Interactive influence of phosphorus and iron on nitrogen fixation by soybean

Adequate supplies of phosphorus (P) and iron (Fe) to legumes have been shown to be crucial in obtaining high nitrogen fixation rates and growth. These responses are anticipated as a result of the high requirement for P in energy transfer processes in the nodule and for Fe as a constituent of nitrogenase and leghemoglobin. However, little attention has been given to documenting the response of nitrogen fixation rates resulting from concentrations of P and Fe that actually exist in nodules. In particular, an open question is whether there is an interaction between nodule P and Fe concentrations that maximize nitrogen fixation activity. This study was designed to induce various concentrations of P and Fe in the nodule and to measure the resultant nitrogen accumulation and nitrogen fixation rates. Plant nitrogen accumulation was linearly correlated with both nodule P and Fe concentration, and with total plant nitrogen fixation rate as measured by acetylene reduction rate. Therefore, total nitrogen fixation rate was also correlated with nodule P and Fe concentrations, but a higher linear correlation was obtained for Fe as compared to P concentration. Surprisingly, nodule ureide concentration, which is generally assumed to be a positive index of nitrogen fixation rate, was negatively correlated with nodule P and Fe concentrations. These results indicated that higher concentrations of P and Fe in the nodules not only stimulated higher nitrogen fixation rates, but were associated with an enhanced ability to export ureides from the nodules. Since there was a linear response to both P and Fe over the range of nodule concentrations induced in these experiments, no evidence for optimum interactive concentrations of these two elements in the nodules was obtained.