Abstract:

Inocculum of Sinorhizobium meliloti strain QL2 was applied to plants of eight alfalfa varieties （three imported varieties： WL168HQ， WL298HQ， and WL319HQ； three domestically bred varieties： Gannong No. 3， Gannong No. 5， and Gannong No. 9； and two local varieties： Qingshui and Longzhong） to clarify the differences in nitrogen fixation effects， with a view to improving the nodule formation and nitrogen fixation capacity of alfalfa and the efficiency of natural nitrogen utilization. Indicators such as nodule formation， nitrogen fixation， forage biomass， and nutritional quality were evaluated to study the differences in symbiosis， nitrogen fixation， and growth-promoting effects after inoculation with S. meliloti QL2. Results demonstrated that in domestically bred varieties， inoculating with S. meliloti QL2 significantly increased the weight of individual root nodules， as well as the root nodule diameter and the number of infected cells in the root nodules of by over 70% compared to both local and imported varieties. Regarding nitrogen fixation efficiency， the imported varieties （52.62% to 63.49%） and domestically bred varieties （53.30% to 62.41%） exhibited significantly higher fixation rates compared to local varieties （43.05% to 46.72%）. Hierarchical segmentation analysis showed that nodulation factors （the number of infected cells in the root nodules， the number of effective root nodules per plant， the single effective root nodule weight and root nodule diameter） and nitrogen fixation factors （nitrogenase activity， nitrogen fixation percentage and nitrogen fixation amount） explained 63% of the variation in the above-ground dry weight. Of the explained variation， 89.56% was contributed by nitrogen fixation factors， and only 13.55% by nodulation factors. A correlation analysis revealed that after inoculation with S. meliloti QL2， the nitrogen fixation percentage of different categories of variety was significantly positively correlated with above-ground dry weight. That is， the growth rate （as reflected by above-ground dry weight） of imported varieties was the largest compared with CK， of which WL319HQ-QL2 had the highest growth rate of 48.08 %， followed by domestic bred varieties and local varieties. The nitrogen fixation percentage was also significantly positively correlated with the forage nutritional quality. Inoculation with rhizobia generally increased the crude protein yield of alfalfa with the highest increase being 51.08% （P<0.05）， while reducing the yields of neutral detergent fiber and acid detergent fiber by 40.55% and 53.96%， respectively. These changes in nutritional composition resulted in an elevation of the forage quality score by 1 to 2 grades. In summary， a high nitrogen fixation rate has a significant positive impact on both above-ground dry weight and forage nutritional quality， meaning that it plays the critical role in improving the yield and nutritional quality of alfalfa. This study lays a foundation for development of materials to optimize nitrogen fixation efficiency in legume-rhizobia symbioses and improve alfalfa forage quality.

Key words: alfalfa, rhizobia, nitrogen fixation percentage, alfalfa variety effect, forage quality