis had a strong positive cor relation with soil organic matter and clay, and were abundant in the long-t erm natural fallows. The correlations between soil physico-chemical properties and nematodes ind icate that, apart from the direct influence of the host plant, the soil cha racteristics play an important role in the abundance, distribution and stru cture 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.