Subsoil Nitrate and Bromide Uptake by Contrasting Alfalfa Entries

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. Joint publication of the USDA-ARS and the Minn. Agric. Exp. Stn. Paper no. 981250044 of the MAES scientific journal series.