Below Ground Control of Ecosystem Carbon Sequestration under Elevated CO2

随着大气中二氧化碳浓度的上升，植物可以进行更多的光合作用，并从大气中吸收更多的二氧化碳。这些碳（C）可以储存在植物和土壤中，减少大气中的二氧化碳和全球变暖。然而，我们预测全球碳吸收量增加的能力受到严重限制，因为关于CO2升高（eCO 2）对营养磷（P）限制植物生长的生态系统（P限制生态系统）影响的研究很少-但世界上超过40%的生态系统是P限制的。此外，这是地下过程，包括植物，土壤微生物和土壤化学之间的相互作用，将控制这些生态系统储存更多的C的能力，但我们很少了解这些过程如何响应eCO 2在磷有限的生态系统。了解地下响应是至关重要的，因为土壤是陆地生态系统中最大的C储存，因此我们必须了解CO2浓度升高如何改变土壤和植物中的C储存。此外，土壤C输入影响P的循环，因此可以决定植物是否可以获得增加生长和C增益所需的额外P，而C在土壤中的命运将决定它是否有助于长期C库存或释放。该项目将使用我们独特的实验来解决这些知识需求，该实验使用自由空气CO2富集技术（FACE）在自然室外环境中将两种对比鲜明的草原生态系统（石灰岩和酸性草原）暴露于升高的CO2。草原都是P-有限的，但关键的是，植物响应eCO 2在直接相反的方式与石灰石草原增加生物量，和酸性草原减少生物量。值得注意的是，草原具有与发育年龄相关的对比土壤，这使得它们非常适合获得可以广泛应用于P限制土壤的机械见解，并可以解释相反的反应。CO2富集与P和N营养操纵相结合，使我们能够调查P限制的重要性和全球重要的大气氮沉降的影响，也可以影响P循环。有一个强有力的理论基础假设，这是在土壤生物化学的差异，解释了对比植物生物量响应eCO 2：在发育年轻的土壤（如石灰石土壤），增加碳输入可能会增强风化的主要磷矿物，如磷酸钙，并刺激有机磷的酶矿化，导致更大的植物生长和碳螯合磷的可用性。相比之下，在土壤形成的后期阶段（如我们的酸性土壤），磷被锁定在次生矿物中，土壤微生物和植物难以获得。虽然升高的CO2可以为酶提供C以增加有机P的矿化，但这主要是由土壤微生物完成的，这将降低酸性土壤中的功效，导致植物和微生物之间对P的强烈竞争，限制了升高的CO2对植物生长的益处。关键的是，这些理论还没有得到验证。我们将使用稳定同位素和放射性同位素C示踪剂的组合，追踪C的短期和长期命运及其通过植物，微生物和土壤的途径。我们将确定新的植物C输入的命运如何控制P的动态，通过使用P同位素示踪剂，以确定CO2如何影响P的可用性，植物吸收，以及与土壤微生物和土壤基质对P的竞争。最后，我们将使用先进的分子方法来了解土壤微生物如何对C输入做出反应，以及这如何影响它们的P循环及其对植物的可用性。这项工作的目的是确定机械的原因，在两个草原的对比植物生产力的反应，并在这样做，发展所需的广泛的理解，以预测未来的碳吸收率在P-有限的生态系统。

As atmospheric CO2 concentrations rise, plants can photosynthesise more and take more CO2 out of the atmosphere. This carbon (C) can then be stored in plants and soils, reducing atmospheric CO2 and global warming. However, our ability to predict global increases in C uptake are severely constrained because there are so few studies on the effects of elevated CO2 (eCO2) on ecosystems where the availability of the nutrient phosphorus (P) restricts plant growth (P-limited ecosystems) - yet more than 40% of the world's ecosystems are P-limited. Furthermore, it is below ground processes, including interactions between plants, soil microbes and soil chemistry, that will control the ability of these ecosystem to store more C, yet we have very little understanding of how these processes respond to eCO2 in P-limited ecosystems. Understanding below ground responses is essential because soils are the largest store of C in terrestrial ecosystems so we must understand how elevated CO2 changes stores of C in soils as well as in plants. Furthermore, soil C inputs influence the cycling of P and so can determine whether plants can gain the extra P needed for increased growth and C gain, while the fate of C in soil will determine whether it contributes to long-term C stocks or is released. This project will address these knowledge needs using our unique experiment that exposes two contrasting grassland ecosystems (a limestone and an acidic grassland) to elevated CO2 in a natural outdoor setting using Free Air CO2 Enrichment technology (FACE). The grasslands are both P-limited but, critically, plants have responded to eCO2 in directly opposing ways with the limestone grassland increasing biomass, and the acidic grassland reducing biomass. Significantly, the grasslands have contrasting soils linked to developmental age that makes them well suited to gaining mechanistic insights that can be applied broadly across P-limited soils, and may explain the opposing responses. The CO2 enrichment is in combination with P and N nutrient manipulations that allow us to investigate the importance of P-limitation and the influence of globally important atmospheric N deposition that can also influence P cycling. There is a strong theoretical basis for hypothesising that it is the differences in soil biogeochemistry that explains the contrasting plant biomass responses to eCO2: in developmentally young soils (such as our limestone soil), increased C inputs may enhance weathering of primary P-minerals like calcium phosphates and stimulate enzymatic mineralization of organic P, leading to greater plant P availability for growth and C sequestration. In contrast, in soils at later stages of pedogenesis (such as our acidic soil), P is locked up in secondary minerals that are difficult for soil microbes and plants to access. While elevated CO2 may provide the C for enzymes to increase mineralisation of organic P, this is predominantly done by soil microbes which will have reduced efficacy in acidic soil, leading to strong competition for P between plants and microbes, limiting the plant growth benefit of elevated CO2. Critically, these theories remain untested.Using a combination of stable- and radioisotope C tracers, we will trace the short and long-term fate of C and its pathway through plants, microbes and soil. We will determine how the fate of new plant C inputs controls P dynamics by using P isotope tracers to determine how CO2 influences P availability, plant uptake, and competition for P with soil microbes and the soil matrix. Finally, we will use advanced molecular approaches to understand how soil microbes respond to the C inputs and how this influences their cycling of P and its availability to plants. This work aims to determine mechanistically the reasons for the contrasting plant productivity responses in the two grasslands and, in so doing, develop the broad understanding needed to predict future rates of C uptake in P-limited ecosystems.