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Subsoil Nitrate and Bromide Uptake by Contrasting Alfalfa Entries

Panhandle Res. & Ext. Ctr., Univ. of Nebraska, 4502 Avenue I, Scottsbluff, NE 69361-4939
Plant Sci. Res. Unit, USDA-ARS, U.S. Dairy Forage Res. Ctr. (Minnesota Cluster), and Dep. of Soil, Water, and Climate, 439 Borlaug Hall, Univ. of Minnesota, St. Paul MN 55108-6028
Plant Sci. Res. Unit, USDA-ARS, and Dep. of Agronomy and Plant Genetics, 411 Borlaug Hall, Univ. of Minnesota, St. Paul, MN 55108-6028

^* Corresponding author (russelle{at}soils.umn.edu).

Alfalfa (Medicago saliva L.) is a deeply rooted perennial legume that can protect the environment by absorbing nitrate (NO^–₃) both better than annual crops and from deeper in the soil. Our objectives were to characterize subsoil NO^–₃ removal by eight alfalfa entries differing in symbiotic efficiency, root system architecture, forage quality, and leaf morphology, and to evaluate Br^– as an alternative tracer to ¹⁵N for monitoring NO^–₃ uptake. Low (=0.3 mM) or high (20 mM) NO^–₃-N concentrations were supplied through a subsoil irrigation system installed in a Hubbard loamy sand soil (sandy, mixed Udorthentic Haploboroll) at Becker, MN. Nitrate uptake and N₂ fixation were evaluated during two regrowth periods using ¹⁵N. We also added small concentrations of Br^– to the subsoil NO^–₃ during three regrowth periods. Subsoil NO^–₃-N removal was similar for the seven N₂-fixing entries, suggesting that selection for traits such as root system architecture, high forage quality, or multiple (>3) leaflets does not necessarily confer an advantage in NO^–₃ absorption. Even though ‘Ineffective Agate’ yielded less herbage than the N₂-fixing entries, this non-N₂ fixing cultivar removed about 38% more subsoil NO^–₃ than N₂-fixing entries over the growing season. These results confirm our earlier findings that non-N₂-fixing alfalfa cultivars are likely to be more effective in phytoremediation of NO^–₃-contaminated sites than are standard cultivars. It also confirms our earlier conclusion that the simple difference technique may underestimate rates of symbiotic N₂ fixation when inorganic N supply is large. A strong correlation between ¹⁵N and Br^– uptake in the herbage (mg excess Br^– = 25.2 x mg excess ¹⁵N, r² = 0.97) led us to conclude that Br- can be used as a tracer of NO^–₃ absorption by alfalfa in the field. This provides a new, dual-tracer approach for such studies.

Received for publication May 4, 1998.

This article has been cited by other articles:

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