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

Abstract

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.