Bromide as a tracer for nitrate-N uptake in alfalfa herbage

The N-15 isotope is a reliable way to measure nitrate-N (NO3--N) uptake in N-2-fixing legumes, but is too expensive for plant breeding programs. Our goal is to develop an affordable method of selecting alfalfa (Medicago saliva L.) for NO3--N uptake under field conditions. In this research, we tested whether the anion bromide (Br-), which is used to trace NO3--N movement in soils and is inexpensive to analyze, reflects NO3--N uptake in alfalfa. 'Webfoot' (1994 and 1995), 'Agate' (1995), and 'Ineffective Agate' (1995) alfalfa were established in pots in the greenhouse. After removing herbage, daily additions of solutions containing Br- and N-15-labeled NO3- were made to the pots. Uptake of Br- and N derived from NO3- was determined in herbage regrowth sampled after 15, 25, and 35 d and the molar ratio of NO3--derived N:Br- was calculated. In both years, when a constant molar ratio in solution was provided at variable tracer concentrations, NO3--N uptake and Br- uptake both increased with increasing applied tracer concentration, but ratios in herbage were not constant for treatment, germplasm, or days of regrowth, These results imply that selection must be carried out under well-controlled tracer supply and crop management. At a constant NO3--N concentration, but increasing solution Br- concentration, NO3--N uptake remained constant and Br- uptake increased, Molar ratios of NO3--derived N:Br- in herbage directly reflected applied solution ratios in a soil-sand mixture (1994), but were less responsive in sand (1995) above molar ratios of 100:1 in solution. Individual plant analysis showed close agreement in Brand NO3--N uptake among plants in all germplasms across a wide range of tracer supply, and indicated that selection for NO3--N uptake using Br- uptake would result in minimal error. We conclude that Br- is a promising alternative tracer for use in selecting alfalfa for NO3- uptake.