ISOFLUXES: Direkt eddy covariance measurements of the stable isotope composition of CO2 and H2O fluxes between forest and atmosphere

稳定的同位素13C和18O可以用来约束局部到全球水平上的碳和水通量，为挑战生物地球化学模型提供独立的数据，并向我们提供关于汇和源以及潜在的生理过程的信息。然而，由于缺乏长期和连续的数据集，对陆地生态系统与大气之间的碳和水通量的稳定同位素数据的解释受到限制。由于技术挑战，到目前为止还没有在年度时间尺度上同时测量二氧化碳和水中的13C和18O通量。在这里，我们建议克服这一知识鸿沟，并利用涡旋协方差技术和最先进的激光光谱分析技术，在两年多的生态系统尺度上研究二氧化碳和水蒸气中的同位素通量的变异性。我们将使用用于二氧化碳同位素的连续波量子级联激光光谱仪和用于H2O蒸气同位素的离轴集成腔输出光谱仪进行高频在线测量(最高可达10赫兹)。测量将在我们位于德国中部一片30米高的山毛榉森林上方的一个流量塔上进行。结合气象测量，我们将确定同位素通量可变性的环境驱动因素。我们将进一步使用水同位素通量测量来量化生态系统尺度上的叶水浓缩，这将与直接的叶水同位素测量进行验证。对于叶片、木质部、土壤水分和降水，将通过三个短期的密集活动和季节性的双周方法进行采样。采用Péclet修正的Craig-Gordon叶水富集模型，将叶水的同位素组成与生态系统尺度水汽通量的同位素组成联系起来。此外，这些数据将被用来开发和约束生态系统规模的水同位素通量的多层数值模型。最后，我们想要表征18O在CO2和H2O中的交换，旨在提高我们对碳酸酐酶(CA)在生态系统尺度上的作用的理解。钙是光合作用过程中的重要酶，促进二氧化碳和水之间的~(18)O交换。测量生态系统尺度上18O在CO2和H2O中的通量将为我们提供独特的机会来间接估计整个生长季生态系统中整合的CA活性。总体而言，该项目将提供全球独一无二的数据，这些数据将提高我们对生态系统规模上稳定同位素相关过程的理解，并为生态系统碳和水模型提供独立的约束。

The stabile isotopes 13C and 18O can be used to constrain carbon and water fluxes at local to global levels, to provide independent data to challenge biogeochemical models, and to inform us about sink and sources as well as underlying physiological processes. The interpretation of stable isotope data in carbon and water fluxes between terrestrial ecosystem and the atmosphere is, however, limited by the missing availability of long-term and continuous datasets. Due to technical challenges, simultaneous measurements of 13C and 18O in CO2 and H2O fluxes have not been carried out so far on annual time scale. Here we propose to overcome this knowledge gap and to investigate the variability of isotopologue fluxes in CO2 and water vapor at ecosystem scale over two years using the eddy covariance technique and state-of-the-art laser spectrometry. We will employ a continuous wave quantum cascade laser spectrometer for CO2 isotopologues and an off-axis integrated cavity output spectrometer for H2O vapor isotopologues for high frequency online measurements (up to 10 Hz). Measurements will be carried out at one of our flux towers above a managed 30 m tall beech forest in Central Germany. Along with meteorological measurements, we will identify environmental drivers on isotopologue flux variability. We will further use the water isotopologue flux measurements to quantify leaf water enrichment at ecosystem scale, which will be validate against direct leaf water isotope measurements. For that leaf, xylem, soil water, and precipitation will be sampled with three short-term intensive campaigns and a seasonal biweekly approach. The Péclet-modified Craig-Gordon leaf-water enrichment model will be used to link the leaf water isotope composition with the isotope composition of ecosystem scale water vapor fluxes. Additionally, the data will be used to develop and constrain a multi-layer numerical model for water isotope fluxes at ecosystem scale. Finally, we want to characterize the exchange of 18O in CO2 and H2O aiming to improve our understanding of the role of the enzyme carbonic anhydrase (CA) at ecosystem scale. CA is an important enzyme for photosynthetic processes and facilitates the exchange of 18O between CO2 and H2O. Measuring ecosystem scale fluxes of 18O in CO2 and H2O will provide us the unique opportunity to indirectly estimate CA activity integrated over the ecosystem continuously over the entire growing season. Overall, this project will provide worldwide unique data that will improve our understanding of stable isotope related processes at ecosystem scale and that provides independent constrains for ecosystem carbon and water models.