Subsoil Nitrate and Bromide Uptake by Contrasting Alfalfa Entries
- Jürg M. Blumenthal,
- Michael P. Russelle and
- JoAnn F.S. Lamb
- P anhandle Res. & Ext. Ctr., Univ. of Nebraska, 4502 Avenue I, Scottsbluff, NE 69361-4939
P lant 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
P lant Sci. Res. Unit, USDA-ARS, and Dep. of Agronomy and Plant Genetics, 411 Borlaug Hall, Univ. of Minnesota, St. Paul, MN 55108-6028
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 × 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.
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