Nitrogen Fixation in Some Prairie Legumes

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Abstract
Acetylene reduction was compared in 5 important leguminous species of the central North American grasslands. Nodulated seedlings were obtained by natural or artificial means, watered with N-free nutrient solution, and maintained under uniform conditions in growth chambers. Seedlings 9-13 wk old were assayed weekly by the C2H2-C2H4 technique using gas chromatography. Most species differed significantly in total N2 fixation expressed as .MU. mol of C2H4 daily/plant, and in nodule efficiency expressed as .eta. mol 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 (6.2-11.1), C. fasciculata (3.1-4.7), A. canescens (2.6-4.2), S. uncinata (1.3-4.1) and P. argophylla (1.5-2.7). Measurements of growth rates in most cases paralleled the fixation rates. The possibility is suggested that species (C. fasciculata and L. capitata) which occupy niches as pioneers 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).

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