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

温带草原的固有水分利用效率(IWUE)在上个世纪有了很大的提高，但这种气候变化的响应是由于水汽净同化速率的增加还是气孔导度的降低尚不清楚。这种不确定性阻碍了我们对气候变化对温带草原碳循环和水循环的影响的理解。植物纤维素的氧同位素组成(纤维素中的18O)携带着来自气孔导度的信息，但气孔导度与纤维素中的18O之间的机理关系复杂，并且受到蒸汽压差(VPD)和形态生理植被参数的强烈影响。本项目试图通过实验、用同位素启动的土壤-植被-大气模型(MUSICA)模拟以及比较模拟的和详细的经验数据集来解开这种关系。本试验以温带草原最重要的饲草黑麦草为研究对象，研究了CO2浓度(200、400和800微摩尔-1)和VPD(0.47和1.17 kPa)对气孔导度、纤维素、叶片和林分水分利用效率中18O的直接和交互作用，以及将18O信号(叶片水和叶生长带水分中的18O富集物，以及源叶和叶生长带中的蔗糖)与气孔导度联系起来的关键参数。模拟模型将预测气孔导度和纤维素中的18O，并跟踪从其来源(降水和大气湿度)通过土壤、木质部和叶水到光合作用产物和纤维素的18O信号。模型预测得到了以下详细的数据集的检验：(I)Grünschwaige(弗里辛/德国)温带草原生态系统土壤、茎和叶水的同位素组成(18O，2H)、纤维素的18O和iWUE的日、季和多年变化的详细数据集，(Ii)受控环境实验的结果(见上文)，以及(Iii)在英国Rothamsted/Rothamsted的Park Grass实验中不同范围的草原生态系统的18O的纤维素和iWUE的世纪尺度时间序列。此外，该项目还将利用来自明尼苏达理工学院长期草原管理实验的物种特有的18O纤维素数据，探索不同的共生植物物种、功能群组成(草/豆类/双子叶)和管理(刈割频率和肥料供应)对群落水平纤维素18O的影响。预计这项工作将显著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个世纪温带草原的碳通量和水通量的理解。

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.