Effects Of Biological Soil Crust On The Distribution Of Microbial Quantity In Loess Plateau Region, China

Large area of biological soil crusts(biocrusts) were developed on the revegated grasslands after the implementation of the Grain for Green ecological project in the hilly Loess Plateau of China, which exerted a profound effect on soil nutrient content, moisture permeability, structure and temperature. Thus, biocrusts may further cause a variation in the quantity, spatial and temporal distribution of soil microorganisms. Combined field surveyed with laboratory analysis, we identified the influence and mechanism of biocrusts on soil microbial distribution characteristics. Then, we identified the characteristic of seasonal dynamic and spatial distribution of biocrustal microbial quantity by measuring the spatial and temporal distribution of soil microorganisms in biocrusts. The main results are as follows: 1、 Biocrusts significantly affected the microbial quantity, and the intencity of the influence were related to biocrustal types, seasons and soil deepness. Moss crust significantly increased soil microbial quantity compared with other types of biocrust. There was different influence on microbial quantity between moss crust and light cyanobacteria crust in different seasons. Biocrusts had different effect on different groups of microorganisms in the vertical distribution.Biocrusts could significantly increase the quantity of total microorganisms, bacteria and fungi, and reduced actinomycic quantity. The total microbial quantity and bacterial quantity in biocrustal layer, 0-2 cm soil layer, 5-10 cm soil layer of moss crust were significantly higher than slope farmland, and respectively increased 22.92%、21.23%、6.12% and 44.01%、33.39%、10.98% at the same soil layer. Actinomycic quantity in 2-5 cm soil layer of moss crust was 20.43% lower than slope farmland. Fungal quantity in biocrust and 2-5 cm soil layer of moss crust were increased 30.03% and 14.65% than slope farmland, fungal quantity in 0-2 cm soil layer of moss crust was 33.34% lower than slope farmland. The variable coefficient of total microbial quantity, bacterial quantity and fungal quantity in biocrust layer and 0-2 cm soil layer were 4.66, 3.72 and 3.44 times than substratum, which was implied that the effect of moss crust on total microbial quantity, bacterial quantity and fungal quantity in surface was bigger than substratum.The effect of biocrusts on soil microbial quantity was related to biocrustal types. Microbial quantity of moss crust was highest than the other types of biocrust. In biocrust layer, the quantity of total microorganisms and bacteria in moss crust were 56.40%, 36.81% and 71.33%, 32.88% higher than those in cyanobacteria crust and mixed crust, the quantity of actinomycetes and fungi in moss crust were significantly higher than other biocrusts. In 0-2 cm soil layer, different group of microbial quantity in moss crust were higher than other biocrusts, there was no significant difference between the other four types of biocrust in microbial quantity, except of cyanobacteria with sparse moss crust. And then, in biocrust layer, the phenomenon was various difference in soil microbial quantity between moss crust and light cyanobacteria crust of different seasons. Microbial quantity of light cyanobacteria crust was significantly higher than moss crust in spring. Total microbial and bacterial quantity of light cyanobacteria crust was significantly higher than moss crust in summer, no significantly difference were found in fungal and actinomycic quantity. In winter, bacterial quantity of light cyanobacteria crust were significantly higher than moss crust, the quantity of fungi and actinomycetes in moss crust were significantly higher than light cyanobacteria crust. 2、 The quantity of microorganisms in biocrusts showed significantly seasonal dynamic in the region.The seasonal dynamic of soil microbial quantity were difference between moss crust, light cyanobacteria crust and slope farmland. Total microbial quantity in slope farmland was highest in spring, followed with autumn and winter, and least in summer. The quantity of total microorganisms were showed like summer > spring > autumn and winter in light cyanobacteria crust. The quantity of total microorganisms were showed like spring, summer, autumn > winter in moss crust. The seasonal dynamic of three groups of microbial quantity were difference between different types of biocrust. The quantity of microorganisms of moss crust had a lower seasonal fluctuations compared with slope farmland and light cyanobacteria crust, the seasonal dynamic of soil microbial quantity was more stably. The development of biocrusts weakened the spatial heterogeneity of microbial quantity, and changed the vertical distribution characteristics of soil microorganisms. 3、 The vertical distribution characteristics of soil microorganisms were varied compared with farmland soil since the development of biocrusts, different spatial heterogeneity was found for different group of microbial quantity.In biocrust layer, the total microbial quantity and actinomycic quantity in middle position of shady slope were higher than sunny slope, no significant difference in bacterial quantity and fungal quantity were detected between different slope aspects; no significant difference in microbial quantity were detected between different slope positions; no significant difference in total microbial quantity, microbial quantity and fugal quantity in different azimuth of higher plants, the quantity of actinomycetes in the southeast and southwest was significantly different from northwest and northeast.The vertical distribution of the soil microorganisms were varied between moss crust and slope farmland. The total quantity of soil microorganisms in slope farmland was highest in 2-5 cm and 5-10 cm soil layer, 1.07 times than 0-2 cm soil layer; total microbial quantity was highest in biocrust and 0-2 cm soil layer, 1.18 and 1.07 times than 2-5 cm and 5-10 cm soil layer. The vertical distribution of three group of soil microbial quantity were varied between moss crust and slope farmland. 4、 The quantity of soil microorganism in biocrusts was mainly related to the biocrustal composition, biocrustal thickness and moss coverage, and no related to the spatial variation. Biocrusts could accelerate the growth of bacteria and fungi, by increased soil organic matter, total nitrogen, T/N ration and electrical conductivity, reduced soil pH. The higher bacterial proportion in the soil may be the reason for the reduction of actinomycic proportion.Total microbial quantity, bacterial quantity and proportion, fungal quantity were significantly positively correlated with moss coverage and biocrustal thickness, no significant correlation between different groups of microbial quantity with slope aspect and position.The quantity of total microorganisms, bacteria and fungi, and the percentage of bacteria and fungi were positively correlated with soil organic carbon, total nitrogen, ratio between carbon and nitrogen, and electrical conductivity, while negatively correlated with soil pH; actinomycic proportion was significantly negatively correlated with bacterial proportion.