黄土高原典型草原生物土壤结皮固碳潜力研究

Biological soil crusts widely exist in arid and semiarid regions, and are significant components and features of arid and semiarid ecosystems. Therefore, the vital roles of biological soil crusts in the processes of community succession, soil and plant ecological, soil hydrological, soil biological and geochemical processes, as well as in the practice of ecological rehabilitation for arid and semiarid regions have been widely emphasized. Grazing and the Establishing fenced enclosure are as the main utilization and management method used in the arid and semiarid ecosystem, which directly determines developmental stage and structure of the biological soil crusts. It related to the process of biological carbon sequestration. In addition, arid and semi-arid regions are characterized by episodic precipitation inputs, which occurs in isolated events or pulses, and is accompanied by large inter pulse periods with little or no rainfall, and different precipitation patterns significantly influence the effects of biological soil crusts on the carbon cycle. The Loess Plateau is known as a region where soil erosion is extremelly serious in China, even in the world, and its ecological environment is also most vulnerable. In recent years, overgrazing has exacerbated semiarid grasslands' degradation on the Loess Plateau. After the implement of re-grass project, the characteristic of the vegetation has been improved, and biological soil crust has been developed broadly in the restored grasslands. Therefore, we will conduct the study in the steppe of Loess Plateau, aiming at studying the influence of grazing disturbance and fenced enclosure on the process and mechanisms of the development of biological soil crusts in this region, and revealing the relationships between biological soil crusts development and the growth of higher plants and soil physical and chemical traits. At the same time, we plan to continuously measure the CO2 exchange rate of biological soil crusts under the different precipitation pattern, in order to further explore the process of biological soil crust CO2 exchange controlled by the precipitation patterns under grazing or enclosure conditions and the contribution of CO2 flux by biological soil crust to the carbon sequestration in the whole grassland system. Finally, the function of biological soil crust carbon source and sink under grazing and enclosure conditions, the contributions to the whole grassland system CO2 exchange process and carbon source and sink function can be clarified. The result is helpful to understand the function of carbon sequestration by biological soil crust in the context of global climate change.

生物土壤结皮在干旱和半干旱区群落演替、生物地球化学循环、土壤生态水文及生态修复过程中发挥着重要作用。放牧与围封作为草地的主要利用管理方式，直接影响草地生物土壤结皮的发育程度及其结构组成，最终影响其固碳过程。干旱半干旱地区的降水格局变化对生物土壤结皮的固碳过程也有重要的调控作用。黄土高原是我国乃至世界上水土流失最严重、生态环境最脆弱的地区，退耕（牧）还草工程实施后，植被有所改善，且退化恢复的草地出现大面积的生物土壤结皮。鉴于此，本研究以黄土高原典型草原为研究区域，研究放牧与围封条件下生物土壤结皮发生发育过程，以及降水量格局变化下土壤生物结皮及其整个草地系统CO2交换量，以期阐明放牧与围封条件下生物土壤结皮CO2交换过程及机制、其碳源/汇功能以及对整个草地系统源/汇功能的贡献，对认识生物土壤结皮的生态固碳功能及对草地生态系统碳平衡的准确估算具有重要意义。

生物土壤结皮是干旱、半干旱生态系统中的重要生物组成部分，但是，当前很多关于干旱、半干旱区系统碳源/汇的研究都忽略了生物土壤结皮的净CO2通量。本研究以黄土高原典型草原为对象，通过4年的野外观测和室内试验，测定了围封草地生物土壤结皮的地表CO2通量的季节动态和全年释放量，量化了生物土壤结皮在系统碳过程中的贡献；研究了模拟降水对生物土壤结皮净CO2通量和土壤呼吸及其组分的影响，以及生物土壤结皮的净CO2通量和土壤呼吸与微生物呼吸及其温度敏感性对放牧的响应；力图阐明生物土壤结皮净CO2通量在系统碳循环中的贡献，及降水和放牧对其的影响及其机制，以期为准确预测全球气候变化下黄土高原典型草原碳源汇特征的变化和该区草地的可持续利用提供科学依据。主要研究结果如下：.1. 生物土壤结皮在调节地表CO2通量中起到了重要的作用。与裸地相比，生物土壤结皮显著降低了地表CO2通量，藻类结皮和藓类结皮的降低幅度分别为18.2%和12.4%。藻类结皮和藓类结皮的总光合速率相似，分别为61.6 g C m-2 y-1和60.0 g C m-2 y-1；净固碳速率分别为36.5 g C m-2 y-1和24.8 g C m-2 y-1，藻类结皮具有更高的固碳效率。该区生物土壤结皮对系统固碳的贡献率约为22.4% - 32.2%。生物土壤结皮对于地表CO2通量的影响，主要是通过增加土壤的SOC、TN、TP、SMBC、和SMBN含量，改变真菌/细菌比来实现。.2. 降水量的大小是影响生物土壤结皮碳通量的最主要因素。当降水量低于这个值时，生物土壤结皮将表现出净碳释放，藻类结皮固碳的阈限值小于2 mm，而藓类结皮的阈限值在2 mm和5 mm之间。当达到降水量的阈限值时，生物土壤结皮种类和生物土壤结皮盖度对其碳的固定能力没有影响。.3. 模拟践踏显著降低了生物土壤结皮地表CO2通量和水分入渗率。随着践踏强度的增加，生物土壤结皮水分入渗率持续下降；生物土壤结皮的存在提高土壤表面水分入渗率的趋势。模拟践踏降低了生物土壤结皮的地表CO2通量，也显著地降低了生物土壤结皮的呼吸，减少了碳排放，但践踏强度对地表CO2通量无影响。该区藻类结皮在践踏约10天后可恢复其原有的碳过程。模拟践踏对生物土壤结皮地表CO2通量与土壤温度的相关性无影响，但是生物土壤结皮的存在显著升高了地表CO2通量的温度敏感性(Q10值)。