Short-term rather than long-term exclusion of grazing increases soil bacterial diversity in an Inner Mongolian steppe

Cessation of grazing is an important management practice in restoration of grassland ecosystem productivity and function. However, little is known about the effects of long-term exclusion of grazing on soil bacterial community structure and diversity in grassland ecosystems. This study utilized three grassland sites over two consecutive years (2004 and 2005) in a semi-arid Inner Mongolia steppe; there were a free grazing site (FG), fenced site since 1999 (UG99) and fenced site since 1979 (UG79). Soil moisture content, organic carbon (C) and nitrogen (N), NH₄⁺-N and NO₃^--N concentrations were measured across the treatments. Bacterial community structure and diversities were assessed with PCR amplification of genomic DNA extracted from soils and following denaturing gradient gel electrophoresis (DGGE) separation. Results showed that the UG99 soil had higher moisture, organic C, organic N and NH₄⁺-N concentrations than the other soils. Principal components analysis of DGGE patterns showed that soil bacterial community structure sampled in 2004 was different from that in 2005, and the UG99 soil was significantly different from the FG and UG79 soils across the two consecutive years. In addition, the UG99 soil had significantly higher bacterial diversity and evenness compared with the FG and UG79 soils. These results indicate that long-term exclusion of grazing decreases bacterial diversity, which has significant implication for grassland ecosystem management.