Year-round soil moisture and temperature, Toolik Field Station, Alaska, 2017-2018

Arctic soils contain very large amounts of organic carbon most of which is frozen in permafrost and has not participated in the global carbon cycle for thousands of years. Perturbations to carbon storage in permafrost soils have the potential to significantly increase the amount of carbon in the atmosphere and contribute to global climate change. This data set contains volumetric soil moisture and temperature data near Toolik Field Station in moist acidic tussock tundra - a common tundra system in Northern Alaska. This data was collected in support of a project that aims to understand the sources of carbon dioxide emitted from Arctic tundra year-round. Carbon dioxide is a greenhouse gas produced in soils by the respiration of roots and of microorganisms decomposing soil organic matter, and both processes are sensitive to changes in temperature and water content. Increases in carbon dioxide emissions can be related to increased plant productivity (photosynthesis and storage of carbon in plants and soils) or increased microbial activity (loss of carbon previously stored in soils). However, measuring the radiocarbon content (age) of carbon dioxide emissions can be used to understand how Arctic ecosystems are responding to climate change, because roots and microorganisms respire carbon with distinct isotopic signatures. In this project, our team built and deployed new technology to characterize the sources of carbon emissions from Arctic tundra year-round, with a special focus on winter emissions. Specifically, we developed a sampling system that continuously collects carbon dioxide over a period of 1-4 weeks. The system is passive (no power requirements, ambient pressure and temperature), rugged (suitable for well-aerated, waterlogged, and frozen soils), light-weight (<0.5 kg/sample), and isotopically-clean (i.e. the recovered carbon dioxide is suitable for radiocarbon analysis and the sampler itself does not emit carbon). The samples are shipped to the W. M. Keck Carbon Cycle Accelerator Mass Spectrometer facility at the University of California, Irvine, where they are analyzed for their radiocarbon content. Their isotopic information allows us to elucidate which soil carbon pools are being consumed by microorganism during the winter, and to quantify what proportion of the carbon originates from microorganisms decomposing organic matter (as opposed to from the roots of plants that are fixing carbon from the atmosphere) during the summer.

北极土壤储存有巨量有机碳，其中大部分封存于多年冻土（permafrost）中，数千年来均未参与全球碳循环。多年冻土土壤碳库的扰动，有可能大幅提升大气碳含量，进而加剧全球气候变化。本数据集包含阿拉斯加北部典型苔原生态系统——湿润酸性草丛苔原（moist acidic tussock tundra）内图利克野外站（Toolik Field Station）周边的体积土壤含水量与温度数据。本数据采集自一项旨在全面解析北极苔原全年二氧化碳排放来源的研究项目。二氧化碳是土壤中根系呼吸与微生物分解土壤有机质过程产生的温室气体，两类过程均对温度与水分含量的变化极为敏感。二氧化碳排放量的提升，既可能源于植物生产力提升（光合作用及碳在植物与土壤中的储存），也可能源于微生物活性增强（土壤中此前封存的碳发生流失）。不过，通过测量排放二氧化碳的放射性碳含量（年龄），可解析北极生态系统对气候变化的响应机制，这是因为根系与微生物呼吸释放的碳具有独特的同位素特征。本项目团队研发并部署了全新技术，以全年表征北极苔原的碳排放来源，研究重点尤其聚焦冬季碳排放。具体而言，我们开发了一套可连续1至4周采集二氧化碳样本的采样系统。该系统采用被动式设计（无需外接电源，适配环境压力与温度），坚固耐用（可适用于通气良好、积水及冻土环境），轻量化设计（单样本装置重量低于0.5千克），且同位素本底纯净（即回收的二氧化碳可直接用于放射性碳分析，且采样器自身不会释放碳干扰检测）。样本将被送往加州大学欧文分校的W.M.凯克碳循环加速器质谱（Accelerator Mass Spectrometer）设施，进行放射性碳含量分析。通过上述同位素信息，我们可阐明冬季时哪些土壤碳库被微生物消耗，并量化夏季碳排放中，源自微生物分解有机质的碳占比（相较于从大气固碳的植物根系所释放的碳）。