INEFFECTIVELY AND EFFECTIVELY NODULATED ALFALFAS DEMONSTRATE BIOLOGICAL NITROGEN-FIXATION CONTINUES WITH HIGH-NITROGEN FERTILIZATION

Estimates of biological nitrogen fixation (BNF) require a non-N-2-fixi ng control preferably of the same species as the crop under evaluation . A field experiment was conducted to evaluate two ineffectively nodul ated alfalfa (Medicago sativa L.) germplasms as non-N-2-fixing control s for estimating BNF under increasing N fertilizer applications. 'Inef fective 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 earl y 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 fi xation in the 1986 plots was estimated by both the N-15 isotope diluti on technique and the difference method. In the presence of adequate in organic N, ineffectively nodulated plants were comparable to their eff ective counterparts in herbage yield and N content. No difference in h erbage yield or N content at any N fertilizer treatment was observed b etween Ineffective Agate and Ineffective Saranac, indicating that the two entries would be comparable non-fixing controls for held assessmen t of BNF. The isotope dilution technique and the difference method gav e 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. Th is is the first report that BNF continues with high levels of N applic ation.