| Biological soil crusts are the common features on soil surface in the arid and semiarid regions throughout the world,and have important ecological functions in these ecosystems,such as wind prevention and sand—fixation,reducing soil erosion,improving soil nutrients circulation,etc．It has been reported that the coverage ofbiological soil crust(BSC)is above 70％in the Loess Plateau of China,where the soil and physiognomy types are very differentfrom other regionsintheworld．ThewidespreadBSC hasmuchimportantinfluence onmany ecological processes in this region,such as hydrology,erosion,nutrients cycling,etc．However,by far there are few related researches that have been done to recognize the spatial distribution and development characteristics,and also ecological functions of BSC in this region．Therefore,there are still many scientific problems needed to be solved urgently．In this paper,the study area is located in Liudaogou small watershed in the wind-water erosion crisscross region on the Loess Plateau of China．Using the combination method of literature review,field survey and sampling,fixed location observation and laboratory analysis,we studied the spatial distribution characteristics and developmental process of BSC,and discussed the factors that affected the BSC'S distribution and development characteristics,including the soil types,vegetation conditions and slope aspects．Meanw~le,the effects of BSC on soil enzyme(~ease,catalase and alkaline phosphatase)activities and nutrients content(soil organic matter,total N,available N,P and K)had also been analyzed in order to explain its formation mechanisms and ecological functions in research area．The main conclusions were as follows:(1)The stabile and mature moses crust Was the most widely distributed in study area,and it Was likely to form in hill slope or slope top for less erosion or less disturbance,andbetter soil water conditions．The crust'S coverage Was mostly from 60％to 70％．Sandy BSChad a concentrated distribution and mainly formed and developed at leeward slopes and themiddle or lower parts of slopes,and also the lowland among sand dunes under higher soil moisture conditions．Because it was affected by soil types and vegetation conditions,the loessial BSC mainly covered the areas deprived of vegetmion cover,and showed a patchy distribution pattem．(2)There were 2 families,8 genera and 15 species of mosses in BSC in the area,of which the family Pottiaceae accounted for 6 genera and 10 species and the dominant genera was Didymodan with 4 species being Didymodon constrictus(Mitt．)Saito,Didymodon nigrescens(Mitt．)Salto,Didymodon vinealis Brid．,and D,igidulus Headw．var．ditrichoides(Hedw．)Zand．．For the family Bryaceae,the dominant genera was Bryum,of which the common species were Bryum argenteum Hedw．,Bryum algovicum Sendt．,Bryum caepiticium Hedw．,and Bryum funkii Schwaegtr．,and others．Aloina rigida(Hedw．)Limpr．and Aloina brevirostris(Hook．&Grev．)Kindb．were the dominant moss species of BSC in the initi~stage．However,when BSC entered stabile or mature stage,the dominant moss species would become Bryum algovicum Sendt．,Bryum argenteum Hedw．,and Didymodon conslrictus(Mitt．)Salto,and the following dominant mosses were Aloina rigida(Hedw．)Limpr．,Barbula fatlax Hedw．,and Didymodon nigrescens(Mitt．)Saito．Meanwhile,the results also showed that moss species increased with the development and succession of BSC．(3)There were 7 phyla,26 families,35 genera and 50 species ofalgae in the early BSC,with Cyanophyta and Bacillariophyta being the best represented phyla including 16 families,23 genera and 38 species,which accounted for 76．O％of the total algae species．The phylum Chlorophyta had 5 families,6 genera and 6 species,which took up 12．O％of the total algae species．However,the phyla Euglenophyta,Dinophyta,Xanthophyta and Cryptophyta had only rare species of algae f1~2 species)in BSC．In the phylum Cyanophyta,the dominantgenera ofNostocaceac were Anabaena,Anabaenopsis and Aphanizomenon,and the followingdominant genera of Merismopediaceae were Synechocystis and Merismopedia．In the phylumBacillariophyta,the dominant genera of Coscinodiscaceae were Coscinodiscus,Cyclotella,Thalassiosira,and Melosira,and the following dominant genera of Nitzschiaceae wereNitzachia and Pseudo-Nitzschia．The dominant species of Chlorophyta were Chlorellavulgaris,Palmellococcus miniatus,Chlorococcum humicola,and Chlamydomonas sp．,amongothers．The algae species would increased first and then decreased with the development andsuccession of BSC．(4)Overall,the sandy BSC with a higher degree of the development and successiondistributed more widely than loessial soil crust,which was expressed in the crust's coverageand thickness,but there were no significant differences in crust's shear strength,which wasfrom 25．2～27．8kPa．The bulk density of sandy BSC was greater than loessial crust,effectedby soil type,and their bulk densities were 1．30 andl．15 g·cm一,respectively．While the bulk density'S variation coertlcients of both them were 6 8％and 14．4％．it 1ndicated that the development and succession degree of loessial crust had much greater variability．There were great differences among these properties of BSC under different vegetation types,and the development degrees of BSC generally followed the order of the arbor and shrub vegetation>perennial herbaceous plants>annual herbaceous plants in the research area．The formation and development of BSC was significantly affected by slope aspects,and the crust'S cover and thickness in the shady slope were significantly higher than thats in the sunny slope．These were mainly caused by light,soil moisture content and nutrient conditions in the different slope aspects．(5)With the development and succession of BSC,the crust'S thickness increased gradually,but the increasing extent was very slow．Meanwhile,the bulk density of BSC notably decreased with the increase of crust'S thickness,SO the porosity and structure of soil was improved．The bulk density ofstabile or mature BSC varied froml．0 tO 1．3 g．cm。。,but inthe early stage,BSC'S bulk density had no obvious changes,which varied from 1．40tol 43g'cm-3．The shear strength and thickness of BSC showed obviously positive correlation．With the increase of crust'S shear strength,the effects of BSC on controlling erosion were enhanced．(6)Compared to no crust,BSC could significantly improved soil enzyme activities in crust,but had no significant increase effect on soil enzyme activities in subsoil．The activitiesof soil urease and catalase in crust layer were 1．56 and 1．31 times as high as that in 0-2cmtopsoil under crust,respectively．The increasing extent of soil alkaline phosphatase activity in crust was the biggest,which was 3．72 times of that in 0-2cm topsoil under crust．Soil nutrients accumulation effects of BSC were obvious mainly in crust soil．In this study,BSC significantly increased the content of soil organic matter,total N,and available N in crust,butthe increase effects of available P．K were not significant．The soil pH was 8．08 in crust,which Was lower than that in 0-2cm topsoil under crust(pn=8．32)．The decrease effect ofBSC on soil pH Was helpful to improve the soil nutrients bioavailability．Therefore,it can beconcluded that the formation and development of BSC would be beneficial to betteringsurface soil biochemical properties,which call promote vegetation restoration andreconstruction in the arid and semiarid regions．... |
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