Heavy grazing over 64 years reduced soil bacterial diversity in the foothills of the Rocky Mountains, Canada

Grazing is one of the most widespread grassland management strategies. However, the effects of over six decades of different grazing intensities on soil bacterial community composition in the foothills of the Rocky Mountains are uncertain. We analyzed the bacterial community composition in soil samples collected in both summer and fall, 64 years after a long-term grazing intensity study was initiated in 1949. Grazing intensity treatments were (i) 0 animal-unit months (AUM) ha(-1) (ii) 2.4 AUM ha(-1) and (iii) 4.8 AUM ha(-1), which represented the control, moderate and heavy grazing intensities, respectively. The evenness and diversity indices decreased with heavy grazing intensity relative to the other treatments in both summer and fall. In summer and fall, heavy grazing significantly shifted the bacterial community composition compared to the other treatments. Heavy grazing intensity significantly decreased the relative abundances of Bacteroidetes, Chlorobi, Nitrospirae and Proteobacteria, but significantly increased the relative abundance of Actinobacteria. Principal Coordinate Analyses revealed that available nitrogen, moisture content, total nitrogen and organic carbon were the primary environmental factors affecting the soil bacterial community composition. This study suggests that the effects of grazing on soil bacterial community composition are largely dependent on changes in soil physicochemical properties induced by the intensity of grazing over periods of six decades.