Yield, nodulation, and N-2 fixation by cowpea cultivars at different phosphorus levels

A greenhouse experiment was conducted to investigate the effect of a P application (0 vs. 50 me P kg(-1)) on yield. nodulation. and N-2 fixation by three cowpea cultivars (Soronko, Amantin, and IT81D-1137) using the N-15 isotope-dilution method. When P was not applied the inoculated cowpea genotypes showed significant differences (Soronko>Amantin> IT81D-1137) in N accumulation, in contrast to the uninoculated cowpea cultivars, which accumulated similar amounts of N. The differences in shoot N in inoculated plants were thus caused by differences in N-2 fixation. The average values of N fixed (for both P levels) were 74% in Soronko, 59% in Amantin, and 42% in IT81D-1137, corresponding to 80, 51, and 24 mg N plant(-1) respectively. inoculation increased the total shoot-N accumulation in cv, Soronko by 270% without P and by 204% with P, cv. Amantin by 152 and 104%, and cv. IT81D-1137 by 74 and 58%, respectively. With P, the % N derived from atmosphere (%Ndfa) was 42%, for IT81D-1137, 62% for Amantin, and 76% for Soronko. The high value for Soronko indicates that in a soil of medium fertility, certain cowpea cultivars are capable of satisfying their total N requirement through N-2 fixation. The P effect on N-2 fixation was mainly in the total amount of N fixed rather than on the percentage derived from the atmosphere.