The Response Of Soil Microorganisms To Grazing In The Loess Plateau Grassland

Since the implementation of the Loess Plateau Returning Cropland to Forestry(Grass)Project,about 2.32 million hm~2 of grassland had been formed,accounting for 33%of the total area of the region.Due to the implementation of the enclosed grazing ban policy,these fallow pastures were in a state of abandonment and grazing prohibition.Studies have shown that moderate grazing of grasslands played an important role in conserving biodiversity and main-taining soil nutrients,and was conducive to improving grassland productivity,while the re-sponse process and mechanism of soil microorganisms,as important players in biogeochemi-cal cycles,to grazing remained unclear.In this study,five different grazing intensities(2.2,3.0,4.2,6.7 and 16.7 sheep/hm~2,considered as light grazing,light to moderate grazing,mod-erate grazing,moderate to heavy grazing and heavy grazing respectively)and two different grazing patterns(one-month rotational grazing and growing season grazing only)were set up in test plots in the Loess Plateau grasslands that had been closed for more than 20 years by fallowing,with no grazing as the control.The effects of grazing intensity and grazing pattern on soil enzymes activity,bacterial and fungal diversity,community composition and functional groups were investigated to reveal the driving mechanisms of soil microbial community struc-ture in grazed grasslands and to provide scientific basis for grassland management and man-agement in the Loess Plateau.The main results are as follows:(1)Soil enzymes activity increased and then decreased with increasing grazing intensity,light and moderate grazing significantly increased N-converting enzymes(β-1,4-N-acetylglu-cosaminidase,L-leucine aminopeptidase and Alanine transaminase)and P-converting enzyme(Alkaline phosphatase)activity,and moderate grazing significantly increased C-converting enzyme(β-1,4-glucosidase)activity.Both one-month rotational grazing and growing season only grazing significantly increased the activity of N-converting enzyme(β-1,4-N-acetylglu-cosaminidase),C-converting enzymes(β-1,4-xylosidase andβ-1,4-glucosidase)and P-con-verting enzyme(Alkaline phosphatase)compared to no grazing,but the effect of one-month rotational grazing on soil enzymes activity was more pronounced.(2)Moderate grazing significantly increased the richness and diversity of bacteria in the topsoil layer,by 3.97%and 0.94%respectively compared to no grazing,but also decreased with increasing grazing intensity,and different grazing intensities had no significant effect on theα-diversity of fungi in the topsoil layer.The two grazing patterns of one-month rotational grazing and growing season grazing only had no significant effect on the microbialα-diversity compared to no grazing.Under different grazing intensity,β-diversity of bacteria had little difference,while that of fungi had great difference.There were significant differences inβ-diversity of bacteria and fungi under different grazing patterns.The community structure of bacteria varied significantly with soil depth,while fungi tended to be similar.(3)Light grazing significantly increased the relative abundance of Actinobacteriota and its genera X67-14 and Gaiella in the topsoil layer by 5.95%,1.93%and 0.91%respectively compared to no grazing,but significantly reduced the relative abundance of Chloroflexi JG30-KF-CM45.Both grazing patterns significantly increased the relative abundance of Actinobac-teriota and its genus Gaiella,and one-month rotational grazing significantly reduced the rela-tive abundance of Chloroflexi JG30-KF-CM45 and TK10.The relative abundance of the fun-gal Ascomycota Archaeorhizomyce increased and then decreased with increasing grazing in-tensity,and light grazing and one-month rotational grazing significantly increased the relative abundance of the Archaeorhizomyce and decreased the relative abundance of the Ascomycota Leohumicola.(4)Tax4Fun2 functional predictions indicated that metabolism,environmental infor-mation processing and cellular processes were the main primary functions of the bacterial community,and that grazing intensity and grazing pattern significantly influenced fungal func-tional groups,including pathotroph,pathotroph-saprotroph and symbiotroph.For example,light and moderate grazing significantly reduced pathotroph fungi in the topsoil layer and in-creased the resistance of grassland to pathogens.Light grazing and light to moderate grazing significantly reduced the presence of symbiotroph fungi in the topsoil layer,such as epiphyte fungi growing on the plant surface.One-month rotational grazing significantly reduced pathotroph and pathotroph-saprotroph fungi in the soil,increasing the resistance of grassland to pathogens and making grassland ecosystems healthier and more stable.(5)Microbial diversity was mainly influenced by soil factors and was closely related to factors such as total nitrogen,quick acting phosphorus,quick acting potassium and soil organic matter.Microbial community composition was driven by a combination of vegetation factors,including vegetation aboveground biomass and vegetation height,and soil factors,including p H,total nitrogen and soil organic matter.Fungi were more susceptible to vegetative factors than bacteria,and fungi were also influenced by bacteria.In conclusion,light and moderate grazing can increase the activity of carbon,nitrogen and phosphorus converting enzymes and microbial diversity of grassland soils in the Loess Plateau,enhance the resistance of grassland to pathogens.Compared to growing season graz-ing only,one-month rotational grazing had a more significant effect on the enhancement of soil carbon,nitrogen and phosphorus conversion enzymes activity and resistance to pathogens in grassland.