Abstract:

Aim: This study aims to clarify the response of soil bacterial species composition, pattern, and functional structure to a range of grassland degradation in alpine meadows.
Methods: Degraded alpine meadows at five stages (including non-degraded, light-degraded, moderate-degraded, severe-degraded, and extreme-degraded) were selected in 2017 by the classification standard in the Three-River-Source. Four different plots at each degraded stages were set as replications, and the area of each plot is about 80 m². Soil microbial characteristics were analyzed by high-throughput gene detection, and the soil physical and chemical properties were analyzed by conventional methods.
Results: Acidobacteria, Actinobacteria, Planctomycetes, Proteobacteria and Verrucomicrobia were the dominant bacteria in alpine meadow soil, accounting for 16%‒18%、9%‒12%、12%‒14%、23%‒29% and 11%‒12%of the total soil bacteria abundance, respectively. The soil bacterial species compositions changed substantially with increasing grassland degradation: Proteobacteria abundance decreased, while Acidobacteria and Planctomycetes abundance increased significantly with increasing degradation. Furthermore, the number of taxa (at the family level) differed across the diverse soil layers in degraded alpine meadows. Grassland degradation had no effect on bacterial diversity, as measured by the Chao1 index, but alpine medow that were lightly degraded increased bacterial diversity, as measured by the Simpson index. Soil bacterial diversity, as measured by the Shannon-Wiener index, was greatest in severe-degraded grasslands. The Faprotax functions were mainly composed by the processes of Chemoheterotrophy, nitrification, nitrite oxidation, and sulfur metabolism. Grassland degradation altered the carbon, nitrogen, sulfur, iron and manganese cycles, which were mediated by microorganisms. Severe and extreme degradation increased the bacterial ammonia-oxidizing function, and decreased the functions of sulfide, nitrite oxidation, and ureolysis. Across the gradient of grassland degradation, the bacteriological Chemoheterotrophy, aromatic compounds degradation, and the denitrification functions all initially decreased before subsequently increasing; moderate stages of degradation stage represented turning points of the bacterial community in terms of ecological functional structure changes.
Conclusion: Alpine meadow degradation altered the soil bacterial community and functional structures. The main driving factors for differences in soil bacterial community and functional structures were soil moisture content, pH, total organic carbon, total nitrogen, total potassium, and the ratio of available nitrogen and phosphorus.

Key words: grassland degradation, soil bacteria, species composition, diversity, community characteristics, ecological functional structures