Effects Of Degradation On Soil Aggregates Characteristics And Microbial Community In Alpine Meadow

Alpine meadow is one of the most important terrestrial ecosystems over the world,with various ecological service functions,and plays extremely important role in biodiversity protection,water conservation,climate regulation etc.In recent years,due to the jointing interference of natural and human factors,alpine meadow has been seriously degraded.How to restore and manage the degraded alpine meadow is an serious problem to be solved.Soil degradation is the core issue of grassland degradation,mainly manifested in the destruction of soil structure,decline in erosion resistance,and loss of soil nutrients,resulting in lower soil fertility.Soil aggregates are the basic unit of soil structure and the place where material and energy are transformed in soil.Their composition and stability are closely related to soil physical and chemical properties.Grassland degradation significantly affects the soil aggregates,soil bulk density,soil nutrients and soil microbial communities.The characteristics of soil aggregates and microbial communities under different degrees of degradation in alpine meadow remains obscure,and the effects of degradation on soil aggregates and microbial community and their relationships also waiting for further research.In this study,the alpine meadow in the Jinqianghe watershed in Tianzhu County,Gansu Province was selected as the research area and four different degradation gradients were designed,including:non-degradation（ND）,light degradation（LD）,moderate degradation（MD）,and severe degradation（SD）.Soil aggregates composition andstability,aggregate nutrients,and stoichiometric characteristics of soil aggregates under different degradation were explored.Illumina Miseq high throughput sequencing technology was carried out to explore the response mechanism of soil aggregates and soil microbial communities to the degradation of alpine meadow.Statistical methods including spearmman analysis,rebundancy analysis（RDA）and structural model（SEM）were performed as well.The main results are as following:（1）As the degradation of alpine meadow intensifing,the dominant aggregates transform from large to small particle sizes,and the stability of soil structure decreases.During the degradation process,the content of aggregate with>5 mm particle size in each soil layer decreased to varying degrees,aggregate with>5 mm particle size in the degraded grassland was decreased by 48.59-81.73%compared with the undegraded grassland,while the content of aggregate with<0.25 mm particle size increased,the degraded grassland increased by88%-159%compared with the non-degraded grassland.With the aggravation of degradation,the content of large aggregates(R_0.25),mean weight diameter（MWD）,and geometric mean diameter（GMD）of aggregates decreased significantly,and the fractal dimension（D）gradually increased.MWD and GMD increased and then decreased with soil depth increased.The degradation of alpine meadow leads to a decline in the stability of soil structure and a decrease in the proportion of macroaggregates.（2）With the aggravation of degradation,the content of SOC,TN in soil aggregates with different particle sizes increased and then decreased,with the soil depth increased,it gradually decreases.Soil organic carbon,nitrogen,and phosphorus were concentrated in aggregates with>5 mm particle size in undegraded alpine meadow,however,in degraded alpine meadow,these nutrients were concentrated in microaggregates with<0.25 mm particle size.The contribution rate of>5 mm aggregates to carbon,nitrogen,and phosphorus gradually decreased with the aggravation of degradation,while the contribution rate of microaggregates<0.25 mm gradually increased.The highest content of SOC,TN,and TP are mainly distributed in aggregates with 0.25-0.5 mm particle size,and they are the main carriers of soil carbon,nitrogen,and phosphorus nutrients.Degradation caused a decrease in soil aggregate C/N,while C/P and N/P first increased and then decreased with the aggravation of degradation.Nutrient supply of degradation degree was mainly restricted by N and followed by P.（3）Degradation had no effect on the composition of soil microbial communities in alpine meadow,but had a significant impact on relative abundance.Actinobacteria,Proteobacteria,Acidobacteria,and Gemmatimonadetes are the dominant bacterial populations in alpine meadow soil;Basidiomycota,Ascomycota,Mortierellomycota,and Glomeromycota are the dominant flora of soil fungi.The alpha diversity of soil microbial communities first increased and then decreased with the aggravation of degradation,reached a maximum at the stage of light degradation,and dereased with the soil depth increased.LEf Se analysis found that there were significant differences between 50 bacterial species and 41 fungal species in different degrees of degradation.The potential function prediction of soil microorganisms found that there were differences in the metabolic function of bacterial species in alpine meadow soils with different degrees of degradation.Bacterial community is mainly the functional characteristics related to the metabolism of carbon and nitrogen organic compounds,while fungal community is mainly the functional groups of saprophytic fungi,ectomycorrhizae,plant pathogens and animal pathogens.Lightly degraded meadow has the largest microbial network and the most complex interactions among species.The degradation of alpine meadow had a greater impact on the fungal community.（4）The Gemmatimonadetes and the Basidiomycota had a very significant positive correlation with the soil stability indicators MWD,GMD,and R_0.25.There was a strong correlation between other dominant populations and other soil factors such as SOC,TN,AP,SM,etc.The stability of aggregate had a significant effect on the abundance of dominant species,but had no significant effect on the diversity of microbial communities.The alpha diversity of soil microbial during grassland degradation was mainly affected by soil factors such as SOC,MBC,MBN,AP,SM,etc.（5）The distribution of nutrients in aggregates with different particle sizes had differe nt effects on the dominant bacterial and fungal populations in soil.The carbon and nitrogen in the aggregates with 0.25-0.5 mm size had a great effect on it.Structural equation modeling analysis found that changes of soil bulk density and soil nutrient s caused by alpine meadow degradation had a direct impact on microbial community.Grassland degradation affected the microbial communities mainly through soil bulk density and soil nutrients rather than soil aggregates.To sum up,the degradation of alpine meadow leads to a decline in the stability of soil structure and soil nutrients,and changed the distribution and stoichiometry of soil aggregates.The degradation of alpine meadow leads to the change of aggregate structure,which indirectly affected the microbial community structure,especially the fungal community,by changing soil nutrients.