Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/5/1998
Publication Date: N/A
Citation: Interpretive Summary: There is increased evidence that ground water and surface water supplies are becoming contaminated with a form of nitrogen called nitrate. Nitrate can be toxic to human infants and young livestock when consumed in excess. Because nitrate moves through soil easily, methods are needed to remove excess nitrate from soil before it contaminates water supplies. Alfalfa, a avery important forage crop for milk and meat production, can obtain the nitrogen it needs for growth from the atmosphere, but also is capable of taking up and using nitrate-nitrogen from the soil. In earlier research, we discovered that certain kinds of alfalfa are superior to other alfalfa in absorbing nitrate. We plan to develop new alfalfas with increased capacity to take up nitrate, but we needed to find an affordable way to do this. The problem has been that it is difficult and expensive to differentiate between nitrogen in the plant that came from the atmosphere and the nitrogen that was taken up by the plant as nitrate from the soil. In this research we found that nitrate and a naturally occurring ion called bromide are taken up similarly by alfalfa. This means that bromide can be used as an economical way to estimate nitrate uptake in alfalfa. This is a breakthrough that will allow us to begin developing improved, high yielding, and high quality alfalfa that is efficient in cleaning up nitrate-contaminated sites. Planting alfalfa to remove nitrate from soil and water supplies will be much more economical than standard clean up techniques, which depend on digging up and removing contaminated soil or cleaning water in large, expensive treatment facilities.
Technical Abstract: The 15N isotope is a reliable way to measure nitrate-N (NO3-N) uptake in N2-fixing legumes but is too expensive for plant breeding programs. Our goal is to develop an affordable method of selecting alfalfa (Medicago sativa 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, reflected NO3-N uptake in alfalfa. 'Webfoot' (1994 and 1995), 'Agate' (1995), and 'Ineffective Agate' (1995) alfalfa were grown from seed in pots in the greenhouse. After clipping herbage to 5 cm, solutions containing Br and 15N-labeled NO3 were added to the pots daily. Uptake of NO3-N and Br was determined in herbage regrowth sampled after 15 to 35 d and the molar ratio of NO3-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 hincreased with applied tracer concentration, but ratios in herbage were no constant for treatment, germplasm, or days of regrowth. These results imply that selection must be done 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-N:Br in herbage directly reflected applied solution ratios in soil/sand mixture (1994) but were less responsive in sand (1995) above molar ratios of 100 in solution. Individual plant analysis showed close agreement in Br and 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 selecting alfalfa for NO3 uptake.