Response Of Soil Microbial Biomass And Community Structure To Grazing And Fencing In Shrub Grassland In Eastern Qilian Mountains

Grazing is an important factor that affects the soil system of grassland and causes degradation of grassland.Fencing is beneficial to the restoration of the ecological function of the degraded grassland.Soil microbial communities can indicate changes in soil quality and are early warning and sensitive indicators of changes in soil ecosystems.This experiment was conducted with east Qilianshan shrub high-cold grassland of different intensity grazing(light grazing,moderategrazing,heavy grazing)and fenned for 2 years.The basic principles and methods of grassland ecology were used to study the above-ground vegetation and soil physical and chemical properties.Through correlation analysis;Using chloroform fumigation and phospholipid fatty acids(PLFA)to explore the characteristics of soil microorganisms and their changing patterns;The relationship between microbial communities and soil nutrients and vegetation was explored;Using stepwise regression analysis and path analysis methods,the main factors affecting the microbial community and the most direct influencing factors of the microbial community were identified.1 With the increase of grazing intensity,the herb cover,shrub coverage,herb aboveground biomass,height of each functional group were significantly reduced(P<0.05);the important values and biomass of different plant groups responded to grazing intensity differently,the biomass and important values of gramineous plants and forb biomass decreased with the increase of grazing intensity,the important values and biomass of sedgesfamily plants were the highest in the shrub grassland of moderategrazing,and the important values of forb wasthe lowestin theshrub grassland ofmoderategrazing;The underground biomass increased first increased and then decreased with the grazing intensity.After the grazing intensity shrubs were fenced,the herb speicies coverage,shrub speicies coverage,above-ground biomass,height of functional groups and above-ground biomass of herbaceous functional groups all increased significantly.Different grazing intensities responded differently to the enclosures for functional groups.Coverage of gramineous family increased significantly after fenced,and sedges family were significantly reduced,coverage of forbl was reduced after fenced only in heavy grazing shrubs grassland,afterwards,the coverage of the shrubs of Rosaceae(Postentilla fruticosa,Spiraea alpina)increased after fenced in heavy grazing shrubs grassland.At different grazing intensities,Above-ground biomass of herbaceous functional groups accounted for total above-ground biomass differently responsed to enclosures in different grazing intensities.The proportion of above-ground biomass of grass family increased significantly after fenced in each grazing intensity polt,the proportion of sedge biomass increased after fenced only in lingt grazing,and the ratio of forb biomass decreased significantly after fenced in heavey grazing intensity.2.Under the same intensity of grazing and fecing,the soil moisture content,organic carbon content and total nitrogen content in the same soil layer are all 0-10 cm>10-20 cm,and the p H value is 0-10 cm<10-20 cm.With the gradual increase of grazing intensity,the content of soil organic carbon and total nitrogen in the same soil layer decreased significantly,and the soil moisture content first increased and then decreased,and the pH value increased significantly.After each grazing intensity shrub enclosure,the soil moisture content of the same soil layer was significantly higher than that of grazing treatment,and the pH value was significantly lower than that of grazing.The SOC and total nitrogen in the same soil layer were higher than its grazing treatment,the difference of light grazing grassland did not reach significancet level.3.There was a significant positive correlation between herb aboveground biomass and pH(P<0.01).There was a very significant positive correlation between herbaceous underground biomass and soil water content,and a very significant negative correlation between organic matter content and total nitrogen content.Shrub coverage were positively correlated with organic carbon content and total nitrogen content,and herbage coverage was significantly negatively correlated with pH value.There was a significant negative correlation between shrub coverage and soil moisture content.4 Grazing and fenced grasslands at the same intensity,the amount of microbial biomass carbon and microbial biomass nitrogen in the same soil layer are all 0-10cm>10-20 cm.With the gradual increase of grazing intensity,the amount of microbial carbon and nitrogen in the same soil layer decreased significantly.The content of soil microbial biomass C and N in the same soil layer was significantly higher than thoseafter fenced in moderate and heavy grazingshrubs grassland,and no significant change was observed in the light-grazing shruland after fenced.5.The soil total PLFA,bacterial biomass,actinomycetes biomass,and fungi biomass were significantly reduced with an increase in grazing intensity(P<0.05).At heavy grazing intensity,soil total PLFA,bacterial biomass,actinomycetes biomass,G~+/G~-ratio,and the stress index after the fenced treatment were significantly higher than those after the grazing treatment,whereas the bacteria/fungi ratio after the fenced treatment was significantly lower than that after the grazing treatment.At medium grazing intensity,soil total PLFA,bacterial biomass,and bacteria/fungi ratio were significantly higher than those after the grazing treatment,whereas fungal biomass and the stress index were significantly lower than those after the grazing treatment.At light grazing intensity,biomass and biomass ratio after the fenced treatment and those after the grazing treatment were not significantly different.component analysis(PCA)showed that 53.68%of PC1 was composed of straight-chain saturated and unsaturated fatty acids 14:0,15:0,10Me16:0,and18:1ω9c,whereas 51.34%of PC2 was composed of branched-chain saturated and cyclopropane fatty acids i16:0,16:1ω7c,i17:0,cy17:0,17:0,18:1ω9t,18:0,and cy19:0.Grazing grasslandand fenced grassland of different grazing intensities had similar soil microorganisms.The fenced treatment affected the microbial community structure to a greater extent than did the grazing treatment.Correlation analysis showed that soil total PLFA,bacteria biomass,fungi biomass,and actinomycetes biomass were significantly and positively correlated with soil organic carbon and total nitrogen at p<0.001,whereas bacteria/fungi had a significant and negative correlation with soil organic carbon and total nitrogen(P<0.001).6.Correlation analysis showed that the total amount of PLFA,bacterial PLFA,fungal PLFA,actinomycete PLFA and microbial biomass carbon and microbial biomass nitrogen were extremely significantly positively correlated;Gram-positive bacteria PLFA amount and microbial biomass carbon Significantly positive correlations were observed between nitrogen and microbial biomass nitrogen,and positive correlations between PLFA and gram-positive bacteria/microbial carbon were positively correlated with microbial biomass nitrogen;bacterial/fungic ratios of PLFA and microorganisms were positively correlated.There was a significant negative correlation between amount of carbon and microbial nitrogen.7.Stepwise regression analysis and path analysis showed that underground biomass had the maximal impact on total microbial and Gram-positive bacteria,and above-ground biomass had the greatest impact on bacteria and fungi,and the effects of pH on gram-negative bacteria,G+/G-,bacteria/fungi,and pressure index were the greatest,while organic carbon had the greatest effect on actinomycetes.