三峡库区生物结皮对土壤分离过程的影响及其动力机制

The formation and development of biological soil crusts (BSCs) cause great changes in soil surface characteristics and then will inevitably affect the soil detachment process, but the influence extent and its mechanisms are still unclear. Furthermore, studies concerning the dynamic response of soil detachment process to the seasonal variations of BSCs in terms of the community composition, structure, coverage, and thickness were rarely report yet. Therefore, in this study a series of overland flow scouring experiments by a hydraulic flume will be carried out for different types of BSCs (selecting phycophyta, lichen, and moss as dominant species, respectively) covered soil in the Three Gorges Reservoir area. Via in-situ dynamic monitoring, indoor-outdoor measurement analyses, and mathematical statistics methods, the effects of BSCs type, coverage, thickness, and vertical structure on soil physicochemical properties and its response of soil detachment process will be systematically studied. The quantitative relationship between BSCs coverage and soil detachment process will be established. The quantitative effects of BSCs aboveground top crust (reflecting the surface covering effect) and belowground sub-crust (reflecting the binding effects of cyanobacteria filaments and the anchoring structures of mosses and lichens rhizoids as well as the bonding effects of exopolysaccharide) on soil detachment process will be explored. The dynamics of BSCs, soil properties, and soil detachment process as well as their quantitative relations will be thoroughly analyzed and revealed. The dynamic mechanism of BSCs influencing soil detachment process will be ultimately clarified. The results will provide scientific basis for deeply understanding the mechanism of BSCs influencing soil erosion process and developing process-based soil erosion models. It also has very important theoretical significance and practical value for soil and water conservation and ecological environment construction in the Three Gorges Reservoir area.

生物结皮的形成和发育引起地表特性变化，势必会对土壤分离过程产生重要影响，但其程度和机制尚不清楚，土壤分离过程对生物结皮季相变化（组成、结构、盖度、厚度等）的动态响应鲜有报道。本项目以三峡库区不同类型（藻类、地衣类、苔藓类）生物结皮为研究对象，通过径流冲刷试验、定位动态监测、室内外测定分析并结合统计学方法，系统研究生物结皮类型、盖度、厚度、结构对土壤理化性质的影响及土壤分离过程的响应，建立土壤分离过程与生物结皮盖度间的定量关系，量化生物结皮地上部分（地表覆盖作用）和地下部分（菌丝、假根等缠绕捆绑及分泌物胶结作用）影响土壤分离过程的贡献率，揭示土壤性质、生物结皮、土壤分离的季节变化规律及定量关系，阐明生物结皮影响土壤分离过程的动力机制。研究结果为深入理解生物结皮对土壤侵蚀过程的影响机理、建立或完善土壤侵蚀过程模型提供科学依据，对三峡库区水土流失治理、生态环境建设具有重要理论意义和实践价值。

土壤分离是土壤侵蚀的起始阶段，研究土壤分离发生、发展的水力、地形、土壤、等临界条件及其相互影响、相互制约的动力机制，是建立土壤侵蚀过程模型的基础；而利用实测土壤分离能力分析土壤侵蚀阻力时空变化及其动力机制、进一步构建土壤侵蚀阻力预报模型或修订方程，是土壤分离过程研究的终极目的。生物结皮的发育和演替引起植物群落近地表特性变化，进而对土壤分离过程产生深刻影响。三峡库区生物结皮发育显著提高了土壤黏粒含量、水稳性团聚体和土壤养分含量，显著降低了土壤容重和砂粒含量，微生物生物量和胞外酶活性均随结皮盖度的增大而显著增加；生物结皮促进了土壤水分入渗，土壤入渗性能随结皮盖度先增加后减小，土壤水分入渗过程可用Horton模型拟合。生物结皮发育显著增强了土壤侵蚀阻力，抑制了土壤分离，相对土壤分离速率和细沟可蚀性均随生物结皮盖度的增加呈指数减小。生物结皮地上部分（地表覆盖作用）和地下部分（假根菌丝物理捆绑和分泌物化学胶结作用）抑制土壤分离的作用均随结皮盖度的增大而增大，但地上部分的作用要强于地下部分，二者的贡献与结皮盖度相关但并非线性，在一定的水力、土壤条件下可能存在生物结皮组分影响土壤分离的临界盖度。随着季节变化，生物结皮盖度先增大（8月达到最大）后减小，土壤粘结力随结皮盖度的增加呈线性增大；生物结皮发育的土壤分离能力总体呈下降趋势，且整个观测期均显著低于对照裸地；粘结力是影响细沟可蚀性季节变化的主要因素，随着粘结力增大，细沟可蚀性呈指数函数减小。综合分析了生物结皮盖度、厚度及其季节变化驱动的内在土壤属性变化对土壤分离过程的影响，发现土壤分离能力主要受结皮盖度、土壤粘结力和砂粒含量的影响，细沟可蚀性主要受结皮盖度和土壤容重的影响；土壤分离能力可用水流剪切力、土壤粘结力和生物结皮盖度的幂函数进行模拟。研究成果为揭示土壤侵蚀过程动力机制、构建生物结皮发育条件下的土壤侵蚀过程模型提供科学依据和数据支撑。