Selection of the most appropriate reference crop for the assessment of nitrogen fixation by soya bean in Anambra State, Nigeria

Nitrogen fixation by various soya bean cultivars was estimated using upland rice, maize and sesame as reference non-legume crops to enable the selection of the most appropriate reference crop for Nigerian soil. The per cent (15)N atom excess of rice was consistently higher than that of the three soya bean cultivars at all sampling dates, indicating that the soya bean cultivars fixed N whereas rice did not. By contrast, the per cent (15)N atom excesses of sesame and maize were smaller than those of the soya of the soya bean cultivars at certain days after planting, giving a false impression of nitrogen fixation by sesame and maize. Using rice to calculate the per cent nitrogen derived from the atmosphere (NDFA) gave larger and consistently positive values. The use of maize and sesame as reference crops gave inconsistent negative and positive values for the NDFA. Thus, upland rice was a better reference crop for the calculation of NDFA under the prevailing conditions of the experimental site.