Effect Of Nitrogen Forms And Supply Levels On The Growth Characteristics , Nodulation And Nitrogen Fixation Of Alfalfa

As a kind of qualified leguminous forage grass, alfalfa (Medicago sativa L.) absorbs the molecular nitrogen (N) in the air by means of symbiotic N fixing bacteria in nodules, whereas the effective symbiosis procedure between alfalfa and rhizobia always been affected by ecotype, especially for the external N environment. In order to reflect the effect of N supplying on nodulation and N fixing accurately and objectively, and ascertain the appropriate N form and level for alfalfa growth, pot culture with sand and Hoagland's solution was adopted, and Gannong NO.3 was used as test material, 5 concentrations (0, 105, 210, 315, 420 mg·L^-1) of the 2 N forms that could be assimilated directly by plants (NH₄⁺_N and NO₃^-_N) were adopted, to study various N levels and forms on growth characteristics, N fixing and nodulation capability of alfalfa inoculated with 12531 and LW107. The results are listed as follows:1. On condition of the 2 N forms (NH₄⁺_N and NO₃^-_N), for both inoculated and un-inoculated plants, the shoot height, leaf number, leaf area, chlorophyll content, root volume, root activity, biomass, total N content and N assimilation were found higher than N free treatments. Therefore, the appropriate application of N on alfalfa could promote the production characteristic.2. Compared with NO₃^-_N, for both inoculated and un-inoculated plants that treated with NH₄⁺_N, the shoot height, leaf area, root length, root volume, root activity, biomass, total N content, N assimilation, nodule number, nitrogenase activity and N fixation content performed better, indicating that NH₄⁺_N is an appropriate form for alfalfa growth, therefore, ammonium or amide state nitrogen (viz. urea) is suggested in alfalfa production.3. When the N level was at 315 mg·L^-1, the biomass of alfalfa plants performed best on condition of the 2 N forms and 5 concentrations; whereas nodule number and nitrogenase activity performed best at 210 mg·L^-1 N level, indicating that 315 mg·L^-1N level promoted the accumulation of alfalfa dry matter while 210 mg·L^-1 N level promoted the nodulation and N fixation of alfalfa.4. Compared with un-inoculated plants, the plants inoculated with LW107 and 12531 rhizobia performed better in growth, nodulation and N fixation; the optimum nodule number and nitrogenase activity were found at 210 mg·L^-1 N level, especially when being treated with NH₄⁺_N. Therefore, rhizobia inoculation with appropriate exogenous N promoted the growth, nodulation and N fixation of alfalfa. According to the inoculation comparison of the 2 rhizobia, the effect of 12531 was found better in accelerating Gannong NO.3 growth, nodulation and N fixation than LW107.