Mechanisms Of Plant Community Assembly And Productivity Formation Under Different Grazing Intensities In The Stipa Breviflora Desert Steppe

Grassland is one of the terrestrial ecosystems with the largest distribution area on earth,providing a variety of services such as material basis and living environment for human beings.Desert steppe is not only an important part of Inner Mongolia steppe,but also a natural ecological barrier and animal husbandry base in northern China,which is sensitive to interference response due to its dry climate,less precipitation,less species number of plant community and fragile ecological environment zone.Grazing is the main utilization way of the desert steppe,however,to date,the mechanisms of grazing intensity on plant community assembly and productivity formation in the desert steppe remains unclear.In this study,taking the Stipa breviflora desert steppe as the research object,four grazing treatments（i.e.,enclosure without grazing（control）,light,moderate and heavy grazing）were set,based on the long-term different grazing intensity platform（2004 to present）.Community structure,functional traits,above-ground net primary productivity,soil physical and chemical properties,microbial community structure and diversity were measured and analyzed with the basis of field vegetation survey and laboratory experiment analysis from 2020 to 2022,to uncover the change trend of plant diversity（species diversity,phylogenetic diversity and functional diversity）in different dimensions and ecosystem productivity under different grazing intensities.Based on the null model,this study aims to explore the mechanism of plant community assembly under different grazing intensities,to clarify the relationship among ecosystem productivity,biodiversity and soil environment,and further to reveal the internal mechanism of ecosystem productivity in the grassland communities,providing a theoretical basis for grassland grazing management and sustainable use.The main results were as follows:1.There were 52 species in the Stipa breviflora desert steppe community.Among them,there were 51,41,41 and 30 species in the control,light,moderate and heavy grazing treatments,respectively;and S.breviflora was the most frequent species under different grazing intensities.With the increase of grazing intensity,Cleistogenes songorica showed a gradually increasing trend,while Artemisia frigida and Convolvulus ammanni showed a gradually decreasing trend.The occurrence frequency of shrub/semi-shrub plants such as Kochia prostrata,Caragana stenophylla and Ceratoides latens showed a decreasing trend with the increase of grazing intensity,while annual/biennial plants such as Chenopodum aristatum and Euphorbia humifusa showed an increasing trend.The number of species in plant communities gradually increased with the increase of sampling area under different grazing intensities,which conformed to the logarithmic growth model.With the increase of grazing intensity,species abundance tended to decrease rapidly with the increase of species sequence,and the heterogeneity of abundance among species increased gradually.2.Grazing significantly reduced species richness,Simpson index,Shannon-Wiener index and Pielou index（p<0.05）.The mean nearest taxon distance（MNTD）increased significantly in the light and moderate grazing treatments,but decreased significantly in the heavy grazing treatment（p<0.05）.Both moderate and heavy grazing treatments significantly reduced functional richness(FR_ic),functional dispersion(FD_is),functional evenness(FE_ve),functional divergence(FD_iv)and Rao Q quadratic entropy（Rao Q）index（p<0.05）.Grazing significantly decreased the Community-weighted-mean（CWM）values of leaf-stem-root carbon content(CWM_LC,CWM_SC,and CWM_RC),but significantly increased CWM of stem nitrogen content(CWM_SN)（p<0.05）.With the increase of grazing intensity,the CWM of specific leaf area and plant height(CWM_SLA,CWM_Height)showed a gradually decreasing trend,while the CWM of specific root length(CWM_SRL)showed a gradually increasing trend.3.The plant community assembly based on phylogenetic structure and functional traits showed that the nearest taxa index（NTI）was greater than zero under different grazing intensities,indicating that the process of plant community assembly was mainly dominated by deterministic processes.NTI increased with the increase of grazing intensity,indicating that the relative importance of deterministic processes increased gradually in the process of plant community assembly.The plant community assembly based on the niche-dispersal process showed that the plant community was mainly affected by the niche process,while the important value of dispersal process was less.The plant community assembly based onβ-diversity showed that the deterministic process driven by environmental filtering gradually dominated the community with the increase of grazing intensity,while the relative importance of stochastic process gradually decreased.Evaluation of species co-occurrence mode and ecological stochasticity research based on C-score and standardized effect size（SES）showed that plant communities were mainly dominated by deterministic processes,while stochastic processes relatively less important.Overall,deterministic processes dominated plant community assembly in the S.breviflora desert steppe,and the relative importance of deterministic processes gradually increased with the increase of grazing intensity.4.As the grazing intensity increased,the above-ground net primary productivity（ANPP）decreased gradually（p<0.05）.Both light and moderate grazing treatments significantly increased ANPP of perennial grasses（p<0.05）;however,both moderate and heavy grazing treatments significantly decreased ANPP of shrubs/semi-shrubs and perennial forbs（p<0.05）,while grazing had no significant effect on ANPP of annual/biennial plants（p>0.05）.Grazing significantly increased the ANPP of S.breviflora but decreased the ANPP of Artemisia frigida（p<0.05）,and heavy grazing significantly decreased the ANPP of Cleistogenes songorica and Convolvulus ammannii（p<0.05）.The ANPP of the whole community was highly significantly positively correlated with the ANPP of perennial grasses,shrubs/semi-shrubs and perennial forbs,and the ANPP of four dominant species（p<0.01）.Besides,community ANPP was highly significantly positively correlated with species diversity（species richness,Simpson index and Shannon-Wiener index）（p<0.01）,and significantly（p<0.05）or extremely significantly（p<0.01）positively correlated with functional diversity（functional richness and functional evenness index）,while no significantly correlated with phylogenetic diversity（p>0.05）.Likewise,community ANPP was significantly（p<0.05）or extremely significantly（p<0.01）positively or negatively correlated with the CWM of plant traits(e.g.,CWM_SLA,CWM_SRL,CWM_RC and CWM_LN).The relative importance of ANPP of the three dominant species（i.e.S.breviflora,A.frigida and C.songorica）was higher（p<0.05）,followed by species richness,Shannon-Wiener index and ANPP of C.ammannii（p<0.05）.5.Both moderate and heavy grazing significantly decreased soil field water capacity,soil organic carbon,total nitrogen,dissolved organic carbon,available nitrogen,available phosphorus,and richness of both bacterial and fungi community（p<0.05）,but significantly increased soil p H value and bulk density（p<0.05）.Light grazing only reduced soil dissolved organic carbon and available phosphorus（p<0.05）.Plant community ANPP was significantly negatively correlated with soil bulk density（p<0.05）,but significantly（p<0.05）or extremely significantly（p<0.01）positively correlated with field capacity,soil total nitrogen,dissolved organic carbon,available nitrogen content,bacterial richness,and fungal richness（p<0.01）.The random forest model showed that the relative importance of bacterial richness,soil available nitrogen and soil total nitrogen content to plant community ANPP was higher（p<0.05）,while the relative importance of soil dissolved organic carbon content and fungal richness was second（p<0.05）.