Influence of phytoparasitic nematodes on symbiotic N-2 fixation in tropical herbaceous legume cover crops

Populations of plant parasitic nematodes and their effects on symbiotic nit rogen (N) fixation in herbaceous legumes and on some selected characteristi cs of other plant species associated with such cover crops were studied. Tw o legume species [mucuna, Mucuna pruriens (L) DC. var. utilis (Wright) Bruc k and lablab, Lablab purpureus L. Sweet], one grass/weed species [imperata, Imperata cylindrica (L.) Rauschel] and a cereal (maize, Zea mays L.) were used. There were three soil treatments (fumigation, fumigation plus inocula tion with Meloidogyne species, and an untreated control). Plant parasitic n ematode populations in soil, roots and nodules were determined at 4, 8 and 12 weeks after planting. The response of the phytoparasitic nematodes to so il treatments varied according to the plant species present. The predominan t nematodes in soils, roots and nodules of legumes were of the genus Meloid ogyne, whereas other genera of parasitic nematodes dominated the fauna in s oils and roots of maize and imperata. Biomass yield of mucuna was not signi ficantly affected by either Meloidogyne spp. or the other genera of phytopa rasitic nematodes. In contrast, the dry matter yield of lablab measured at 12 weeks was reduced by 16% in inoculated compared with fumigated soils. Si milarly, the biomass yields of maize and imperata were reduced by 10% and 2 9%, respectively, in unfumigated rather than fumigated soils. The amounts o f N accumulated in mucuna, maize and imperata were not significantly affect ed by the two groups of plant parasitic nematodes. However, at 12 weeks, la blab grown on inoculated soils accumulated only 69% of the N found in plant s grown on fumigated soils. Inoculation of soil with Meloinogyne spp. signi ficantly increased the number of nodules on lablab roots compared with the noninoculated treatments, whereas nodulation in mucuna was not affected by soil treatment. After 12 weeks, the quantity of N-2 derived from symbiotic fixation in mucuna was not significantly affected by soil treatments wherea s the amount of fixed N in lablab was 32% lower in inoculated than in fumig ated soils. Possible mechanisms for the non-suppressive effect of plant par asitic nematodes on mucuna are discussed.