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

Abstract

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.