Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/12/2007
Publication Date: 3/1/2008
Citation: Lamb, J.F., Russelle, M.P., Fenton, D.M. 2008. Field-based selection method creates alfalfa populations that differ in nitrate nitrogen uptake. Crop Science. 48(2):450-457. Interpretive Summary: New alfalfa cultivars that reduce nitrate-N losses to ground water and decrease fertilizer N requirements would be environmentally and economically beneficial to sustainable cropping systems. Alfalfa cultivars with greater capacity to remove nitrate would be useful for environmental protection and remediation, whereas alfalfa that is less competitive for nitrate may be useful in mixed plantings with grasses. The traditional method used to measure nitrate-N uptake in alfalfa is too expensive to use in a plant breeding program where thousands of plants must be measured individually. We developed a new and affordable field-based method to identify alfalfa plants that differed in their capacity to take up nitrate-N from the soil. We used this new method to successfully create alfalfa populations with either high and low amounts of nitrate-N uptake in the harvested hay. This new method can be used by commercial plant breeders to create new alfalfa cultivars with improved capacity to remove excess nitrate from over-fertilized fields or fertilizer spill sites to protect ground water from N contamination, or with decreased capacity for nitrate-N uptake to improve the nutrient quality of mixed species pastures for animal production systems.
Technical Abstract: Development of new alfalfa (Medicago sativa L.) germplasms to reduce nitrogen (N) losses to ground water and to decrease N fertilizer inputs would economically and environmentally benefit many agricultural systems. Alfalfa germplasm developed for differences in N2 fixation and nitrate uptake in the greenhouse did not express these traits in the field. The 15N isotope is a reliable, but expensive, way to measure nitrate-N uptake in N2-fixing legumes. Bromide (Br) is a promising alternative tracer for nitrate that is less expensive to quantify. Our objectives were to develop and refine an inexpensive selection method using Br as an analog for nitrate and to produce divergent alfalfa populations with altered nitrate-N uptake under field conditions. Two different selection schemes using divergent herbage Br concentration and/or divergent herbage Br uptake in combination with high herbage N content produced two Cycle 1 and four Cycle 2 alfalfa populations that were evaluated for both Br and 15N-nitrate uptake for one regrowth period in two locations in each of 2 yr. During the establishment year, Br and N derived from fertilizer (Ndff) in the herbage were not associated among the selected populations, whether expressed on a tissue concentration or uptake basis. In contrast, strong correlations were evident between Br and Ndff concentration (r=0.82 to 0.85) and uptake (r=0.85 to 0.90) among the selected populations during the first production year. Populations selected for high Br concentration or uptake had greater herbage yield, tracer concentration and uptake, and N uptake compared to populations selected for low Br. This new selection method, based on differences in herbage Br concentration or uptake in combination with high herbage N content, successfully produced alfalfa populations that differed in nitrate-N uptake. Selection and evaluation must be conducted after the establishment year in this perennial forage legume.