| Plant community succession is a process,in which one plant community is replaced by another plant community and finally develops to the climax community.Many previous researches studied the change in soil quality in the process of vegetation restoration in the Loess Plateau.However,the niche differences among different species and factors affecting niche in the transitional stage of grass-shrub succession are largely unknown.Thus,the plant-soil-microorganism system of different plant species appeared in the transitional stage of grass-shrub succession was selected as our research object.Aiming at studying plant-soil-microorganism characteristics and how they drive the niche difference among different plant species in the transitional stage of grass-shrub succession,we conducted field investigation,rhizosphere sampling and laboratory analysis to systematically analyze the niche changes,plant chemical property characteristics,soil chemical property characteristics,soil microbial characteristics and microbial community composition and diversity of four dominant shrub species(Sophora viciifolia,Periploca sepium,Buddleja alternifolia,Clematis fruticose)and six dominant grass species(Bothriochloa ischaemum,Artemisia sacrorum,Stipa bungeana,Cleistogenes squarrosa,Stipa grandis,Lespedeza bicolor).Our main findings are as follows:(1)The relative important value(RIV)and niche width of shrub species were overall higher than that of grass species(except for Bothriochloa ischaemum).Sophora viciifolia,Periploca sepium,Buddleja alternifolia had relatively higher RIV than other species(>0.30),while the RIV of other species was between 0.10 and 0.24.Meanwhile,the aboveground plant nitrogen(N)concentration of Sophora viciifolia,Clematis fruticose and Artemisia sacrorum was significantly higher than other species.The belowground plant N concentration was the highest in Sophora viciifolia,and was 1.67-3.21 times that of other species,but the plant N concentration of all species was below the national average level(18.6 g kg-1).Additionally,the aboveground and belowground plant phosphorus(P)concentration exhibited no significant differences among the 10 different plant species and that of all species was at a relatively low level.The aboveground and belowground non-structural carbohydrate(NSC)of the 4 shrub species varied from 116.09-173.99 g kg-1to 132.91-191.93 g kg-1 and that of the 6 grass species vary from 145.83-201.18 g kg-1 and131.71-176.89 g kg-1.Besides,Buddleja alternifolia,Bothriochloa ischaemum,Stipa bungeana and Cleistogenes squarrosa had relatively higher aboveground NSC concentration and was 1.01-1.73 times of other species,while Periploca sepium,Buddleja alternifolia and Bothriochloa ischaemum had relatively higher belowground NSC concentration and was 1.02-1.46 times of other species.Thus,compared with grass species,shrub species had overall higher carbon sequestration capability,nitrogen absorption capability and stronger resistance to stress,occupying a relatively more important position in the plant community,with stronger ability to acquire environmental resources.(2)The SOC,total N content and microbial biomass N and P content of grass species were overall higher than those of shrubs,but the soil P availability and soil enzyme activities of shrub species were overall higher than those of grass species.The SOC,total N content,SOC:TP and TN:TP of grass species are higher than that of shrub species.The rhizosphere SOC contents of Bothriochloa ischaemum,Artemisia sacrorum,Stipa bungeana and Cleistogenes squarrosa were 8.27 g kg-1,8.53 g kg-1,8.35 g kg-1 and 8.78 g kg-1,respectively and their total N contents were 0.84 g kg-1,0.87 g kg-1,0.87 g kg-1 and 0.89 g kg-1,respectively,while the SOC and total N content of other plant species were all lower than 8.00 g kg-1 and 0.80 g kg-1.In addition,the content of available P and available N in rhizosphere soil did not show significant differences among different plant species.The rhizosphere microbial biomass N and microbial biomass P content of the grass species were overall higher than that of the shrub species,and the soil enzyme activity of the shrub species was overall higher than that of the grass species.(3)The bacterial diversity of grass species was overall higher than that of shrub species,but the negative correlation of rhizosphere soil bacteria of among grass species was stronger.At the class level,the 10 plant species all had?-proteobacteria,Subgroup-6 and Blastocatellia as the dominant classes of bacteria.The relative abundance of?-proteobacteria of 4 shrub species was all higher than 15%,while that of 6 grass species(except for Artemisia sacrorum and Stipa grandis)was all below 15%.Among shrub species,the bacterial correlation network produced 45 nodes(number of species)and 90edges(connecting lines).The negative correlation between species in the network was more obvious,and the proportion of negative correlation was 76.67%.Among grass species,the bacterial correlation network produced 40 nodes and 82 edges,of which the proportion of negatively correlated edges was 85.37%.The?diversity of rhizosphere soil bacteria exhibited no separation effect between shrub and grass.(4)The fungal OTUs,ACE,Chao1 and simpson index did not show significant differences among different plant species.The 10 plant species were all dominated by Dothideomycetes,Agaricomycetes,Sordariomycetes,and Eurotiomycetes.Shrub species had higher the abundance of Dothideomycetes,Sordariomycetes,while grass species had higher abundance of Agaricomycetes and Eurotiomycetes.The correlation network of plant-soil-fungi had 29 nodes and 109 edges.The positive correlation between the variables in the network was obvious,and the proportion of positive correlation was 53.21%.The?diversity of rhizosphere soil fungi did not show obvious separation effect between shrub and grass.(5)The niche was mainly positively correlated with plant aboveground and belowground plant carbon concentration,plant belowground phosphorus concentration,plant aboveground and belowground C:N,soil C:N,soil enzyme activity,Alphaproteobacteria,Betaproteobacteria,Gammaproteobacteria,Gemmatimonadetes,Leotiomycetes;and negatively correlated with SOC,total nitrogen,microbial biomass,bacterial diversity;insignificanly correlated with plant nitrogen concentration and fungal diversity.Meanwhile,niche was mainly driven by plant carbon concentration,plant C:N,plant phosphorus concentration,soil enzyme activity and microbial composition. |
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