The significance of cellulose-18O for understanding water-use efficiency of grassland: Evidence from experimental, observational and process-based modeling studies

近一个世纪以来，温带草地的内在水分利用效率（iWUE）显著提高，但目前尚不清楚这种气候变化响应是由于净同化率的增加还是气孔导度的降低。这种不确定性阻碍了我们对气候变化对温带草原碳和水循环影响的理解。植物纤维素的氧同位素组成（纤维素中的18O）携带着气孔导度的信息，但气孔导度与纤维素中的18O之间的机制关系是复杂的，并且受到蒸汽压亏缺（VPD）和植被形态生理参数的强烈影响。本项目试图通过实验、利用同位素土壤-植被-大气模型（MuSICA）进行模拟以及比较模拟数据集和详细的经验数据集来揭示这种关系。本实验将评估CO2浓度（200、400和800 micromol mol-1）和VPD（0.47和1.17 kPa）对控制环境下黑麦草气孔导度、纤维素18O、叶片和林分尺度水分利用效率的直接和交互影响，以及连接18O信号（叶片水分和叶生长期水分中18O的丰富程度、源叶片和叶生长期中蔗糖含量）和气孔导度的关键参数。温带草原最重要的牧草。模拟模型将预测纤维素的气孔导度和18O，并跟踪18O信号从其来源（降水水和大气湿度）通过土壤、木质部和叶片水到光合产物和纤维素。模型预测使用以下数据进行测试：(i)土壤、茎叶水分、纤维素中18O和iWUE的同位素组成（18O, 2H）的日、季节和多年变化的详细数据集：gr<s:1> nschwaige （Freising/德国）温带草地生态系统的同位素组成（18O, 2H）； （ii）受控环境实验的结果（见上图）；（iii）英国洛桑研究所（Rothamsted/UK）公园草地实验中多种草地生态系统的纤维素中18O和iWUE的世纪尺度时间序列。此外，该项目还将利用Technische Universität m<e:1> nchen Veitshof长期草地管理实验的物种特异性18O纤维素数据，探索不同共优势植物物种、功能群组成（禾本科/豆科/双科）和管理（刈割频率和肥料供应）对纤维素中18O的影响。预计这项工作将大大提高我们对环境和植被因素如何决定纤维素中的18O以及气候变化如何影响上世纪温带草原的碳和水通量的理解。

Intrinsic water use efficiency (iWUE) of temperate grassland has increased strongly in the last century, but it is unknown if this climate-change response originated from increased net assimilation rate or decreased stomatal conductance for water vapor. Such uncertainty hampers our understanding of climate-change effects on carbon and water cycles in temperate grasslands. It has been proposed that the oxygen isotope composition of plant cellulose (18O in cellulose) carries information from stomatal conductance, but the mechanistic relationship between stomatal conductance and 18O in cellulose is complex, and strongly influenced by vapor pressure deficit (VPD) and morpho-physiological vegetation parameters. The present project seeks to unravel that relationship by a combination of experimentation, simulation with an isotope-enabled soil-vegetation-atmosphere model (MuSICA), and comparison of modeled and detailed empirical data sets. The experiment will assess the direct and interactive effects of CO2 concentration (200, 400 and 800 micromol mol-1) and VPD (0.47 and 1.17 kPa) on stomatal conductance, 18O in cellulose, leaf- and stand-scale WUE and the key parameters that link the 18O signal (18O enrichment in leaf water and leaf growth zone water, and sucrose in source leaves and the leaf growth zone) and stomatal conductance in controlled environment studies with Lolium perenne, the most important forage grass of temperate grassland. The simulation modeling will predict stomatal conductance and 18O in cellulose, and track the 18O-signal from its sources (precipitation waters and atmospheric humidity) through soil, xylem and leaf water to photosynthetic products and cellulose. Model predictions are tested with (i) detailed data sets of diurnal, seasonal and multi-annual variations of the isotopic composition (18O, 2H) of soil, stem and leaf water, 18O in cellulose and iWUE of a temperate grassland ecosystem at Grünschwaige (Freising/Germany), (ii) the results of the controlled environment experiment (see above), and (iii) century-scale time series of 18O in cellulose and iWUE of a diverse range of grassland ecosystems at the Park Grass Experiment in Rothamsted/UK. In addition, the project will also explore effects of different co-dominant plant species, functional group composition (grasses/legumes/dicots) and management (cutting frequency and fertilizer supply) on community-level 18O in cellulose using species-specific 18O cellulose data from the Veitshof long-term grassland management experiment at Technische Universität München. It is expected that the work will significantly improve our understanding of how environmental and vegetation factors determine 18O in cellulose and how climate change has affected last-century carbon and water fluxes in temperate grassland.