The Response And Impact Of Bioligical Soil Crust On Rainfall Erosivity In The Loess Hilly Region

Biological soil crusts (biocrusts) were occured in the arid and semiarid region extentivly,and had been a general study topice by many scholars. Biocrusts could improve soil nutrient,promote soil development and reduce soil erosion. Its prospect on soil and water conservationand desertification control was important. In view of the biocrust response and influence torainfall erosion, based on GIS, through the field sampling, this study clearified spatialdistribution of biocrusts in the loess plateau region. Then, response and impact of differenttype biocrusts with varied biomass to rain kinetic energy and surface erosion, by using thedisc infiltration apparatus, field washout test and indoor single raindrop strike in the hillyloess plateau region for representative. This paper research further clarified the mechanismthat biocrusts can reduce the soil erosion, and provides theory basis for the crusts scientificmanagement and use of the biological crust for erosion resistance. The results were:(1) Biocrusts consisted by algae and moss were widely distributed in Loess Plateauregion, its distribution was mainly effected by rainfall and land use type. In the northwesternloess plateau where rainfall less than200mm, biological crusts were mainly algae crusts; Inthe central Loess Plateau where rainfall in250mm to400mm, the biocrusts presents a kindof mixed growth state; In the southeast of the Loess Plateau in which rainfall per year>500mm, the biocrusts mainly are mosses; Biocrusts coverage vary based on the land use type.Biocrusts' coverage on sand surface can achieve more than80%; The grass coverage alsoinfluence biocrusts's coverage. In high coverage grass lands, biocrusts coverage was40%, and50%in moderate grass coverage, low coverage even more than70%; In forest land crustsdistribution least, coverage about40%.(2) Biocrusts can enhance soil resistance to rain spattered corrosion to reduce the soilerosion. Raindrops kinetic energy of breaking down cyanobacteria dominted biocrusts (1cmthickness) was2.17J, which was about12times higher than that of bare soil; moss dominatedbiocrusts (1cm thick) could withstand the rain kinetic energy of62.03J, which was344and28times of the energy of bare soil and cyanbacteria dominated biocrusts, respectively.Biocrust's resistance to raindrop splash erosion was relevant its biological composition andbiomass: cyanbacteria dominated biocrusts biomass(with the unit area of crusts content of chlorophyll a measure, similarly hereinafter)increased from1.81u g·g^-1to3.83u g·g^-1, therain kinetic energy is corresponding from0.22J to2.17J; Moss dominated biocrustsbiomass(with the unit area of mosses dry weight measure, similarly hereinafter) increase from2.03g·dm^-2to6.73g·dm^-2, its corresponding rain kinetic energy increasing from12.11to210.70J.(3) In different season, the response of biocrust on raindrops splash erosion kineticenergy varied. Before the rainy season, algae crust can bear0.86J raindrops splash erosionkinetic energy.50%coverage moss crust could bear12.16J; In the rainy season, value ofnumber increased a lot. Cyanbacteria dominated biocrusts2.19J,50%moss cover biocrustwas24.95J; At the end of the rainy season, value increase a little.(4) Biocrusts soil resistance to splash erosion was relevant with the below soil texture.Cyanbacteria dominated biocrusts resistance's order was: sandy loam (0.99J)> clay loam(0.86J)> sandy soil (0.52J). Moss dominated biocrusts's order was: sand (177.11J)> sandyloam (62.03J)> clay loam (55.66J). The numerical resistance of biocrusts on differenttexture of surface soil increase with biomass as exponentially function relation (ysandy loam=5.0384e0.7316x, R2=0.966; yclay loam=0.9757e0.9423x, R2=0.9639).(5)) Biocrusts resistance to raindrops spattered relates to the slope. When gradient wasunder10°, numerical resistance varies between12.14J¹4.76J, no significant differencebetween the different gradient; When gradient between10°²5°numerical resistanceincreased significantly, change between28.96J¹39.85J, significantly higher than whatbelow10°; When more than25°numerical resistance increase as exponential functionrelations with gradient increasing(y=7.0264e0.4686x, R2=0.9794).(6) Moisture didn't affect the biological crust's numerical resistance of raindropsspattered erosion significantly.(7) Biocrusts could significantly reduce soil water infiltration, increase the intensity ofrunoff, and affect runoff kinetic energy. Water infiltrating30min on moss dominatedbiocrusts achieved stability infiltration rate, that was13.3mm·min^-1; water infiltratingachieved stability infiltration rate in22min on cyanbacteria dominated biocrusts, tant was18.5mm·min^-1. Moss's biocrusts reducing flow velocity was stronger than cyanobacteriacrust's, average flow velocity on moss crust is0.17m·s^-1, on cyanobacteria crust was0.21m·s^-1. The growth of biocrust increased the soil surface drag coefficient, decreased the runoffkinetic energy, and decreased the run off erosion on surface soil.