Effects of Ungulate grazing on soil microbial diversities in inner Mongolian steppes of Northern China

Ungulate grazing plays a crucial role in regulating nutrient cycling and controllingplant diversity and productivity in grassland ecosystems. However, little is known aboutthe effects of grazing on soil bacterial and methanotrophic community structure anddiversity. We selected two experimental sites, i.e., different grazing intensities and longtermgrazing grasslands, in the Inner Mongolia of northern China. There were fourgrazing intensity treatments, i.e., non-grazed (CK), low-intensity grazing (LG), moderateintensitygrazing (MG), and high-intensity grazing (HG) at the experimental field 1. Onthe experimental field 2, there were fenced site since 1979 (UG79), fenced site since1999 (UG99) and outside them a freely grazing site (FG). Community structure anddiversities of soil bacteria and methanotrophs were assessed with PCR amplification ofgenomic DNA and following denaturing gradient gel electrophoresis (DGGE) separationand clone library analyses.Principal component analysis of DGGE patterns showed that the LG and MGtreatments were different from the CK and HG treatments. Soil bacterial diversities in theLG and MG treatments were significantly higher than those in the CK and HGtreatments. The relationships between environmental variables and soil bacterialcommunity structure were assessed using redundancy analysis, and we found that soil moisture content, Artemisia frigida biomass, and pH were the best indicators of thechanges in soil bacterial community structure among all the treatments. Additionally, theUG99 soil had significantly higher bacterial diversity as compared with the FG and UG79 soils. Moreover, 55 clones from the UG79 soil and 56 clones from the FG soilwere selected and sequenced. Phylogenetic analysis of all clone sequences indicated thatbacterial communities were dominated by the groups of Actinomycetes, Proteobacteriaand Bacteroidetes, but there were no significant differences in bacterial diversity betweenthe UG79 and FG sites, consistent with the results obtained from DGGE.Additionally, there were no significant differences in methane flux among fourgrazing intensity treatments, but methanotrophic communities showed significantdifferences. Diversity indices of type II methanotrophs were higher at the LG and MGtreatments than at the CK and HG treatments. Though grazing intensities had an effect onthe structure of the methanotrophic community, changes in the community structure anddiversity of methanotrophs did not reflect methane consumption capacity across thetreatments. Cluster analysis indicated that the methanotrophic communities structure ofUG 99 and FG soils were similar to each other but different from that of UG79 soil.Sequence analysis showed that most bands belonged to the cluster of USCγ. This is thefirst report showing that USCγ cluster is dominant in grassland soils.Overall, our results indicated that intermediate grazing intensities (LG and MG)increased soil bacterial diversities and short-term rather than long-term exclusion ofgrazing increased bacterial diversity, which will be helpful for managements of grasslandecosystems. On the other hand, we also suggest that methane consumption is a complexprocess in soil, and we should be cautious when speculating on methane consumptionrates based on changes in methanotrophic community structure.