Document Type : Research Article
Authors
1 University of Tehran
2 Iranian Research Organization for Science and Technology (IROST)
Abstract
Introduction: Physical and biological soil crusts are the principal types of soil crusts. Physical and biological soil crusts are distributed in arid, semi-arid and sub-humid regions which constitute over 40% of the earth terrestrial surface. Biological soil crusts (BSCs) result from an intimate association between soil particles and cyanobacteria, algae, fungi, lichens and mosses in different proportions which live on the surface, or in the immediately uppermost millimeters of soil. Some of the functions that BSCs influences include: water absorption and retention, nutrient retention, Carbon and nitrogen fixation, biological activate and hydrologic Status. BSCs are important from the ecological view point and their effects on the environment, especially in rangeland, and desert ecosystems and this caused which researchers have a special attention to this component of the ecosystems more than before.
Materials and Methods: This study carried out in the Qara Qir rangelands of Golestan province, northeast of Iran (37º15′ - 37º23′ N &54º33′ -54º39′ E), to investigate the effects of BSCs on some of soil biological properties. Four sites including with and without BSCs cover were selected. Soil biological properties such as microbial populations, soil respiration, microbial biomass carbon and nitrogen, as well as, other effective properties such asorganic carbon percent, total nitrogen, electrical conductivity, and available water content were measured in depths of 0-5 and 5-15 cm of soil with four replications. The gathered data were analyzed by nested plot, and the mean values were compared by Duncan test.
Results and Discussion: The results showed that organic carbon and water content were higher at the surface under BSCs, followed by 5-15 cm soils under BSCs. Both soil depths of uncrusted soils showed substantially lower organic carbon and water content than the BSC-covered soils. Total nitrogen was far higher in BSC-encrusted surface soils than uncrusted surface soils or BSC sub-surface soils. All Electrical conductivities were lower in surface soils covered with BSCs than sub-surface soils. The values for non-BSC covered soils were far higher than values for soils covered with BSCs. The values of soil biological properties such as microbial populations, soil respiration, microbial biomass carbon and nitrogen were higher at the surface under BSCs, followed by 5-15 cm soils under BSCs. The values for non-BSC covered soils were far lower than values for soils covered with BSCs at 0-5 cm depth but these properties in the uncrusted soils did not differ with BSCs covered surface at 5-15 cm depth. The amount of organic carbon was higher in BSC-covered surface soils at both measured depths, likely due to the ability of BSCs to fix atmospheric carbon. This leads to enhanced BSCs biomass and thus organic carbon especially in the soil surface layer (0-5 cm). An extensive cover of even a thin layer of photosynthetically active organisms can be an important basis for carbon input into the soil. BSCs also produce and secrete extracellular polysaccharides into surrounding soils, increasing the soil carbon and nitrogen pool. In general, there is a positive correlation between C and N fixation by BSCs. Also distribution of soil microbial population is positively correlated with the distribution of organic carbon and nitrogen. Microbial population is reduced following increase at depth, which is proportional to reduce of the concentration of nutrient and suitable conditions such as water content for growing them. Therefore proportionate to Microbial population, the properties such as soil respiration and microbial biomass carbon and nitrogen were reduced following increase at depth, because it did not provide the conditions for living organisms. These conditions were more inappropriate for non-BSC covered soils due to lower water content, organic carbon, total nitrogen and much higher electrical conductivity at both depths especially at 5-15 cm depth.
Conclusion: Biological soil crusts can play a key role in the biological properties of soil. Our data showed that organic carbon percent, total nitrogen, and available water content and biological properties such as microbial populations, soil respiration and microbial biomass carbon and nitrogen were increased significantly in two mentioned depths especially in 0-5 cm depth on sites covered with BSCs, relative to without BSCs. Electrical Conductivity had a reverse trend. In general, it can be concluded that BSCs improve soil conditions and provide suitable habitats for heterotrophic microorganisms and increase soil microbial activity. As the presence of BSCs generally increased the positive qualities of the soil, it is suggested that they can be used as a qualitative indicator of soil quality in rangelands.
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