Submitted to: Physiologia Plantarum
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 12, 1996
Publication Date: N/A
Interpretive Summary: Excessive nitrogen fertilizer applications to agricultural soils have been implicated in nitrate contamination of surface and ground water. Alfalfa is a deeply rooted, perennial forage crop that is able to remove nitrate from the subsoil effectively; therefore, it might be used to clean nitrate contaminated sites. Nitrogen acquired by alfalfa originates not only from soil but also from atmospheric nitrogen gas that is fixed by soil bacteria in root nodules through a process called symbiotic nitrogen fixation. Total symbiotic nitrogen fixation by the plant depends on the weight of the root nodules and nitrogen fixing activity per unit weight. In this research we determined the effect of deep vs. shallow exposure of alfalfa root systems to nitrate on symbiotic nitrogen fixation and nitrate uptake. We found that the higher the nitrate concentration in the solution surrounding the roots, the more nitrate was taken up by alfalfa. As the plants took up more nitrate, the lower were both alfalfa root nodule weight and the nitrogen fixing activity per unit weight. Neither nitrate uptake nor nitrogen fixing activity per unit nodule weight were affected by the site of nitrate exposure (deep vs. shallow). The site of nitrate exposure, however, had a strong effect on root nodule mass. Exposure of the shallow, nodulated root system to nitrate for only 10 days resulted in a 20% lower nodule mass than exposure of the deeper, non-nodulated root system. Our results demonstrate the effects of localized exposure to nitrate on nitrogen acquisition in alfalfa and will help in design and development of bioremediation strategies for nitrate contaminated soils.
Technical Abstract: Alfalfa is a deeply rooted perennial legume that may be exposed to varying NO3 concentrations with depth. Our objective was to characterize the effect of localized (deep vs. shallow) exposure of alfalfa root systems to NO3 on symbiotic N2 fixation and NO3-N uptake. Cuttings of a single alfalfa plant were grown in vertical split root systems in a controlled environment chamber. The split root system was a rigid acrylic tube filled with silica sand and divided into upper and lower sections by a wax layer. Roots penetrated the wax layer, but mixing of nutrient solutions between the sections was prevented. Nodulation was restricted to the upper section. The plants were subjected for ten days to the following treatments: both sections of the split root system received nutrient solution containing either 0.5, 5.25, or 10 mM NO3; the upper section received 0.5 mM NO3 while the lower section received 10 mM NO3; or the upper section received 10 mM NO3 while the lower section 0.5 mM NO3. Increasing supply of NO3 in the nutrient solution to both sections resulted in higher NO3-N uptake, lower nodule mass, and lower specific nitrogenase activity. Although NO3-N uptake did not differ, plants exposed to 10 mM NO3 for ten days in the upper, nodulated section of the root system had 20% lower nodule mass than plants exposed to the same NO3 concentration in the lower, non-nodulated section of the root system. Specific nitrogenase activity was not different between these two treatments. Therefore, we conclude that: 1) nodule mass was dependent on two factors, the amount of NO3-N taken up and the concentration of NO3 within the nodulated root zone; and 2) specific nitrogenase activity was not affected by concentration of NO3 surrounding the nodules, but only by the amount of NO3-N taken up.