高寒地区不同退化草地植被特性和土壤固氮菌群特性及其相关性

选取东祁连山不同退化程度的高寒草地为研究对象,调查研究其植物种类、植被盖度、高度、地上生物量等植物指标以及土壤好气性自生固氮菌和嫌气性自生固氮菌数量,在此基础上,采用real-time PCR的方法扩增nifH基因,测定不同退化程度草地土壤中固氮菌相对于土壤总细菌的量,以探讨草地退化过程中植被及土壤固氮菌群的变化规律,结果发现:随着退化程度的加深,草地植物种类逐渐减少,并且优势植物发生变化,毒杂草逐渐增多,植被的高度、盖度、地上生物量都逐渐降低。对土壤固氮菌的研究则表明,土壤好气性自生固氮菌和嫌气性自生固氮菌的数量在不同退化草地随草地退化程度的加重而减少,在同一退化程度草地土壤则是随土层深度加深而下降。对土壤固氮菌nifH基因扩增的结果也表明随着退化加剧,土壤固氮菌相对于土壤总细菌的比例在降低,进一步说明草地退化过程中土壤固氮菌不仅是数量上的下降,更是群落结构层面的变化。对植被特性和土壤固氮菌含量的相关分析表明,植被特性和土壤中固氮菌含量呈显著相关。研究从土壤固氮菌群的角度研究了草地退化的过程,说明了二者具有协同性,研究和治理草地退化必须重视土壤功能菌群尤其是固氮菌群的作用。

Characteristics of, and the correlation between, vegetation and N-fixing soil bacteria in alpine grassland showing various degrees of degradation

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 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.