Soil bacterial community responses to short-term grazing exclusion in a degraded alpine shrubland-grassland ecotone

Grazing exclusion by fencing is a common and effective grassland management strategy for degraded grassland restoration. Soil bacteria, the most important decomposers of soil organic matter, is the linkage between plant and soil. However, soil bacterial alpha-diversity and community composition in the recover process of degraded shrubland - grassland ecotone on the Qinghai-Tibet Plateau has not been well understood. Thus, in this research, lightly and heavily degraded shrubland - grassland ecotone and its adjacent communities (alpine shrubland and alpine meadow) during recover process on the Qinghai-Tibet Plateau were selected to study alpha-diversity and community composition of soil bacteria, as well as the relationships among soil bacterial community, plant community and soil properties. Our results demonstrated that: 1) Dominant taxa at the phylum and genera level in all samples were similar. Proteobacteria, Acidobacteria and Bacteroidetes were dominant phyla, RB41, H16 and Ferruginibacter were dominant genera. 2) Grazing exclusion had no effect on soil bacterial alpha-diversity. However, grazing exclusion significantly decreased relative abundance of the Chloroflexi phylum, but grazing exclusion significantly increased the Elusimicrobia phylum relative abundance in the alpine meadow plot. 3) There were no significant differences on soil bacterial alpha-diversity between the heavily and lightly degraded grassland. However, relative abundance of the Planctomycetes phylum, Deltaproteobacteria class and RB41 genus in the alpine meadow plot of heavily degraded grassland were significantly lower than those in the lightly degraded grassland. 4) The higher plant community alpha-diversity and soil nutrition of shrubland - grassland ecotone did not result in a higher soil bacterial alpha-diversity. However, relative abundances of Chloroflexi phylum, Deltaproteobacteria class and Nitrospira class in the shrubland - grassland ecotone were significantly higher than those in the alpine meadow and alpine shrubland plots inside fence of the lightly degraded grassland. 5) Structural equation model (SEM) showed that soil organic carbon and soil total nitrogen were dominant factors affecting soil bacterial alpha-diversity. 6) Redundancy analysis (RDA) indicated that soil nitrate nitrogen and soil total nitrogen were the main driving factors for soil bacterial community composition. Our research found that the changes of soil bacterial alpha-diversity is incongruous with plant community alpha-diversity, while soil bacterial community composition is paralleled with plant community alpha-diversity and soil characteristics, especially the soil chemical property. These findings can provide management support for the restoration of degraded alpine shrubland.