Measuring Symbiotic Nitrogen Fixation in Rangeland Plots of Trifolium subterraneum L. and Bromus mollis L.1

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Abstract
Accurate assessment of seasonal nitrogen fixation in improved rangeland is an important objective for researchers investigating N budgets and management impact. The 15N A‐value technique and the acetylene reduction method were compared for their accuracy in determining seasonal symbiotic N2 fixation on a ground area basis in rangeland plots of Trifolium subterraneum L. and Bromus mollis L. To simulate range management practices, seeds were planted at 10, 141, or 1970 seeds/m2 in pure stands or 50:50 mixtures in Laughlin loam, a member of the fine‐loamy, mixed, mesic family of Ultic Haploxerolls. Amounts of N2 fixed, calculated by the 15N A‐value technique, varied significantly with planting density and proportion of clover at different densities. Seasonal, symbiotic N2 fixation, calculated from the acetylene reduction method, did not vary significantly with either planting density or proportion of clover. It is concluded that with appropriate studies on possible differences in isotope accumulation by a grass and a legume, including morphological, physiological, and biochemical differences, the 15N A‐value technique should be more quantitative and discriminatory than the acetylene reduction technique. The single seasonal sampling requirement of the 15N A‐value technique also is more amenable to the overall goal of determining the effect of range management systems on symbiotic N2 fixation.

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