Continental drying and carbon sequestration along a subambient to elevated CO2 gradient

自末次盛冰期以来，大气中的二氧化碳浓度急剧增加。正在进行的研究的一个主要重点是陆地生态系统在固碳方面的当前和未来作用。在草原，这是北美最大的潜在植被类型，水的可用性与增加的CO2的相互作用可以作为重要的直接影响的CO2在塑造初级生产，分解，和C存储。拟议的研究和建模综合是杜克之间的合作（杰克逊，Maherali），科罗拉多州（帕顿），和堪萨斯州（欧文斯比）大学和USDA-ARS（波利，约翰逊）预测大气二氧化碳浓度增加和气候变化的后果（夏季干旱增加）为美国南部大平原地区的田间试验在一个完整的C3/C4草原在得克萨斯州中部是独一无二的，在提供一个连续的梯度钙从200至550 umolmol-1，允许检查的临界阈值和非线性响应过去，现在和未来的大气CO2。嵌套在CO2梯度是一个复制的因子水操纵环境和60%的环境降水，后者与夏季干旱预测美国中部的大气环流模型（GCM）在未来世纪和类似的扩展历史干旱从20世纪50年代在该地区。本提案中所述的实验将允许严格核算CO2对碳固存和土壤水分平衡的影响，这一因素在解释CO2富集对草原的综合影响方面是从以前的研究中出现的一个关键因素。H-1生态系统对大气CO2的响应是非线性的，在未来的CO2浓度下，碳固存比最近的过去观察到的要少。2虽然在美国中部GCM预测的夏季干旱下，随着CO2的增加，植物产量的刺激将更大，碳储存最终将取决于土壤水的有效性和分解的平衡。H-3水的有效性通过与土壤氮的有效性的相互作用在草原碳与CO2的固存中起着关键作用。DAYCENT生态系统模型将用于评估大气CO2增加和气候变化对南部大平原草原的潜在长期影响。大量的数据已经可以用来描述在得克萨斯州，科罗拉多，和堪萨斯的长期CO2实验中，大气CO2增加对草原生态系统在南部大平原的潜在影响。将评估不同环境条件和气候变化对地上和地下植物生产、土壤C和N水平以及生态系统净生产的影响。因此，拟议的研究和建模集成直接解决了生态系统在未来世纪继续作为碳汇的能力。拟议的项目与美国碳循环科学计划（CCSP）概述的五个目标中的三个相关：1）通过结合操纵实验和模型开发，改进对未来大气中二氧化碳浓度的预测，2）准确估计历史和当前土地利用模式和趋势对不断变化的C预算的影响，（3）准确估算北方陆地碳汇的潜力及其调节机制，为全球变化与陆地生态系统（GCTE）提供参考国际地圈生物圈计划（IGBP）水文循环的生物圈方面（BAHC）核心项目。

The concentration of CO2 in the atmosphere has increased dramatically since the last glacial maximum. A major focus of ongoing research is the current and future role of terrestrial ecosystems in sequestering CO2. In grasslands, which are North America 's largest potential vegetation type, the interactions of water availability with increased CO2 can be as important as the direct effects of CO2 in shaping primary production, decomposition, and C storage. The proposed research and modeling synthesis is a collaboration among Duke (Jackson, Maherali), Colorado State (Parton), and Kansas State (Owensby) Universities and the USDA-ARS (Polley, Johnson) to predict the consequences of increased atmospheric CO2 concentrations and climate change (increased summer drought) for the Southern Great Plains region of the U.S.The field experiment in an intact C3/C4 grassland in central Texas is unique in providing a continuous gradient of Ca from 200 to 550 umol mol -1, allowing the examination of critical threshold and nonlinear responses to past, present, and future atmospheric CO2. Nested within the CO2 gradient is a replicated factorial water manipulation of ambient and 60% ambient precipitation, the latter consistent with the summer droughts predicted for the central U.S. by general circulation models (GCMs) in the coming century and similar to an extended historical drought from the 1950s in the region. The experiments described in this proposal will permit rigorous accounting of CO2 effects on carbon sequestration and the soil water balance, a factor that has emerged from previous studies as critical in explaining the integrated effects of CO2 enrichment on grasslands.Key hypotheses to be addressed include:H-1 Ecosystems respond nonlinearly to atmospheric CO2, with less carbon sequestration at future CO2 concentrations than observed in the recent past.H-2 Although the stimulation of plant production with increasing CO2 will be greater under the summer droughts predicted by GCMs for the central U.S., carbon storage will ultimately depend on the balance of soil water availability and decomposition.H-3 Water availability plays a key role in grassland carbon sequestration with CO2 through interactions with soil N availability.The DAYCENT ecosystem model will be used to evaluate the potential long-term impact of increasing atmospheric CO2 and climate change for grasslands in the Southern Great Plains. A substantial amount of data is already available to describe the potential impact of increasing atmospheric CO2 on grassland ecosystems in the Southern Great Plains from long-term CO2 experiments in Texas, Colorado, and Kansas. The impact of different environmental conditions and climate change will be evaluated for above-and belowground plant production, soil C and N levels, and net ecosystem production. Therefore, the proposed research and modeling integration directly addresses the ability of ecosystems to continue as carbon sinks in the coming century.The proposed project is relevant to three of the five goals outlined in the U.S. Carbon Cycle Science Plan (CCSP): 1) Improve projections of future atmospheric concentrations of CO2 through a combination of manipulative experiments and model development, 2) Establish accurate estimates of the impact of historical and current land use patterns and trends on the evolving C budget, and 3) Establish accurate estimates of the potential Northern Hemisphere terrestrial C sink and the underlying mechanisms that regulate it. The proposed research also contributes to the Global Change and Terrestrial Ecosystems (GCTE) and Biospheric Aspects of the Hydrological Cycle (BAHC) core projects of the International Geosphere Biosphere Programme (IGBP).