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Nitrogen Transfer from Siratro to Kleingrass: Competition for Mineral Nitrogen and Isotopic Fractionation¹

Availability of combined N limits productivity of Kleingrass (Panicum coloratum L.) when grown with siratro (Macropitilium atropurpureum D.C. Urb.). An experiment was undertaken in a glasshouse to measure the quantity of N transferred from siratro to Kleingrass and the competitive relationship between siratro and Kleingrass for mineral N. Plants were grown for 187 days in pure stand and in mixture and fertilized with two rates of ¹⁵N-labelled (NH₄)₂S0₄ provided in split applications. The only sources of N to the plants was from the labelled fertilizer and biologically fixed N by siratro. Kleingrass obtained approximately 60% of the N recovered when grown with siratro at either rate of N fertilization. Siratro transferred biologically fixed N to Kleingrass, which accounted for 14 and 5% of the total N in the Kleingrass for the low and high rates of N fertilization, respectively. Isotopic fractionation of N occurred between shoots and roots of Kleingrass and was not a result of fractionation during N uptake. Caution needs to be exercised in assuming isotopic fractionation does not influence results of experiments using isotope dilution to measure N fixation.

Key Words: Panicum coloratum L. • Macropitilium atropurpureum (D.C. Urb.) • Isotope discrimination • Nitrogen fixation • Nitrogen-15

² Graduate research assistant and professor, respectively, Soil & Crop Sciences Dep., Texas A&M Univ., College Station, TX 77843.

Received for publication May 27, 1986.