A METHOD FOR EXTRACTING NEMATODES FROM A TROPICAL FOREST SOIL

The centrifugal flotation method is a common method of extracting nematodes but it has been used infrequently for extracting free-living nematodes from tropical soils. No estimate of its efficiency is available, so an experiment was conducted to optimize the extraction efficiency of nematodes of all sizes and shapes from soil samples previously fixed by addition of formalin. Factorial combinations of the speed and duration of centrifugation, the agent used in the extraction and the density of the agent were tested. Samples were drawn from a homogenized mixture of soil from 24 sites in an experimental forest in Cameroon which covered a range of forest types and ages. More nematodes of all sizes and shapes were extracted by Ludox-TM than MgSO4. A solution of density 1.15, speed of centrifugation of 1500 g and duration of 4 minutes extracted the most nematodes. Furthermore, the proportions of nematodes of certain sizes and shapes extracted were altered by using different parameters. Using the optimum parameters, the efficiency of a single extraction in Ludox was about 70% of the total number of nematodes recoverable. Efficiency was not significantly different for sites under different management regimes, but declined for subsequent extractions after the first. The practicability of a method of extraction depends on the amount of extraneous matter extracted with the nematodes. This was also assessed for all the combinations of parameters. The method most efficient for nematodes retained more extraneous matter than some other combinations of parameters, but was not the worst. To obtain cleaner samples would entail considerably reduced extraction efficiency and a sample of nematodes biased by size and shape.