Use of the 15N isotope dilution technique to study the effect of mineral nitrogen on biological nitrogen fixation in red clover

Red clover is often grown in mixed cultures with cereals. Because of the negative influence of the high mineral N required by cereals on biological nitrogen fixation (BNF) in legumes, such as red clover, it is necessary to select varieties of clover with a high fixing capacity in the presence of inorganic N. The 15N isotope dilution technique was used to study the influence of mineral fertilizer N on BNF. Red clover plants (Trifolium pratense cv. Maneta) were grown in a nutrient solution containing different concentrations of N in the form of labelled ammonium nitrate or nitrate (KNO3). The solution was inoculated with Rhizobium trifolii and the N concentrations were kept constant throughout the experiments. Despite the increase in dry matter yield and N content in plants, the fixation rate was drastically reduced with increasing mineral N concentration. This effect was clearer in ammonium nitrate treated plants than in nitrate treated ones. Although nodules kept a relatively high fixation capacity the proportion of nodule dry matter to total dry matter became very low with increasing mineral N concentration in the nutrients solution, leading to a great reduction in the fixation rate of plants. (author). 10 refs, 3 figs, 1 tab