EFFECTS OF INOCULUM SIZE AND PHOSPHORUS APPLICATION ON NODULATION AND N2 FIXATION IN COWPEA (VIGINA UNGUICULATA (L.) WALP) USING 15N METHODOLOGY.

Abstract

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 IR_C 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.