Effects Of NO₃^--N/NH₄⁺-N Ratios On Growth, Physiological Characteristics And Quality Of Alfalfa

The nitrogen form used in soil was mainly NO3--N and NH4+-N mixed, and it both can be absorbed by alfalfa. But which ratio of NO3--N and NH4+-N mixing is more conducive to alfalfa growth, yet in-depth study. Thus we simulate the different proportions of NO3-- N and NH4+-N in soil in this study. The growth, physiological characteristics and nutritional quality of alfalfa(Medicago sativa L.) were studied based on seven ratios of NO3--N/NH4+-N(1:7, 1:3, 3:5, 5:5, 5:3,3:1, 7:1) under the nitrogen level of 210 mg·L-1 during the whole growth stages(seeding, budding, flowering, seed podding and maturing). Gannong No.3 was selected as material, and the experiment was conducted outdoors(in flashing net room) using sand culture method. The main results are as follows:1. Under the seven ratios of NO3--N/NH4+-N by mixed exogenous nitrogen level of 210 mg·L-1, the height and biomass of alfalfa were the largest at the maturing stage. While the root total length, surface area, volume and root average diameter were all larger at the flowering stage. These indexes showed a maximum at the NO3--N: NH4+-N ratio of 5:2. The chlorophyll content, photosynthetic characteristics(photosynthetic rate, transpiration rate, stomatal conductance and CO2 concentration) and the activity of the enzyme which related to nitrogen metabolism(nitrate reductase and glutamine synthetase) were all maximized in flowering stage, and the above indicators were best at the NO3--N: NH4+-N ratio of 5: 3, except for that glutamine synthetase activity was best at the ratio of 1:7.3. The molybdenum, manganese and iron content of alfalfa in each part were higher from seeding to flowering stage when NH4+-N content was higher, while they had a higher content from seed podding to maturing stage. During the whole growth period,Zinc content was higher when the proportion of NO3--N content was higher, especially at the ratio of 1:7.4. The growth characteristics, photosynthetic characteristics, nitrogen fixation characteristics, nitrogen metabolism and the use of micronutrients of alfalfa in the vegetative growth stage are performed better when the treatment had hinger NH4+-N content, especially at the ratio of 1:7. While the most of these indicators performed best at the ratio of 5:3 in the reproductive stage. Therefore, in the production of alfalfa, if in terms of the forage grass mowing, the soil should maintain a high level of NH4+-N content before the flowering stage of alfalfa, it was more conducive to the growth of the vegetative stage, and increasing of grass yield. If in terms of harvesting the seeds,the soil should maintain a high level of NH4+-N content from seeding to budding stage,while maintaining a high level of NO3--N content after flowering stage.