Compensatory Adjustments in Plant Nutrient Uptake: Varying Responses to Rising Atmospheric CO2

氮(N)和磷(P)是自然和农业生态系统中最常限制植物生产的营养物质。这些营养物质的有限供应也是影响植物和生态系统对大气二氧化碳水平上升的长期反应的一个主要因素，即，除非营养物质的供应和/或获取能力随着碳(C)的增加而增加，否则通常观察到的植物生物量的短期增加可能无法长期持续。因此，了解二氧化碳浓度升高是否能引发代偿性调节，从而规避植物生长的营养限制，是至关重要的。补偿性调节，如(a)增加菌根发育，(b)增加根氮和磷吸收能力，(c)根生长可以使植物满足高CO2条件下增加的养分需求，但这些调节对CO2富集的响应程度尚不清楚。在解决养分有效性如何与植物和生态系统对高二氧化碳的反应相互作用时，必须考虑几个问题：植物对氮和磷的需求是否会随着二氧化碳的富集而增加？（需求作为一个概念需要在时间和空间尺度上仔细定义，这将在这里讨论。）如果养分需求在二氧化碳浓度升高的情况下确实增加，植物会做出哪些补偿性调整来克服这种更大的需求？氮磷吸收是否与测量的相对生长率（RGR）和光合氮磷利用率的变化相适应？生长营养状况是否影响对CO2的补偿调节的幅度？这些补偿机制是否与二氧化碳引起的发育阶段的变化相适应？不同官能团的植物，如C3 vs. C4，木本vs.草本vs.禾草，丛枝菌根vs.外生菌根，对CO2升高的营养吸收反应是否不同？在二氧化碳浓度升高的情况下，植物扩大养分吸收的能力是否增强了竞争能力？提出的研究将解决这些问题，重点是根和菌根氮和磷的吸收动力学。PI将使用现场和实验室方法在最现实的条件下测试我们的假设。实地工作将在六个目前资助的国家FACE站点进行，提供不同生态系统和功能组的全面交叉比较。在每个场址，将测量成熟植物完整根系在菌根存在或不存在两种CO2处理的析因组合上的氮和磷吸收能力。野外实验将提供一个独特的机会来比较不同生态系统、生命形式和植物功能群在竞争环境中对二氧化碳水平升高的这些补偿性调整的反应。温室实验室实验使用与田间相似的物种，将研究与吸收动力学调整、根与菌根吸收动力学的相对变化以及植物生理需求有关的更详细的问题。这些数据将被整合到一个综合功能平衡模型中，该模型将包括生理和生物量分配参数。

BassiriRad 9728785 Nitrogen (N) and phosphorus (P) are nutrients that most often limit plant production in natural and agricultural ecosystems. Limited availability of these nutrients is also a major factor influencing long-term plant and ecosystem responses to rising atmospheric CO2 levels i.e., the commonly observed short-term increase in plant biomass may not be sustained over the long-term unless availability and/or acquisition capacity for nutrients increases in concert with carbon (C) gain. Therefore, it is critical to obtain a mechanistic understanding of whether elevated CO2 can elicit compensatory adjustments that would circumvent nutrient limitation of plant growth. Compensatory adjustment such as (a) increased mycorrhizal development, (b) increased root N and P absorption capacity, and (c) root growth can enable plants to meet increased nutrient demand under high CO2 yet the extent of these adjustments in response to CO2 enrichment is unknown. In addressing how nutrient availability could interact with plants and ecosystem responses to high CO2 several questions must be considered: Will plant demand for N and P increase in response to CO2 enrichment ? (Demand as a concept requires careful definition on temporal and spatial scales which will be addressed here.) If nutrient demand does increase under elevated C02, what compensatory adjustment(s) will plants make to overcome this greater demand? Will N and P uptake scale properly with measured changes in relative growth rate (RGR) and photosynthetic N and P utility? Does growth nutrient status affect the magnitude of the compensatory adjustments in response to CO2? Do these compensatory mechanisms scale with CO2-induced changes in stages of development? Do plants of various functional groups e.g., C3 vs. C4, woody vs. herbaceous vs. grasses, or arbuscular mycorrhizae vs. ectomycorrhizae differ in their nutrient uptake responses to elevated CO2? Does a better plant capacity to scale up nutrient uptake at elevated CO2 confer enhanced competitive ability? The proposed research will address these questions with an emphasis on root and mycorrhizal N and P uptake kinetics. The PI will use both field and laboratory approaches to test our hypotheses under the most realistic conditions possible. The field work will be conducted at six currently funded national FACE sites providing a comprehensive cross comparison of different ecosystems and functional groups. At each field site, N and P uptake capacity will be measured on intact roots of mature plants subjected to factorial combination of two CO2 treatments in the presence or absence of mycorrhizas. The field experiments will provide a unique opportunity to compare the response of these compensatory adjustments to elevated CO2 levels among different ecosystems, life forms, and functional groups of plants in a competitive setting. Greenhouse-laboratory experiments using species similar to those from the field will examine the more detailed questions related to adjustments in uptake kinetics, relative changes in root vs. mycorrhizal uptake kinetics, and plant physiological demand. The data will be integrated into a comprehensive functional balance model which will incorporate physiological and biomass allocation parameters.