Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/5/1998
Publication Date: N/A
Interpretive Summary: Alfalfa is a high value perennial crop used mainly as a quality livestock feed. Its roots can grow quite deep in soil, which allows it to absorb water and nutrients from areas unavailable to annual crops, like corn, wheat, and soybeans. Like other legumes (such as peas, beans, and clover), alfalfa forms a symbiotic association with certain bacteria that provides most of the nitrogen alfalfa needs from the atmosphere. Alfalfa also can absorb nitrogen from the soil. Very large amounts of nitrogen are harvested in every crop of alfalfa, providing a good way to remove excess nitrogen from the soil. This nitrogen is in the form of plant proteins that help produce meat and milk on our farms. Therefore, alfalfa could be used to clean up sites contaminated with too much nitrogen, for example at fertilizer spills, old feedlots, and so on. In this research, we found that most alfalfas that we tested were similarly good at absorbing nitrogen nfrom soil. However, we also found that a new type of alfalfa was nearly 40% better in taking up nitrogen from the soil than normal alfalfas. This confirmed our earlier research, which had suggested that this new alfalfa was superior in nitrogen absorption. In addition, we tested a new, less expensive method of measuring soil nitrogen uptake by legumes. The new method worked very well under difficult field conditions. This new method provides a more affordable and more flexible way for researchers to develop new legume varieties that improve the environment.
Technical Abstract: Alfalfa (Medicago sativa L.) is a deeply rooted perennial legume that can protect the environment by absorbing NO3 from subsoil beneath the rooting zone of annual crops. Our objectives were to characterize the potential for subsoil NO3 removal of eight alfalfa germplasms differing in symbiotic efficiency, root system architecture, forage quality, and leaf morphology, and to evaluate Br as an alternative tracer to 15N for monitoring NO3 uptake. Low (0.3 mM) or high (20 mM) NO3-N concentrations were supplied through a subsoil irrigation system installed in a Hubbard loamy sand soil (sandy, mixed Udorothentic Haploboroll) at Becker, MN. Nitrate uptake and N2 fixation were evaluated during two regrowth periods using 15N, and we added small concentrations of Br- to the subsoil NO3 during three regrowth periods. A strong correlation between 15N and Br uptake in the herbage (mg excess Br = 25.2 x mg excess 15N), led us to conclude that Br can be used as a tracer of NO3 absorption by alfalfa in the field, providing a new, dual tracer approach to such studies. Subsoil NO3-N removal was similar for the seven N2-fixing germplasms, suggesting that selection for traits such as root system architecture, high forage quality, or multiple leaflets does not necessarily confer an advantage in NO3 absorption. Even though Ineffective Agate yielded less herbage than N2-fixing germplasms, this non-N2-fixing cultivar removed about 38% more subsoil NO3 than N2-fixing germplasms over a growing season. These results confirm our earlier findings that non-N2-fixing alfalfa germplasms likely will be more effective in phytoremediation of NO3-contaminated sites than N2-fixing germplasms, and that the simple difference technique may underestimate rates of symbiotic N2 fixation when inorganic N supply is large.