Measurement of N2 fixation in 30 cowpea (Vigna unguiculata L. Walp.) genotypes under field conditions in Ghana, using the 15N natural abundance technique

In 2005 and 2006, 30 and 15 cowpea genotypes were respectively evaluated for plant growth and symbiotic performance at Manga in Northern Ghana, in order to identify N-2-fixing potential of these cowpea genotypes as source of N for cropping systems. The results showed differences in biomass production by the 30 or 15 cowpea genotypes. In 2005, cultivars Fahari, Mchanganyiko, IT97K-499-39, IT93K-2045-29 and IT84S-2246 produced the most shoot biomass, while Apagbaala, Brown Eye, ITH98-46, Vita 7 and Iron Grey produced the least. Of the 15 genotypes tested in 2006, cv. TVu11424 produced the largest amount of biomass, and ITF198-46, the least. Isotopic analysis of N-15 in plant parts also revealed significant differences in delta N-15 of the cowpea genotypes studied. As a result, the percent N derived from fixation (% Ndfa) also differed among the cowpea genotypes tested in 2005, with only 5 out of the 30 cultivars obtaining over 50% of their N from symbiotic fixation. Whether expressed as mg N.plant(-1) or kg N.ha(-1), the levels of N-2 fixation by the cowpea genotypes varied considerably during 2005 and 2006, with values of N contribution ranging from 14.1 kg N.ha(-1) by cv. TVu1509 to 157.0 kg N.ha(-1) by IT84S-2246 in 2005. The amounts of N-fixed in 2006 ranged from 16.7 kg N.ha(-1) by cv. ITH98-46 to 171.2 kg N.ha(-1) by TVu11424, clearly indicating genotypic differences in symbiotic N yield. Re-evaluating 15 out of the 30 cowpea genotypes for N-2 fixation in 2006, revealed higher % Ndfa values (>50%) in all (15 cowpea genotypes) relative to those tested in 2005, indicating greater dependence on N-2 fixation for their N nutrition even though, the actual amounts of fixed-N were lower in 2006. This was due, in part, to reduced plant biomass as a result of very late sampling in 2006, close to physiological maturity (72 DAP in 2006 vs. 46 DAP in 2005) when considerable leaf matter was lost. The amount of N-fixed in 2006 can therefore be considered as being under-estimated.