LAUSR

China has some of the most serious grassland degradation in the world, with about 90% of the available grassland being subject to various degrees of degradation. The grassland shows surface vegetation degradation, and there is a close relationship with soil degradation. Self N⁃fixing bacteria have an important impact on soil quality. In this study, alpine grasslands that had suffered different levels of degradation were selected as study areas in the east Chi⁃lien mountains. The plant species, coverage, average height, and above ground biomass were investigated, and the population of aerobic N⁃ fixing bacteria and anaerobic N⁃fixing bacteria in the soil were measured. The levels of soil N⁃fixing bacteria relative to total soil bacteria were determined by the levels of real⁃time PCR⁃amplified nifH gene present in the different degraded htt p:/ /w ww .ec olo gic a.c n http: / / www.ecologica.cn grasslands. These methods showed how vegetation and soil N⁃fixing bacteria changed and how these changes were regulated as the degradation process progressed. The results showed that plant species abundance decreased; the dominant species varied; poisonous weed numbers gradually increased; and the height, coverage, and above ground biomass of the vegetation all decreased as the degradation process progressed. The lightly degraded grassland had up to 20 species, whereas there were only 11 in the severely degraded grassland. At first, there was only one dominant species, Elymus nutans, but this gradually evolved into two species, Elymus nutans and Potentilla chinensis, as degradation progressed. The poisonous weeds that appeared included Achnatherum inebrians and Stellera chamaejasme. The medially degraded and severely degraded grassland ground biomass was lower by 47.2% and 92%, respectively, than that of the lightly degraded grassland, and the average height of the severely degraded grassland decreased by 42.3 cm. Vegetation coverage also showed similar trends. The population of aerobic self N⁃fixing bacteria and anaerobic N⁃fixing bacteria was affected by the degree of degradation and the soil depth. The further grassland degradation had progressed and the deeper the soil layer, the lower were the number of self N⁃fixing bacteria. The examination of genetically amplified nifH showed that the ratio of soil self N⁃fixing bacteria to total soil bacteria declined. In the surface soil, the proportion of the bacteria made up of self N⁃fixing bacteria in the medium degraded grassland was lower by 26.9% than that of the lightly degraded grassland. In the severely degraded grassland, the proportion of self N⁃fixing bacteria was lower by 13.2% than that in the relatively medially degraded grassland, and the proportions were similar across all soil layers. This showed that the population of soil self N⁃fixing reduced, and that there was a degradation of community structure. The correlation analysis indicated a significant correlation between the vegetation characteristics and the population of soil self N⁃fixing bacteria. In summary, grassland degradation gradually reduced ground vegetation height, coverage, and aboveground biomass, and increased the numbers of poisonous weeds. It also reduced the numbers of soil self N⁃fixing bacteria numbers and had negative effects on soil microbial community structure. This jndicates a correlation between the grassland degradation process and soil self N⁃fixing bacteria. These results show that the soil functional flora, especially the role of soil self N⁃fixing bacteria, must be considered when researching and managing grassland degradation.