黄土高原生物结皮的热特性、热传输过程以及土壤热效应

In arid and semi-arid climates, the effects of biological soil crusts on soil temperature regimes are of fundamental importance to understand the hydrological and ecological functions of biological soil crusts. However, these effects and their mechanisms have not yet been fully investigated. To provide more insights into this issue, we plan to conduct a research on the thermal properties and heat transfer processes of moss-dominated biological soil crusts and their effects on soil temperature regimes on the Loess Plateau of China. The objectives of our research are (1) to evaluate the effects of biological soil crusts on soil thermal properties (i.e., surface albedo, soil heat capacity, thermal conductivity, and diffusivity) and explore their mechanisms and responses to determinant factors (e.g., soil moisture and bulk density); (2) to investigate the regulating effects of the crust layer on the heat transfer processes of biological soil crusts and the underlying soils in soil warming and cooling processes, respectively; (3) to quantify the influences of biological soil crusts on soil temperature regimes and their relationships with micro-meteorological conditions (such as soil moisture, air temperature, and solar radiation); and lastly, (4) to simulate the heat transfer processes of biological soil crusts with a numerical model (e.g., Hydrus-1D) and analyze the surface energy balance of biological soil crusts. From above studies, we will finally clarify the effects of biological soil crusts on soil temperature regimes as well as their mechanisms and processes in semi-arid climates on the Loess Plateau of China. Our results would give more explanations to the hydrological and ecological functions of biological soil crusts in dryland ecosystems; and they would be also helpful for understanding the soil mass and energy balance which regulated by biological soil crusts on the Loess Plateau of China and similar climate regions all over the world.

改变土壤热量是生物结皮生态功能的核心方面，但目前过程和机理均不明确，这导致了对生物结皮水热效应的争议乃至生态功能认识的偏差。本项目以黄土高原藓结皮为代表，使用土壤热量研究的理论与方法，对生物结皮的热性质、热传输以及热效应进行系统研究。分析生物结皮的热特性以及生物结皮改变土壤热性质的途径，阐明生物结皮热性质对关键因素的响应规律；解析生物结皮的吸热与散热过程，揭示生物结皮的热传输特征及结皮层对热传输的瓶颈作用；剖析生物结皮剖面土壤温度的动态变化，明确生物结皮的土壤热效应及其变化规律与驱动机制；使用数学模型刻画生物结皮的热传输过程，并探究生物结皮的地表热量平衡特征。项目深入研究生物结皮的生物属性影响土壤热性质和热传输的内在机制，力求全面揭示生物结皮的土壤热效应及其过程与机理。结果可从土壤热量和土壤温度的角度解释生物结皮的生态功能，对揭示黄土高原以生物结皮为主导的土壤物质与能量平衡过程有科学价值。

改变土壤热量是生物结皮生态功能的核心方面，但目前过程和机理均不明确，这导致了对生物结皮水热效应的争议乃至生态功能认识的偏差。本项目以黄土高原藓结皮为代表，分析了生物结皮的热特性以及生物结皮改变土壤热性质的途径，研究了生物结皮热性质对关键因素的响应规律；解析了生物结皮的吸热与散热过程，量化了生物结皮的热传输特征及结皮层对热传输的瓶颈作用；剖析了生物结皮剖面土壤温度的动态变化，分析了生物结皮的土壤热效应及其变化规律与驱动机制；刻画了生物结皮的热传输过程，探究了生物结皮的地表热量平衡特征。项目经过4年研究：（1）明确了生物结皮对土壤温度的影响，证实生物结皮对土壤温度的作用包括干燥时增温和湿润时降温两方面，其降温幅度可达4.7-11.4℃、增温幅度可达1.3-9.3℃，但降温作用时间更长、强度更大、占据主导地位；（2）揭示了生物结皮对表层土壤热特性的影响及其继而作用于土壤温度的机制，发现湿润时生物结皮通过其较高的持水能力维持较高的土壤含水量，进而使土壤热容量、热导率、热扩散率分别提高了10%、28%、28%，因此在吸收等量太阳净辐射后温度增幅减小，即降低土壤温度；而干燥时生物结皮通过减小土壤容重和增加土壤有机质含量使热容量、热导率、热扩散率分别降低了33%、55%、23%，从而阻碍土壤热量散失，即增加土壤温度；（3）阐明了生物结皮对地表热量平衡的影响及其作用机制，生物结皮颜色更暗、粗糙度更高，因此其地表反照率平均比无结皮低44%（0.11 vs. 0.20），继而其短波反射平均比无结皮低45%、短波净辐射比无结皮高11%、净辐射比无结皮高23%；但增加的净辐射并没有一致提升土壤温度，反而使夏季土壤温度有所降低，这证实了生物结皮影响土壤温度的机制包括改变地表热量平衡和土壤热特性两方面；（4）厘清了生物结皮通过调节土壤温度继而影响土壤关键属性与过程并发挥其生态功能这一因果逻辑关系，确认了生物结皮改变土壤温度是其影响土壤蒸发与水汽凝结、维管束植物退化、土壤呼吸与碳排放、生物固氮、土壤微生物丰度与多样性、土壤酶活性等多方面的核心与基础。研究共发表SCI论文9篇、中文论文14篇，培养毕业研究生7名；研究结果从土壤热量和土壤温度的角度解释了生物结皮的生态功能，对揭示黄土高原以生物结皮为主导的土壤物质与能量平衡过程有科学价值。