Comparison of methods of estimation biological nitrogen fixation using cowpea cultivars from Ghana


  • A.B. Ankomah Department of Crop Science, University of Science and Technology, Kumasi, Ghana, West Africa


A-Value, Ghana cowpeas, Nitrogen fixation measurement, Isotope dilution, Total nitrogen difference


A greenhouse pot experiment was conducted at the FAO/IAEA Biotechnology Laboratory at Seibersdorf in Austria to estimate the biological N2-fixation of three Ghanaian cowpea cultivars (Amantin, IT81D-1137, and Soronko). Plants were grown with or without bradyrhizobia inoculation and unfertilized or fertilized with 50 mg P kg-1 soil in an Austrian Wardviertel surface soil. The 15N isotope dilution (ID), A-Value (AV), and total nitrogen difference (TND) methods were used to quantify the N fixed, using a non-nodulating soya bean isoline (CV 129) and the uninoculated (non-nodulated) cowpea plants as the non-fixing reference crops. There were no significant differences (P = 0.05) in the per cent N fixed (per cent N derived from air, %Ndfa) estimated by the ID, AV and TND methods. However, the experimental error of the TND was higher (C.V. = 32%) than that of the isotope methods (C.V. = 21-25%). The lower precision of the TND than the isotope methods might be ascribed to the more variable yield component in the estimation by the TND method. Per cent N fixed (estimated by ID) was 42, 62 and 76%, and corresponding amounts fixed were 26, 59, and 99 mg N plant-1 respectively, in IT81D-1137, Amantin and Soronko. Amounts of N fixed were significantly correlated with nodule dry weight (r = 0.95; P = 0.01) and with total N in shoots (r = 0.99; P = 0.001). The non-nodulating soya bean was as good a reference crop as the uninoculated (non-nodulated) cowpea plants for measuring N2-fixation in cowpea by the isotope methods.

How to Cite

Ankomah, A. (1995). Comparison of methods of estimation biological nitrogen fixation using cowpea cultivars from Ghana. Tropical Agriculture, 72(1). Retrieved from



Research Papers