DISSERTATION RESEARCH: Carbon Cycle Changes in a Changing Climate: Using 13C and 14C to Partition Ecosystem Respiration in Tundra Undergoing Permafrost Thaw

永久冻土是指连续冻结两年或两年以上的土壤，储存着大量的碳(C)。这些土壤是碳的储存库，因为数千年的低温阻止了土壤有机碳（SOC）的微生物分解，从而阻止了随后从土壤向大气释放的二氧化碳（CO2）。然而，永久冻土非常容易受到变暖的影响；随着气候变暖，永久冻土开始解冻，微生物可以进入并分解储存的有机碳，从而释放二氧化碳。因此，永久冻土解冻有可能成为气候变化的积极反馈：大气中的二氧化碳会加剧变暖，而永久冻土的变暖会通过加速分解和生态系统呼吸而导致更多的二氧化碳释放到大气中。然而，永久冻土解冻也可以促进植物生长，通过光合作用从大气中吸收二氧化碳，并可能在短期内导致对气候变化的负反馈。因此，土壤微生物和植物对永久冻土解冻的相对响应决定了，总的来说，永久冻土生态系统是向大气中添加二氧化碳（微生物响应和正反馈），还是从大气中吸收二氧化碳（植物响应和负反馈）。本研究的主要目的是量化导致二氧化碳向大气释放的四种生态系统呼吸成分（地上植物结构、地下植物结构、表层土壤微生物和深层土壤微生物）对永久冻土融化的响应，以确定是植物响应还是微生物响应主导了阿拉斯加冻土带碳循环的变化。两种碳同位素13C和14C的自然丰度将用于确定呼吸二氧化碳的来源。通过测量CO2源的同位素值和生态系统呼吸，可以计算出土壤微生物和植物对碳通量的相对贡献。在冻土融化自然梯度、冻土带变暖实验、两种不同植被数量和两种不同土壤气候条件下，对冻土带生态系统进行季节性呼吸分配，探讨冻土融化对冻土带生态系统碳平衡的影响。在整个研究过程中，将对本科生进行培训，对八年级学生进行教育，并创建一个永久冻土中C循环的外行人指南。最后，本研究的结果将对气候模式具有重要意义；这些模型目前没有考虑生物对气候变暖的物理反应。

Permafrost soils, which are soils frozen for two or more consecutive years, store vast quantities of carbon (C). These soils are reservoirs of C because thousands of years of cold temperatures have prevented the microbial decomposition of soil organic carbon (SOC), and thus the subsequent release of carbon dioxide (CO2) from soils to the atmosphere. However, permafrost soils are very vulnerable to warming; as the climate warms, permafrost soils begin to thaw, which allows microbes to access and decompose stored SOC , thereby releasing CO2. Permafrost thaw, therefore, has the potential to become a positive feedback to climate change: CO2 in the atmosphere can increase warming, and warming of permafrost soils can result in more CO2 release to the atmosphere via increased rates of decomposition and ecosystem respiration. However, permafrost thaw can also increase plant growth, which takes CO2 out of the atmosphere via photosynthesis, and may, in the short term, result in a negative feedback to climate change. Thus the relative responses of both soil microbes and plants to permafrost thaw determine whether, on balance, permafrost ecosystems add CO2 to the atmosphere (microbial response and positive feedback) or whether the ecosystem takes CO2 from the atmosphere (plant response and negative feedback.) The main objective of this research is to quantify the response of four components of ecosystem respiration, all of which result in release of CO2 to the atmosphere (aboveground plant structures, belowground plant structures, surface soil microbes, and deep soil microbes) to permafrost thaw in order to determine whether plant or microbial responses are dominating the Alaska tundra's changing carbon cycle. Natural abundances of two carbon isotopes, 13C and 14C, will be used to identify the sources of respired CO2. By measuring the isotopic values of CO2 sources as well as ecosystem respiration, the relative contribution of soil microbes and plants to carbon fluxes can be calculated. This ecosystem respiration partitioning will be performed seasonally in a natural gradient of permafrost thaw, in a tundra warming experiment, and in an area with two different amounts of vegetation and two different soil climates to explore the effects of permafrost thaw on the carbon balance of tundra ecosystems.Throughout the course of this research, undergraduate students will be trained, 8th grade students will be educated, and a layperson's guide to C cycling in permafrost will be created. Finally, the results of this study will be important to climate models; these models do not currently consider biological responses to the physics of climate warming.