Ineffectively and Effectively Nodulated Alfalfas Demonstrate Biology Nitrogen Fixation Continues with High Nitrogen Fertilization

Estimates of biological nitrogen fixation (BNF) require a non-N₂-fixing control preferably of the same species as the crop under evaluation. A field experiment was conducted to evaluate two ineffectively nodulated alfalfa (Medicago sativa L.) germplasms as non-N₂-fixing controls for estimating BNF under increasing N fertilizer applications. ‘Ineffective Agate’ and ‘Ineffective Saranac’ and their effective parental cultivars, Agate and Saranac, were planted in field plots at Rosemount, MN, in 1985 and 1986. Forage yields were taken in each study at early bloom for 2 yr. Plots received four rates of ammonium nitrate (0, 70, 140, and 210 kg N ha⁻¹) after each harvest. Biological nitrogen fixation in the 1986 plots was estimated by both the ˡ⁵N isotope dilution technique and the difference method. In the presence of adequate inorganic N, ineffectively noduiated plants were comparable to their effective counterparts in herbage yield and N content. No difference in herbage yield or N content at any N fertilizer treatment was observed between Ineffective Agate and Ineffective Saranac, indicating that the two entries would be comparable non-fixing controls for field assessment of BNF. The isotope dilution technique and the difference method gave comparable estimates of the change in BNF with applied N. Estimates of BNF declined in both Agate and Saranac in response to increasing N fertilization, but continued at high rates at the highest fertilizer N addition. A first production year stand of alfalfa under an annual N application rate of 840 kg/ha obtained 20 to 25% of its N from BNF. This is the first report that BNF continues with high levels of N application. Joint contribution of the USDA-ARS, Plant Sci. Res. Unit., U.S. Dairy Forage Res. Center (Minnesota cluster location), and Minn. Agric. Exp. Stn. Paper no. 21048 of the MAES scienctific journal series.