Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/17/1996
Publication Date: N/A
Citation: Interpretive Summary: Excessive nitrogen applications in agriculture have been implicated in contamination of surface water, ground water, and the atmosphere. Field studies routinely show that soil inorganic nitrogen concentrations and nitrate leaching losses to surface and ground water are smaller under deeply rooted perennial crops, like alfalfa, than under annual crops, like corn. This implies that alfalfa may be able to remove nitrate from the subsoil effectively. Nitrogen removed by alfalfa herbage originates not only from soil nitrogen but also from atmospheric nitrogen fixed by Rhizobium bacteria in root nodules and from nitrogen stored in the roots. In this study we determined how well two contrasting alfalfa cultivars removed nitrate from the subsoil. The cultivars were Agate alfalfa, which is a standard type planted by farmers, and a new alfalfa called Ineffective Agate, which is unable to obtain nitrogen from the air. We found that Ineffective Agate alfalfa removed twice as much subsoil nitrate than Agate alfalfa in the fall harvest of the year they were seeded and 30% more subsoil nitrate in the following summer harvest. These results suggest that special non nitrogen fixing alfalfa cultivars would be even better than standard alfalfa for bioremediation of nitrate contaminated sites. Because alfalfa converts nitrate to highly nutritious plant proteins, farmers will benefit by growing these alfalfas for livestock feed.
Technical Abstract: Alfalfa (Medicago sativa L.) is a deeply rooted perennial that may be able to remove nitrate from the subsoil beneath the rooting zone of annual crops. Our main objectives were to characterize the potential of two alfalfa cultivars (Agate and Ineffective Agate, an ineffectively nodulated, near isoline of Agate) to remove nitrate from the subsoil and to evaluate the influence of available subsoil nitrate on symbiotic N2 fixation. Low (ca. 0.3 mM) or high (20 mM) nitrate 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. During fall regrowth in the establishment year, supply of 20 mM 15N enriched nitrate through subirrigation did not decrease symbiotic N2 fixation, and during summer regrowth of the first production year symbiotic N2 fixation decreased only 40% (from 7.77 to 4.60 g N/m2). Even though it had higher yields, Agate alfalfa removed less subsoil nitrate N (1.52 and 5.51 g N/m2) than did Ineffective Agate (3.12 and 7.13 g N/m2) in each harvest taken in separate years. Root length and mass were not altered by either treatment or cultivar. This is the first time that direct measurements of subsoil nitrate removal have been made in the field for an N2 fixing crop. Because it is unlikely that differences in nitrate uptake efficiency could account for such cultivar differences, we propose that assimilation of atmospheric N2 reduced the capacity of the fixing crop to absorb nitrate. These results suggest that non N2 fixing alfalfa cultivars would be more useful for bioremediation of nitrate contaminated sites than standard cultivars.