Impact Of Grazing Exclusion On Soil Aggregate Stability And Their Microbial Characteristics In Semi-arid Steppe

The exclusion of grazing has become an effective approach to semi-arid restore grassland soil degradation by facilitating natural succession,while improper grazing exclusion can have negative impacts on grassland ecosystems.A systematic understanding of soil biochemical and geochemical processes during relatively long-term grazing exclusion,can not only improve soil ecological functions,but also increase grassland productivity.As the basic composition of soil structure,aggregates not only provide habitat for the survival of microorganisms,their stability also determines the productivity of the soil.At present,systematic studies on the evolution process and mechanism of soil aggregates in grassland,especially during relatively long-term grazing exclusion,have not been well documented.In this study,aggregate size classes were fractionated through dry and fresh soil of semi-arid grassland on the Loess Plateau,respectively.Then,we investigated changes in the water-stability of grassland soil aggregates,and nutrients,microbial activity and diversity in soil microhabitats after 0,11,26,and 36 y of grazing exclusion?GE?.The purpose is to clarify the changes in soil structural stability,the nutrient cycling and microbial community succession processes in soil microhabitats and their driving mechanisms after different periods of grazing exclusion.Results from this study will help to provide an efficient theoretical basis and strategy for the management of grassland soil microhabitats after grazing exclusion.The results were as follows:?1?Compared with the 26-year mid-term grazing exclusion,the 11-year short-term and 36-year long-term grazing exclusion were more conducive to the water-stability of soil aggregates.The water-stability indices including the mean weight diameter?MWD?and geometric mean diameter?GMD?of soil aggregates in the grazing excluded grasslands were as follows:GE11>GE36 or GE0>GE26.The R_0.25?>0.25 mm specific gravity of water-stable aggregate?value of soil aggregates was increased by1.4-23.3%after grazing exclusion,suggesting that aggregate fractions tended to change from macroaggregates?>0.25 mm?to microaggregates?<0.25 mm?.Stable compounds,such as carboxyl-carbon and clay minerals,accumulated more in GE11 and GE36treatments,respectively.The effects of nutrient stoichiometry and glomalin-related soil protein?GRSP?stoichiometry on the aggregates water-stability were greater than the nutrient and GRSP contents.?2?Mid-term grazing exclusion?26 y?was more conducive to the accumulation of nutrient in fresh soil aggregates,and nutrients were mainly distributed in the particle sizes of 0.25-2 mm.With the increase period of grazing exclusion,the content of soil organic carbon?SOC?,total nitrogen?TN?,microbial biomass,and extracellular enzymes in aggregates tended from the increase to the decrease.The highest values of these variables were observed in the GE26 treatment,indicating that the 36-year long-term exclusion of grazing could have a negative effect on the ecological function of soil aggregates in grassland.Higher nutrients,enzymatic activities,and microbial biomass of soil aggregates were mainly distributed in the particle sizes of 0.25-2 mm.Lower nutrients and microbial biomass were mainly distributed in the particle sizes of>2 mm,and enzymatic activities were distributed in<0.25 mm.Partial least squares path modeling?PLS-PM?and variation partitioning analysis?VPA?identified that soil characteristics?i.e.,pH and moisture?,extracellular enzymes?i.e.,?-1,4-glucosidase,?-1,4-N-acetylglucosaminidase,and alkaline phosphatase?,microbial biomass?i.e.,microbial biomass carbon,nitrogen,and phosphorus?and their interactions in soil aggregates were major controlling variables of nutrient?i.e.,C,N,and P?cycling.Compared with enzymatic activities,soil characteristics and microbial biomass had a greater impact on nutrient cycling.?3?The exclusion of grazing altered the composition and alpha-diversity?i.e.,richness and diversity indices?of bacterial and fungal communities in soil aggregates.Grazing exclusion increased the relative abundances of trophozoic bacteria Proteobacteria in soil aggregates,and decreased the abundances of Actinobacteria and Chloroflexi suitable for survival in extreme environments.Ascomycota and Basidiomycota are two kinds of fungi that mainly decompose organic matter.With the increase period of grazing exclusion,in contrast to the relative abundances of Basidiomycota,the abundances of Ascomycota in aggregates tended from the increase to the decrease.The effects of grazing exclusion on the richness of bacterial and fungal communities?ACE,Chao1?were greater than their diversity index?Shannon,Simpson?.And the microbial?-diversity was higher in the particle sizes of 0.25-2 mm.Both grazing exclusion and aggregate isolation altered bacterial and fungal community structure.And the effects of aggregate isolation on bacterial communities were greater than fungal communities.Microbial community structure in the particle sizes of>0.25mm was more stable than that in the particle sizes of<0.25 mm,and a stronger competition and restriction relationship were observed in the particle sizes of 0.25-2mm.The diversity of microbial communities stimulated the cycling and accumulation of soil nutrients in the microhabitat.Simultaneously,environmental factors such as nutrients could drive the succession process of microbial communities.