A functional grouping of nematodes is generally synonymous with allocation into feeding groups. However, soil quality assessment indices based on the presence of all feeding groups still provide insufficient information regarding the functioning of soil ecosystems and their threats. The review here was conducted to plant-parasitic nematodes occur naturally in the rhizosphere. We view this as an essential first step toward understanding the importance of nematodes in Soil.
Nematodes, as one component of the soil ecosystem, interact with biotic and abiotic factors, producing economic crop losses but also having many other effects, largely unknown. The role of the nematode community in ecosystems is essential knowledge for managing plant nematode population (Jones, 1973). Soil nematodes are very small (0.3-5.0 mm long as adults) worm-link animals which are very abundant (commonly millions m-2 and diverse (commonly > 30 taxa) in the soils (yeates, 1979). The use of nematodes as indicators of soil functioning could strongly benefit from the techniques and mass of information available for plant-feeding nematodes (Bongers and Bongers, 1998). Plant parasitic nematodes (phytonematodes) have evolved a range of parasitic habits, with the majority of the species being soil dwelling and parasitic of plant roots. (Sijmons, P.C. et al., 1994 and Hussey, R.S. and Grundler, F.M.W., 1998). The rhizosphere is a zone of complex interactions between the plant and rhizosphere organisms, and these interactions are influenced by environmental factors, e.g. nutrient status of the soil. When the soil nutrient level is low or poorly balanced, plant may be more dependent on symbionts (Fay et al., 1996: Liu et al., 2000) Nematodes influence three key rhizosphere functions. Firstly, plant parasitic nematodes have direct and often detrimental effect on plant performance (Grontoft and Jonasson, 1992: Nicol et al., 1999).
Secondly, feeding by microbivorous nematodes may increase the mineralization of nutrients immobilized in the microbial biomass (Ferris et al., 1998, Chen and Ferries, 1999). Thirdly, predatory nematodes may affect the abundance and composition of the plant feeding and microbivorous nematode assemblages. If top down regulation occurs, changes in the abundance and composition of the nematodes assemblage may mineralization of plant nutrients. Through bottom-up regulation, changes within the nematode assemblage could indicate changes of their food source (Ritz and Trudgill, 1999).