QUANTIFICATION OF THE CONTRIBUTION OF N-2 FIXATION TO FIELD-GROWN GRAIN LEGUMES - A STRATEGY FOR THE PRACTICAL APPLICATION OF THE N-15 ISOTOPE-DILUTION TECHNIQUE

Four experiments were performed at two sites in Brazil to evaluate the use of the 15N isotope dilution technique for the quantification of N2 fixation to inoculated and non-inoculated soybean, cowpea and groundnut using three independent control crops in each experiment. The data showed that the 15N enrichment of the N derived from the soil by the different control crops was different even though slow-release forms of labelled N had been applied. These results illustrate the principal problem associated with the use of the isotope dilution technique in that its application theoretically requires that all test ("N2-fixing") and control crops accumulate N from the soil with the same 15N enrichment. It was also shown that the three control crops behaved differently at the different sites, so that the idea that "appropriate" controls can be universally recommended for use with any particular legume crop is fallacious. The use of several control crops, in combination with slow-release labelled fertilizer, was shown to be a successful strategy for the application of this technique to quantify the contribution of N2 fixation to field-grown grain legumes. © 1990.