The interactive effects of rising atmospheric CO2 and nutrient supply on carbon and water relations of grassland ecosystems

预测气候变化对草地的影响需要了解生态系统对大气CO2上升的响应和反馈响应。通过对草地植物稳定碳氧同位素组成（δ13C和δ18O）的回顾性研究，探讨了植物碳水关系对近期CO2增加的响应。我们将研究养分有效性如何影响大气CO2升高下草地植物碳水交换的调节，以及这种调节是由光合作用(A)还是气孔导度（gs）的变化引起的。氮限制限制了同化对CO2升高的响应，限制了A的增加，C3植物在CO2升高的条件下增加了A，降低了gs。这影响了水的利用效率。因此，我们将研究近一个世纪草原植物的内在和瞬时水分利用效率的变化趋势。瞬时水分利用效率解释了大气蒸发需求的变化，从而估算了实际叶面水分利用效率。相反，内在水分利用效率是衡量在标准蒸发需求下每单位水分损失的碳增益。我们将分析在英国洛桑的公园草实验中，不同施氮量、磷量和钾量的肥料处理中存档的干草样品的δ13C和δ18O。这个实验可以研究1915年至今的趋势。为了验证植物有机物δ18O与蒸腾调节之间的关系，我们将分析最近在gr<s:1> nschwaige草地研究站进行的野外试验数据。在为期4年的研究中，我们对影响有机物δ18O的植物生物量和水体进行了取样。这项工作有助于更好地了解草地对二氧化碳上升的反应，并将改进植被-气候模型。

Prediction of the effects of climate change on grassland requires an understanding of ecosystem responses to rising atmospheric CO2 and feedback responses. This retrospective study of stable carbon and oxygen isotope composition (δ13C and δ18O) in grassland plants investigates the responses of plant carbon and water relations to the recent history of CO2 increase. We will study how nutrient availability influences the regulation of carbon and water exchange of grassland plants under rising atmospheric CO2 and whether this regulation results from changes in photosynthesis (A) or stomatal conductance (gs). Nitrogen limitation constrains the response of assimilation to rising CO2 and could limit the increase of A. C3 plants were shown to increase A and decrease gs under elevated CO2. This influences water use efficiency (WUE). Therefore, we will study the trends in intrinsic and instantaneous WUE of grassland plants during the last century. Instantaneous WUE accounts for changes in atmospheric evaporative demand and thus estimates actual leaf-level WUE. Conversely, intrinsic WUE is a measure of carbon gain per unit water lost under standard evaporative demand. We will analyse δ13C and δ18O of archived hay samples from fertilizer treatments at the Park Grass Experiment in Rothamsted, England, that differ in the amount of applied nitrogen, phosphorus and potassium. This experiment allows study of trends from 1915 to today. To verify the relationship between δ18O of plant organics and regulation of transpiration, we will analyse data from a recently conducted field experiment at Grünschwaige Grassland Research Station. In this 4-year study we sampled plant biomass and water pools influencing δ18O of organics. This work contributes to a better understanding of grassland responses to rising CO2 and will improve vegetation-climate models.