The Development Characteristics And Formation Mechanism Of Saxicolous Bryophyte Crust In The Northern Foothills Of Qinling Mountains

Biological soil crust is presently being utilized in ecological restoration in arid places because of its abilities to control the soil hydrological cycle,increase nutrient content,and stabilize the soil surface to lessen the risks of wind and water erosion.Mineral resource extraction in the Qinling Mountains has resulted in numerous slopes of exposed rock that require immediate attention.The bryophyte crust can be widely developed in the bare rock surface,which may be a viable option for ecological restoration.However,it is challenging to directly generalize the findings of the research on soil-borne biological crust in dry areas to those of saxicolous biological crust in humid areas because of the significant differences in climate,substrate type,and species composition,which resulted in the biological soil crust exhibit obvious developmental complexity and spatial heterogeneity.Therefore,understanding the developmental characteristics and formation mechanism of saxicolous bryophyte crust in the Qinling Mountains can not only make up for the lack of relevant research in the humid zone,but also provide an important reference for the ecological restoration of bryophyte crust.In this study,we took the saxicolous bryophyte crust in a branch ditch west of Cheyu in Qinling Mountains as the research object.Though field surveys,field observations,and indoor experiments to clarify the species composition,species diversity,spatial distribution characteristics and key influencing factors,developmental characteristics of the dominant species,and the formation mechanism of the saxicolous bryophyte crust in the study area.In order to provide a theoretical basis for exploring new ways of ecological restoration of bare rock engineering surfaces in saxicolous mountainous areas.The main conclusions are as follows:（1）The saxicolous bryophyte crust in the study area are abundant in species composition.There are 32 saxicolous bryophytes in the west ditch of Cheyu,which belonged to 21 genera and 14 families,including 30 species of mosses（19 genera,12 families）and 2species of liverworts in 2 families and 2 genera.The dominant families of bryophyte are Brachytheciaceae,Hypnaceae and Pottiaceae,the dominant genera is Brachythecium,and the dominant species are Plagiomnium cuspidatum and Brachythecium plumosum.A total of 2774species in 32 phyla,363 genera of bacteria and 1479 species in 10 phyla,266 genera of fungi were identified from the stable bryophyte crust,the dominant phyla in both were Proteobacteria and Ascomycetes,respectively,and the dominant genera were Ferruginibacter and Minimedusa,respectively.A total of 32 species of algae in 21 families and 25 genera were identified,dominated by Cyanophyta and Chlorophyta.（2）The saxicolous bryophyte crust in the study area was distributed along the river channel and exhibited weak spatial autocorrelation,which was affected by light intensity,air humidity,elevation,p H and nutrient content of the rock surface.The area of saxicolous bryophyte crust in the study area accounted for about 38%of the rock area.On rocks with bryophyte crust development,the average cover of the saxicolous bryophyte crust was about67%,with an average biomass of 138.65 g/m²,and the bryophytes were mostly mixed,usually containing 3～4 species and up to 12 species.When air humidity was less than 50%,the biomass,coverage and bryophyte numbers of saxicolous bryophyte crust showed a declining tendency with elevation,but they did not significantly alter with elevation in the high humidity area（>80%）.Saxicolous moss crust coverage,biomass and species of bryophyte exhibited a positive correlation with air humidity,light intensity,and SOM,TN and AK content but a negative correlation with elevation and rock surface p H（P<0.05）.The key variables influencing the development of the bryophyte crust were elevation and p H of the rock surface.（3）Brachythecium plumosum expanded quickly when the environmental conditions were suitable and had good adaptability to adversity,in addition,both the propagule types and nutrient supply patterns had significant effects on its growth and development.In field,Brachythecium plumosum had densely lateral branches that formed simultaneously with the growth of the stem.Except a significant reduction in light intensity（15～55%）,the undercanopy showed less variation in air humidity and temperature of the rock surface than the intercanopy.However,the growth rate of Brachythecium plumosum was more than 3 times higher under the canopy than in the intercanopy.The gametophytes were regenerated directly from the stems,which had a 100%survival rate.The developing protonema was not differentiated,which might play a role in attachment and nutrient uptake.Only 3.3%of the leaves survived,and the gametophytes differentiated from the protonema.When the produced gametophytes are in contact with the growth substrate,the rhizoids are sequentially distributed along the gametophytes and have forked ends,and when they were not in contact,the rhizoids were distributed at intervals and were not forked at the ends.Long-term nutrient solution supply may have a negative impact on the moss growth on the rock surface.In the early stages of the experiment,the Hoagland nutrient solution promoted the growth of Brachythecium plumosum while simultaneously encouraging the development of pathogenic bacteria/pathomycete and algae,which caused the moss to wither or turn yellow.（4）Selected the suitable colonizing site and dissolved root（extracellular）secretions chemically,microorganisms,algae,and bryophyte alter the structure and function of the microbial community on the rock surface,increase nutrient content,and control the acid-base balance,all of which create the ideal environment for the growth of moss crust;The distinct morphological traits,water-absorbing qualities,and strong adherence of the rhizoid system of the saxicolous bryophyte enhanced the adaptability to harsh environments.The capacity of bryophyte to improve the rock surface gradually improved with the prolonged development time,after then,the stable bryophyte crust formed gradually.Most of the saxicolous bryophyte crust originate from cracks or depressions in the rock surface,which is the basis of their formation.As succession proceeds,the surface structure of the rock is gradually fragmented,and microorganisms,algae,or rhizoid and protonema of bryophytes enter the interior of the rock and cause mechanical damage.A total of 27 sugars,35 organic acids and 7 phytohormone were found in the secretions of bryophytes and algal crusts,and the relative content difference was significant.The leaching solutions of the bare rock,algal crust,and bryophyte crust show a gradual increase in carbonic anhydrase activity,dissolution of K⁺,Ca²⁺,Mg²⁺,Zn²⁺,and Fe²⁺as well as a gradual decrease in p H,indicating that the chemical dissolution of the rock surface layer increased with succession.In comparison to bare rock and algal crust,bryophyte crust had much higher bacterial and fungal diversity.The relative abundance of Proteobacteria,Actinobacteria,Planctomycetes,and Verrucomicrobia of bacteria was higher than that of bare rock and algal crust,and all of them were positively correlated with the TN,SOM,AP,and AK contents of the rock surface.The relative abundance of pathogenic Sarocladium,Verticillium,and Mortierella in fungi community decreased in saxicolous bryophyte crust compared to algal crust.Furthermore,saxicolous bryophyte frequently grew intertwined in the wild which favored the improvement of the surrounding water conditions.When it comes into contact with water,it takes only 10 minutes to reach the saturation point of both internal and external water absorption.The well-developed rhizoid system allows the bryophyte to withstand the 570 and 860 times force higher than its own gravity in the longitudinal and transverse directions,which ensures the bryophytes colonize the rock surface.