The Effects And Mechanisms Of Livestock Grazing On Soil Microbial Diversity In Grasslands

Soil microorganisms are abundant and diverse forms in grasslands,and they are involved in many ecological processes such as carbon,nitrogen,phosphorus and sulfur cycling,as well as greenhouse gas production and emission.Soil microorganisms play a key role in maintaining ecosystem multifunctionality and restoring degraded grasslands.Soil microbial diversity is not only influenced by environmental factors such as climate,plant communities and soil environment,but also regulated by anthropogenic factors such as livestock grazing.Livestock grazing is the most important land use of grasslands,the grasslands degradation due to long-term overgrazing is one of the critical global changes.Livestock grazing not only affects plant and soil properties,but also has an effect on soil microbial diversity,which in turn affects ecosystem function.Therefore,researches about livestock grazing effects on soil microbial diversity in grasslands and its mechanisms is of great scientific value for the conservation and prediction of grassland soil biodiversity,the maintenance of grassland ecosystem functions,and guidance of grassland grazing management.Based on the phenomenon of long-term overgrazing in grasslands of China,this study investigated the effects of overgrazing on soil microbial diversity,the relationships between soil microbial and plant diversity,and also microbial community assembly process across eight vegetation types of three grassland types(meadow steppe,typical steppe and desert steppe)in Northern China through a long-term grazing exclusion experiments.Meanwhile,based on the typical phenomenon of patchy degradation of grassland,this study further examined the changes in soil microbial community and their responses to different grazing types in Songnen degraded saline grasslands using the experimental platform of "Diversified Livestock Grazing in Degraded Grasslands".This study analyzed the changes in soil microbial community composition and diversity during the transition from non-degraded to degraded patches,and the spatial effects of different livestock grazing animals and assemblage(CG,cattle grazing;SG,sheep grazing and MG,cattle and sheep mixed grazing)on soil microbial composition and diversity in different vegetation types(grass patches,Leymus chinensis patches,Leymus chinensis-saline patches and saline patches).The following main findings were obtained in this study.(1)The effects of long-term overgrazing on soil microbial diversity in Northern China strongly depended on grassland type.Overgrazing significantly reduced soil bacterial and fungal diversity only in typical steppes,had no significant effects on bacterial and fungal diversity in meadow steppes,and significantly increased bacterial and fungal diversity in desert steppes.The pathways of overgrazing on microbial diversity also differed among grassland types.In typical steppes,overgrazing reduced microbial diversity both directly and indirectly by reducing soil organic carbon.In meadow steppes,overgrazing reduced microbial diversity directly and increased microbial diversity indirectly by affecting plant attributes,with the direct negative effect and the indirect positive effect offsetting each other,resulting in microbial diversity not affected by overgrazing.In desert steppes,overgrazing indirectly increased microbial diversity by reducing plant aboveground biomass and increasing plant species richness and plant community nitrogen.In addition,in meadow steppes,overgrazing did not change the relative abundance of dominant fungal phyla and bacterial community composition,and only affected a few phyla with less relative abundance,indicating that the resistance of soil microbial community to overgrazing was the highest in meadow steppes.In typical steppes,overgrazing not only significantly reduced bacterial and fungal diversity,but also changed the relative abundance of dominant bacterial and fungal phyla and the intensity of the effects were higher than that of meadow and desert steppes,indicating that the resistance of microbial communities in typical steppes to overgrazing was the lowest.(2)We studied the relationships between plant diversity and microbial diversity and their responses to overgrazing at the regional scale.The results showed that long-term overgrazing did not change the positive relationship between soil microbialβ diversity(including bacterial,fungal,saprophytic,symbiotic fungal and pathogenic fungal diversity)and plant β diversity,but the mechanisms regulating their relationships changed.In ungrazed grasslands,the plant-microbial diversity relationships disappeared when controlling for the environmental factor,suggesting that the consistent changes in response to the same environmental factor was the main reason for maintaining positive relationships between plant and microbial diversity in ungrazed grasslands.While in overgrazed grasslands,even though plant and microbial diversity were still regulated by mean annual temperature,when controlling for the environmental factors,the positive correlations between plant diversity and microbial diversity still existed.In addition,overgrazing increased the correlation coefficients and links between plant species and microbial OTUs,suggesting that the maintaining of their positive relationships were mainly due to their functional associations in overgrazed grasslands.