Evaluation of nitrogen fixation in legumes by the regression of total plant nitrogen withe weight

Abstract

In six greenhouse and two field experiments with beans (Phaseolus vulgaris), soybeans (Glycine max) and Centrosema pubescens, significant regressions were found of the logarithm of total plant nitrogen with nodule weight. With these regressions it became possible to evaluate the amount of nitrogen absorved from seeds and soil (Y intercept α) and the amount of nitrogen fixed per unit nodule tissue (regression coefficient b). The logarithm of N fixed for each nodule weight can then be calculated by NF ═ bX. The regression coefficient was quite constant for each species not being affected by a number of factors such as liming, fertilizing with major and minor elements (except N), pH levels, soil temperatures, plant varieties and most Rhizobium strains. One completely inefficient Rhizobium strain and the addition of mineral nitrogen were the only factors affecting the regression coefficient. It was concluded that most factors affecting nitrogen fixation of legumes act on nodule numbers or sizes but not on the amount of nitrogen fixed/unit nodule tissue.