Inorganic nitrogen supply and symbiotic dinitro‐gen fixation in alfalfa

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Abstract
It may be desirable to minimize dinitrogen (N2) fixation in alfalfa (Medicago saliva L.) when a source of inorganic nitrogen (N), such as manure, is readily available. Our objectives were to determine the N2 fixation response of eight alfalfa germplasms to inorganic N and to characterize plant‐to‐plant variation for this trait. Seed was sown in vermiculite and irrigated with nutrient solution in growth chambers. Herbage was removed at 71 d and treatments of 1, 3, 5, or 10 mM N were applied as 15N‐depleted ammonium nitrate (NH4NO3). After 34 d of regrowth, herbage was removed and analyzed for dry mass, total N concentration, and N isotope ratio. Increased availability of inorganic N resulted in a linear increase in herbage weight, height, shoot number, and N concentration, and consistently decreased N2 fixation for all germplasms. Estimated N2 fixation was greater than zero at the highest rate of inorganic N, which we speculate was due, in part, to remobilized root and crown N, because nodules appeared to be nonfunctional. Across all treatments, N2 fixation correlated best with herbage N concentration, but there was no relationship between these variables within a given N treatment concentration. Significant variation in reliance on N2 fixation in the presence of inorganic N existed in all eight germplasms.