(3)We studied soil microbial community assembly process and their responses to overgrazing.The results showed that overgrazing altered bacterial and fungal community assembly process,but the changes in bacterial and fungal communities were slightly different.Overgrazing reduced the dispersal limitation in bacterial community assembly process,and slightly increased homogeneous selection,heterogeneous selection and homogenizing dispersal.Overgrazing reduced dispersal limitation and increased homogenizing selection.Overgrazing reduced the relative abundance of specialist species and increased the relative abundance of netural species,which in turn increased the niche width and niche overlap of bacterial and fungal communities,ultimately leading to regional biotic homogeneous of soil microbial community.(4)We examined the changes of soil microbial community in saline degraded grasslands.The results showed that the diversity of bacterial abundant taxa and rare taxa increased,while the diversity of fungal abundant taxa and rare taxa decreased during the process from non-degraded to degraded patches.The diversity of bacterial and fungal was negatively correlated with their dominant phylum,Actinobacteria and Ascomycota,respectivity.Soil p H was the main environmental driver of bacterial and fungal diversity,therefore,the promotion and competition between microbial species and soil environment jointly regulated the spatial distribution pattern of microbial diversity during grassland patchy degradation.The relative abundance of microbial abundant and rare species also differed significantly among non-degraded,the ecotone and degraded patches,with the relative abundance of bacterial abundant taxa increasing and rare taxa decreasing from non-degraded to degraded patches,and fungi having a similar pattern,but the differences were not significant.Bacterial rare taxa was indicators in patchy degraded grasslands.The ecotone between non-degraded and degraded patches had the highest bacterial abundant taxa diversity and the lowest relative abundance of the phylum Actinobacteria,indicating that the highest facilitation and lowest competition among bacterial species were found in the ecotone.(5)We investigated the effects of livestock grazing animals and assemblage(grazing types)on the soil microbial community in patchy degraded grasslands.The results showed that grazing types had significantly,but different effects on bacterial and fungal diversity without considering vegetation patch types.Compared with no grazing,bacterial diversity in SG significantly decreased,and bacterial diversity in CG and MG had no significant differences compared with that in no grazing.Fungal diversity in SG had no significantly differences with that in no grazing,while fungal diversity in CG had a tendency to reduce and that in MG has a tendency to increase,resulting in significantly higher fungal diversity under MG than CG.In addition,the effects of grazing types on bacterial diversity depended on vegetation patch types.In Leymus chinensis patches and Leymus chinensis-saline patches,grazing types did not change bacterial diversity.In grass patches,bacterial diversity was lower in SG and CG than that in no grazing,but the differences was not significant,MG increased the relative abundance of the dominant bacterial phylum-Actinobacteria,by increasing the total soil nitrogen content,resulting the decreasing of bacterial diversity due to competition exclusion.In saline patches,the bacterial diversity in SG had a tendency to decrease than that in no grazing,while CG significantly reduced bacterial diversity by increasing soil p H and electrical conductivity.In summary,the results and pathways of overgrazing on soil microbial community composition and diversity are extremely complex and strongly depend on the grassland types,vegetation patch types,livestock grazing animals and assemblages.1)In meadow steppes with high precipitation and plant diversity,soil microbial communities can effectively resist the disturbance of livestock overgrazing;2)long-term overgrazing enhances the functional associations of soil microbial and plant community;3)Overgrazing reduced dispersal limitation in microbial community assembly process,decreased the relative abundance of specialist species,and increased netural species,resulting in increased microbial community niche width and niche overlap,which eventually led to soil microbial communities in the region becoming more homogeneous;4)In patchy degraded grasslands,moderate grazing pressure under mixed grazing of cattle and sheep had an important role in maintaining and improving microbial diversity,and in the vegetation patch types where the dominant plant Leymus chinensis occupies a certain proportion,both cattle grazing and mixed grazing of cattle and sheep can maintain bacterial and fungal diversity.This study obtained an insight into the effects of livestock grazing on soil microbial diversity in grasslands and a better understanding of the mechanism,which provides strong data support for rationalizing grazing management measures.The results in this study emphasize the importance of conserving plant diversity at the regional scale and conserving dominant plants at the local scale for the purpose of soil microbial diversity conservation in the context of long-term overgrazing in grasslands of Northern China.This study recommends that establishing fences to exclude grazing animals at the regional scale can help maintain microbial community β diversity,however,it is appropriate to establish site-specific "flexible fencing system" in different grassland types,for example,in typical steppes with low resistance,it is better to establish fences to exclude grazing animals strictly,while in meadow steppes with high resistance,the fencing time can be appropriately adjusted.In patchy degraded grasslands,moderate mixed grazing of cattle and sheep can maintain or improve microbial diversity,or in areas where dominant species occupy a certain proportion,moderate cattle grazing and mixed grazing of cattle and sheep can be carried out to achieve the purpose of restoring grassland in utilization.