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Comparison of Inoculant and Indigenous Rhizobial Dinitrogen Fixation in Cowpeas by Direct Nitrogen-15 Analyses¹

ABSTRACT

Soil that contained ¹⁵N enriched organic matter (0.461 % ¹⁵N) was used to determine competitiveness of six strains at different logarithmic inoculum densities against indigenous rhizobia and against a previous surviving inoculant (strain P132). Analyses of N content of plant tissues by direct ¹⁵N technique showed that cowpeas (Vigna unguiculata L. Walp.) were capable of deriving 60 to 98% of shoot N from N₂ fixation. The two fast-growing strains (176A26 and 176A28) were poorer competitors and fixed less N₂ compared to the other slow-growing strains. Inoculum density had no effect upon yield response of cowpeas, but inoculation with strains P132, 401, and 22A1 effected greater seed yield, shoot dry matter, total N, and percentage of N derived from fixation (86-98%) than other strains and the uninoculated control (60-73%). By contrast, N₂ fixation and yield parameters in inoculated cowpeas were not significantly different from inoculated controls that contained residual P132 from a previous inoculum study. The higher hydrogen uptake (Hup) efficiency of nodules containing residual P132 (98 ± 2%) facilitated presumptive identification of P132 (100% ± 0 Hup efficiency axenically) as the surviving and infecting inoculant strain since nodules infected by indigenous rhizobia had lower Hup efficiencies (88 ± 2%).

¹ Contribution from the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521.

² Former Graduate Student and Professor of Soil Microbiology, respectively, Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521. Reprint requests should be sent to Dr. Focht.

Received for publication January 25, 1985.