EFFECTS OF INOCULUM SIZE AND PHOSPHORUS APPLICATION ON NODULATION AND N2 FIXATION IN COWPEA (VIGINA UNGUICULATA (L.) WALP) USING 15N METHODOLOGY.
Keywords:inoculum size, nodulation, nitrogen fixation, cowpea, N methodology.
In a greenhouse study at the Institute of Agricultural Research and Training (IAR&T), Ibadan, Nigeria, isotope dilution method was used to evaluate the effect varying inoculum sizes with and without phosphorus (P) application on nodulation and N2 fixation of cowpea (Vigna unquiculata (L.) Walp) cv. Ife Brown. Different inoculum sizes (concentrations) were formulated by severally diluting a re-suspended (2.4 x 108 cells/ml) solution of Bradyrhizomium spp strain IRC 27 and mixing with known (1.5kg) weight of soil; phosphorus was applied to the appropriate pots. The control pots were neither treated with Bradyrhizobium spp nor P. Seeds of Ife Brown and a maize reference crop were sown after treatment application.
Significant (P<=0.05 interaction effects of inoculum size and P application were obtained on nodulation, dry matter yield, percentage nitrogen derived from atmosphere (% Ndfa), nitrogen yield, amount of nitrogen fixed separately but in varying degrees. The inoculum size of 3.2 x 105 cells/g soil plus P appeared to be most beneficial in terms of cowpea N2 fixation (proportion and amount) but on nodule number and dry matter yield variables, 104 cells/g soil plus P was found appropriate.
Therefore, in order to ensure high inoculation success, a range between the two inoculum sizes plus P application (20 ppm) is suggested. Phosphorus application caused significant variable increases in % Ndfa, amount of N fixed and N yield only when inoculum size higher than 103 cells/g soil was used, depicting that it was feasible to improve inoculation success through increasing inoculum concentration.