Community Composition And Formation Mechanism Of Biological Soil Crusts In The Northern Shaanxi Of Loess Plateau

Biological soil crusts（BSCs）result from an intimate association between soil particles and cyanobacteria,algae,microfungi,lichens,and bryophytes（in different proportions）which live within,or immediately on top of,the uppermost millimeters of soil.Soil particles are aggregated through the presence and activity of these biota,and the resultant living crust covers the surface of the ground as a coherent layer.BSCs had many functions,such as,it could resist pressure,wind erosion,water erosion and desertification.The Northern Shaanxi Loess Plateau,one of the most serious soil erosion regions in the world.However,after the implementation of“returning farmland to forestry”,the ecological environment had improved,and various types of biological soil crusts（BSCs）had been widely developed,which played an important role in the ecological environment change of the Loess Plateau in northern Shaanxi.In this research,the community composition of BSCs were measured by high-throughput sequencing analysis,made a profound study of the algae diversity.The relationship between the cyanobacterial communities composition of BSCs and their extracellular polymers,and the effects of physical and chemical properties of their matrices on the cyanobacterial diversity of BSCs were explored.The main results were as follows:1.The composition communities of prokaryotic and eukaryotic in different regions of the northern Shaanxi Loess Plateau was detected by high-throughput sequencing.Cyanobacteria and Proteobacteria were dominant in surface BSCs,and Proteobacteria was major in upper BSCs;Streptophyta and Ascomycota were dominant eukaryotes.The community composition structure of BSCs which of different regions at different levels is similar.2.In BSCs,Cyanobacteria was dominant coummunties,besides,Diatomea,Chlorophyta and others were developed in BSCs.Calculating the biomass of BSCs by Ritchie 2006,which was consistent with the results of high-throughput sequencing,found the relative abundance of cyanobacteria was high.3.Using high-throughput techniques and morphological methods,we found that the community composition of cyanobacteria in biological crusts in different regions of the northern Shaanxi Loess Plateau was obviously different.Microcoleus was dominant in Shenmu,Yijun,Dingbian and Suide,while Mastigocladopsis and Scytonema were dominant in Ansai.Chloropyrula and Myrmecia were dominant in Ansai,while Leptosira was dominant in Shenmu and Yijun.In addition,there were Diatomea,Ochrophyta,Heterokontophyta and Phaeophyta.4.The content of extracellular polysaccharide（EPSs）,colloidal extracellular polysaccharide（C-EPSs）,EDTA-extracted extracellular polysaccharide（TB-EPSs）and glucuronic acid in the BSCs of different area were different,and their composition was highly complex.The distribution of cyanobacteria in biological crusts was related to EPSs.The TB-EPSs was related to Scytonema,Mastigocladopsis and Nostoc.Glucuronic acid was related to Microcoleus.5.The physical and chemical characteristics of different BSCs matrices were different in different regions of northern Shaanxi Loess Plateau.Prokaryotic biodiversity and richness were mainly affected by soil pH;eukaryotic biodiversity was mainly affected by the content of Na⁺,Mg²⁺,and which was mainly affected by soil pH.Cyanobacterial diversity is mainly affected by the EC,negatively affected by Mg²⁺,and cyanobacterial richness was mainly affected by OP;The genetic diversity of cyanobacteria was mainly affected by TN and OM.The distribution of cyanobacteria community was mainly affected by alkali hydrolyzed nitrogen（AN）.Different cyanobacteria species had different functions to adapt to the microenvironment in different regions of the Loess Plateau in northern Shaanxi.6.The BSCs morphology of the Loess Plateau in northern Shaanxi were different,but the surface contained organic layer and inorganic layer,which were covered by more fine substances.The cementing mechanism was mainly that algae use themselves or their secretions to bond soil particle,a large number of algal filaments crawl on the surface of biological crust and interlace with soil particles.The filamentous algae penetrate into the soil and combine with soil particles,and penetrate between the sand grains,thus played a good bonding role to the sand grains.