Nitrogen Fixation in Some Prairie Legumes

Abstract

Acetylene reduction was compared in five important leguminous species of the central North American grasslands. Nodulated seedlings were obtained by natural or artificial means, watered with nitrogen-free nutrient solution, and maintained under uniform conditions in growth chambers. Seedlings 9-13 weeks old were assayed weekly by the C2H2 C24 technique using gas chromatography. Most species differed significantly in total N2 fixation expressed as ,u moles of C24 daily/plant, and in nodule efficiency expressed as v1 moles of C2H4 hourly/mg fresh nodules. Using the ranges of the weekly assay means as indicators, the species ranked in total N2 fixation as follows: Cassia fasciculata (25.9-87.0), Lespedeza capitata (7.7-16.6), Schrankia uncinata (3.1-8.9), Amorpha canescens (3.5-7.0) and Psoralea argophylla (1.1-1.9). The order for nodular efficiency was L. capitata (6.2-1 1.1),, C. fasciculata (3.1-4.7), A. canescens (2.6-4.2), S. uncinata (1.3-4.1) and P. argophylla (1.52.7). Measurements of growth rates in most cases paralleled the fixation rates. The results of this study suggest the possibility that species (C. fasciculata and L. capitata) which occupy niches in pioneer through late seral stages of grassland succession have a greater N2-fixing capacity than species more limited to the climax (A. canescens, S. uncinata, P. argophylla). INTRODUCTION Domestic legumes have been widely studied in regard to nitrogenfixing capacity and stimulation of pasture and forage production (Stewart, 1966). Similar information on legumes of native grasslands of the world is lacking (Coaldrake, 1962), although legumes reach high densities in the central North American grasslands (Weaver, 1954; Weaver and Albertson, 1956). In the true prairie of eastern Nebraska, Warren (1909) studied the extent of nodulation of various species, and Alway and Pinckney (1909) estimated that various legumes added 9.4 kg N/ha/year to the soil. This estimate was based on the harvest method and Kjeldahl analysis of total N of the tops; no data were given for N added from nodules or from root decay. In central Oklahoma, Kneebone (1959) studied nodulation of prairie legume seedlings and found nodules in all species examined; however, in mature individuals there was often a paucity or absence of nodules indicating absence of fixation. In the mixed prairie of Kansas and Nebraska, Weaver and Albertson (1956) reported nodules from depths