Collaborative Research: Climate controls over ecosystem respiration: Using isotopes to determine the sources and age of respired carbon

碳通过光合作用这一单一过程进入生态系统，几乎所有的碳都通过呼吸作用返回到大气中，其中约50%-80%发生在地下。土壤呼吸综合了根系代谢和分解生物的活动。虽然影响植物代谢(自养)呼吸和分解速率(异养呼吸)的主要过程是已知的，但预测土壤呼吸在空间和时间上的变化的能力有限--这是当前和未来碳循环中的一个主要不确定性。本文提出的工作将结合新的测量和建模方法来分离自养和异养呼吸，并确定土壤中C的呼吸年龄。在北美生物群落和气候网络的实地站点，这些方法将包括：(1)频繁、自动化地测量土壤呼吸及相关因素；(2)以测量呼吸中的稳定同位素和放射性碳为基础的同位素质量平衡法；(3)培养以确定异养呼吸成分对土壤条件变化的响应；(4)在一些站点，通过排除降雨来操纵土壤水分含量。产生的数据将被用来将土壤呼吸的碳分为自养和异养成分，确定异养呼吸的碳的年龄，确定土壤有机质的成分，并确定这些关系与控制变量(光合作用速率、土壤条件等)的关系。这些结果将被用来对CASA生态系统模型中一个新的自养呼吸成分进行参数化，并测试该模型在季节到年际时间尺度上对异养呼吸来源平衡的预测能力。CASA模型的预测将在跨越(1)气候序列和(2)从热带森林到冻土带的一系列地点的区域梯度上得到进一步检验。我们将使用大气中碳同位素季节变化的记录(全球网络站点的13C和阿拉斯加州巴罗点大气二氧化碳的14C)作为CASA模型确定北半球陆地生态系统与大气之间的季节性和年际交换C的能力的全球测试。分离生态系统呼吸的组成部分是生态系统科学中最重要的研究挑战之一。这项活动将使用新的工具，特别是在测量和模型中创新使用放射性碳示踪剂，以发展对选定美国熔剂站点的碳通量的过程水平的理解。这项工作将促进对陆地生态系统如何影响全球碳循环的基本理解，并将通过展示异养呼吸如何对温度和湿度的变化做出反应，来改进对未来大气二氧化碳浓度的预测。我们通过在W.M.凯克碳循环加速器质谱仪设施开设的放射性碳短期课程教育学生的计划，将培训下一代科学家在放射性碳应用方面研究陆地和海洋碳循环。

ABSTRACTCarbon enters ecosystems through a single process, photosynthesis, and nearly all is returned to the atmosphere through respiration, some 50-80% of which occurs below ground. Soil respiration integrates root metabolism and the activity of decomposer organisms. While the major processes affecting plant metabolic (autotrophic) respiration and decomposition rates (heterotrophic respiration) are known, the ability to predict variations in soil respiration in space and time is limited - a major uncertainty in the current and future carbon cycle. The work proposed here will combine new measurement and modeling approaches for separating autotrophic and heterotrophic respiration, and determining the age of C respired from soils. At field sites in the Ameriflux network that span a range of North American biomes and climates, these methods will include: (1) frequent, automated, measurements of soil respiration and related factors; (2) isotope mass balance methods based on measurements of stable isotopes and radiocarbon in respired CO2; and (3) incubations to determine responses of heterotrophic respiration components to changing soil conditions; and (4) at some sites, manipulation of soil moisture content through rainfall exclusion. The data generated will be used to partition soil respired C into autotrophic and heterotrophic components, to determine the age of heterotrophically respired C and identify the components of soil organic matter contributing to its production, and to determine how these relationships chance with controlling variables(photosynthesis rate, soil conditions, etc). The results will be used to parameterize a newautotrophic respiration component of the CASA ecosystem model, and to test how well the model predicts the balance of sources of heterotrophic respiration on seasonal to interannual timescales. Predictions of the CASA model will be further tested across regional gradients spanning (1) a climosequence and (2) a suite of sites from tropical forest to tundra. We will use atmospheric records of seasonal variation in C isotopes (13C at the global network sites and 14C in atmospheric CO2 at Point Barrow, Alaska) as a global test of the CASA model's ability to determine the seasonal to interannual exchange of C between northern hemisphere terrestrial ecosystems and the atmosphere.Intellectual merit. Separating the components of ecosystem respiration is one of the fundamentally important research challenges in ecosystem science. This activity will use new tools, in particular, innovative use of the radiocarbon tracer in measurements and models, to develop process level understanding of C fluxes at selected Ameriflux sites.Broader impacts. This work will advance fundamental understanding of how terrestrial ecosystems influence the global carbon cycle, and will improve projections of future atmospheric concentrations of carbon dioxide by showing how heterotrophic respiration will respond to changes in temperature and moisture. Our program to educate students through a short course in radiocarbon at the W.M. Keck Carbon Cycle Accelerator Mass Spectrometry facility will train the next generation of scientists in the applications of radiocarbon to study land and ocean C cycling.