Distinct grazing pressure loads generate different impacts on bacterial community in a long-term experiment in Pampa biome

The Pampa biome is a rich and fertile area of grassland in Brazil, South America, which has been considered ecologically important owing to its enriched biodiversity. However, this landscape has undergone serious fragmentation, with half of its original vegetation being removed due to continuous agricultural activities and its widespread use for livestock production. The excessive cattle farming has generated a significant grazing load on the area, which is reflected in the diminishing soil microbial diversity. With an aim to study the impact of increasing grazing load on microbial diversity of soil of pampas, the present study compared the bacterial communities of grassland-controlled soils of the Pampa biome under different grazing pressures: high (HG), moderate (MG), low (LG), and a control area (CG) in a long-term experiment using high-throughput sequencing. The soil microbial community was not significantly affected by the presence or absence of bacterial taxonomic groups; however, significant changes were detected in each different grazing pasture sample. This observation was especially notable in MG pressure that, when compared to other grazing pressures and control grazing (CG), was found to be associated with an increased prevalence of some specific groups of microorganisms. These include those belonging to the following families: Nitrospirae, DA052, Syntrophobacteraceae, Methylocystaceae, Syntrophaceae, Mycobacteriaceae, and Koribacteraceae. Moreover, the grazing pressure disturbance created by MG was believed to enhance the diversity of a bacterial community, thereby maintaining the stability of soil and indicating that stressed bio-systems might be more stable than those not experiencing stress. Thus, the results of the current study imply that quantitative and qualitative changes in the composition of the soil bacterial community could serve as an important indicator of both short- and long-term changes in soil health.