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Alfalfa

Nitrogen Fixation and Transfer in a Mixed Stand of Alfalfa and Bermudagrass

^a Texas Agric. Exp. Stn., Texas A&M Univ., P.O. Box 200, Overton, TX 75684-0200
^b Los Alamos National Lab., Los Alamos, NM
^c Texas Agric. Exp. Stn., Texas A&M Univ., College Station, TX 77843

^* Corresponding author (v-haby{at}tamu.edu)

Received for publication March 21, 2005. Information about dinitrogen (N₂) fixation and transfer is needed to determine potential benefits from growing binary mixtures of alfalfa (Medicago sativa L.) and hybrid bermudagrass [Cynodon dactylon (L.) Pers.] on Coastal Plain soils of the USA. Our objectives were to quantify N₂ fixed by alfalfa and transferred to bermudagrass. ‘Alfagraze’ alfalfa was grown at row spacings (RS) of 23, 46, 69, and 91 cm in a mixed stand with ‘Coastal’ bermudagrass on a Darco loamy fine sand (loamy siliceous, semiactive thermic Grossarenic Paleudult). The mixed stand was fertilized with 0, 28, 56, 84, and 112 kg N ha^–1 applied for each bermudagrass regrowth. Isotope dilution was used to estimate N₂ fixation and subsequent transfer to bermudagrass in the zero-applied N plots. Wider alfalfa RS significantly increased bermudagrass yields. Higher N rate (NR) significantly improved bermudagrass yield during cool temperature and drought stress periods in 1994 and 1996. Narrower RS and higher NR significantly increased alfalfa production. Legume N derived from the atmosphere ranged from 42 to 91% and the fixed N yield ranged from 80 to 222 kg N ha^–1 yr^–1. Bermudagrass N derived from atmospheric N₂ fixation ranged from 1.0 to 77%, and the transfer N yield (TNY) was 18 kg N ha^–1 yr^–1. Results varied depending on harvest date, year, treatment, and grass yield. Alfalfa RS had little effect on TNY. The low bermudagrass yield and subsequent TNY in this binary forage production system suggest that monoculture alfalfa may be the best approach for managing alfalfa on Coastal Plain soils.

Abbreviations: %Ndfa, percentage N derived from the atmosphere • %Ngdfa, percentage N in grass derived from the atmosphere • DMY, dry matter yield • FNY, fixed-N yield • NR, nitrogen rate • RS, row spacing • TNY, transferred-N yield