Field Evaluation of Reference Crops in the Study of Nitrogen Fixation by Legumes Using Isotope Techniques 1

Nitrogen fixed by a legume crop can be measured in a field fertilized with labeled 15 N when a suitable non‐fixing reference crop is grown also. Legume and reference crop, respectively, are used with 15 N fertilizer to determine available soil plus fixed N and available soil N. Nitrogen derived from fixation is computed from the difference between these two estimates of N availability. Suitability of a reference crop depends on its utilization of fertilizer and soil N in the same proportion as the legume crop. Because this cannot be evaluated directly, the acceptability of reference crop data was examined by determining whether the proportion of fertilizer and soil S taken up by legume and non‐fixing reference crops is the same. Various reference crops for determining N fixation by broadbean ( Vicia faba L.) and by soybean ( Glycine max L. Merr.) were examined by this approach in field experiments on a Typic Eutrocrept using 15 N and 35 S labeled ammonium sulfate to determine plant available amounts (A‐values) of N and S in the soil. Nitrogen A‐values for broadbean were clearly higher than for any of the non‐fixing crops at two harvest dates. At physiological maturity of the legume, N fixation estimated from A‐value comparisons was approximately 140 kg/ha. Sulfur A‐values were reasonably similar among all crops studied but somewhat lower for oil radish ( Raphanus sativus L.) which proved to be an inappropriate reference because it took up much more S than the other crops. Nitrogen A‐values for inoculated soybeans at the R 3 stage of maturity were similar to those for the several reference crops including non‐nodulating soybeans. At stage R 6 , N fixation was clearly evident from A‐value comparisons but no greater than 60 kg/ha. By creating a N deficient condition in the soil using sucrose to immobilize N in an experiment with soybeans, the percentage of N in the crop derived from fixation was increased from 30 to 80%.