Abstract:With the exploitation and utilization of land resources, especially the non-sustainable utilization like overgrazing, the extent of alkalization of Song-Nen grassland has been enlarged and the vegetation has been degraded seriously. The area of preponderant grass Leymus chinense (L. chinense) also reduced, the height of grass and the extent of closed canopy of grassland decreased, and the alkali spot with very little grass occurred in the grassland. Puccinellia tenuiflora (P. tenuiflora) could grow in alkalized soil after artificial planting, and ameliorate the alkalized soil. Then L. chinense and other plants intruded into the alkalized spot where P. tenuiflora had been grown for certain years, making the plant population no longer the singularity population of P. tenuiflora. The mechanism about the vegetation of L. chinense succeeding in recovery was not only due to the improvement of physical or chemical property of the soil, but also due to the anthropogenic nitrogen deposition, which could cause vegetation-type conversion. But the mechanisms of vegetation succession were not very clear. As to the cause of vegetation succession, there were many hypotheses, such as frequent fire and grazing, but nitrogen deposition had only recently been considered. There were two aims of this study on the growth of P. tenuiflora. The first was to reveal that how the P. tenuiflora could meliorate the salt-alkali soil, the second was to quantify the changes of niche after the vegetation succession from P. tenuiflora to other kinds of plants. Niches of various plants in Song-Nen alkaline grassland were calculated. Comparing with the controlled alkali spot soil, all the evidences from the experiment indicated that P. tenuiflora improved the nitrogen nutrition status of salt-alkali soil. There were smaller value of niche separation and bigger value of niche overlap between P. tenuiflora and L. chinense in one dimension niche space (N content of soil) than those among other species, which indicated that L. chinense had stronger competition ability on soil with high nitrogen content. These appear to be the mechanism of L. chinense and other plants intruding into the alkalized spot in which P. tenuiflora has grown for certain years. As the pioneer plant of meliorating alkalized soil and vegetation restoration, the growth of P. tenuiflora could improve the nitrogen mineralization and nitrogen fixation in salt-alkali grassland, and decrease the nitrogen loss with water run-off. Finally, the nitrogen deposition in salt-alkali grassland was improved to the level that other plants (such as L. chinense) could grow normally, and the vegetation in alkali spot was restored.