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

Citation

Zhang, Y., Gao, X., Hao, X., Alexander, T.W., Shi, X., Jin, L., Thomas, B.W. (2020). Heavy grazing over 64 years reduced soil bacterial diversity in the foothills of the Rocky Mountains, Canada, 147 http://dx.doi.org/10.1016/j.apsoil.2019.09.011

Plain language summary

In the foothills of the Rocky Mountains, rough fescue grasslands are primarily managed for cattle grazing. Yet, the effect of long-term cattle grazing on the soil bacterial community and their relationship to soil properties is uncertain. This research took advantage of a long-term grazing study that was established near Stavely, Alberta, Canada in 1949 to assess how more than six decades of different cattle stocking rates (Mid-May to Mid-November) effected the soil bacterial community and their relationship with soil properties. We found that heavy grazing, roughly three times the recommended cattle stocking rate, clearly shifted the bacterial community composition and led to reduced bacterial diversity. Meanwhile, moderate grazing, slightly above the recommended cattle stocking rate, shared a similar bacterial community composition as the section of grassland that was fenced off and excluded from grazing since the beginning of the experiment. Further analyses revealed that the effect of grazing largely depended on how the intensity of grazing altered soil properties. Overall, grazing for more than six decades, at moderate cattle stocking rates, should not be expected to change the soil bacterial community composition in the foothills of the Rocky Mountains, Canada. This information will provide grassland managers with valuable information about how soil biodiversity responds to long-term grazing. The long-term nature of this field experiment is a similar reality faced by many beef producers.

Abstract

© 2019 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.