Phosphorus Limitation And ecosystem responses to Carbon dioxide Enrichment (PLACE)

陆地生态系统吸收了近三分之一的人类活动释放的二氧化碳。作为限制大气二氧化碳浓度增加的唯一最重要因素，这种吸收大大减缓了全球变暖的速度。这种对全球变暖的限制被认为是由于大气中二氧化碳的增加增加了植物的光合作用，最终导致生态系统在植物生物量和土壤有机质中储存了更多的碳(C)。但要产生更多的生物量，植物也需要营养。氮(N)和磷(P)是最常见的限制植物生长的两种营养物质。已经知道，在二氧化碳浓度升高的情况下，氮的低有效性会降低生态系统吸收碳的能力，可能会减少一半的吸收。因此，营养限制对生态系统能在多大程度上限制全球变暖造成了重大限制。然而，我们不知道磷而不是氮限制生产力的生态系统将如何应对二氧化碳增加。重要的是，磷限制的生态系统几乎和氮限制的生态系统一样常见，人类活动持续的氮沉积正在将一些以前的氮限制的生态系统转变为磷限制的生态系统。因此，我们目前缺乏了解意味着我们无法预测生物圈的大片区域将如何应对二氧化碳升高。在政府间气候变化专门委员会的最新报告中，这种不确定性被强调为我们理解上的一个关键差距，使我们的研究变得极其重要和及时。我们将利用一种独特的资源：山顶地区国家公园有限磷的酸性和石灰岩草原的对比，那里的氮和磷投入已经被操纵了20年。最重要的是，长期的养分添加产生了不同程度的磷限制；磷的添加缓解了磷的限制，而氮的添加加剧了磷的限制。这两个草原还允许我们研究土壤中含有不同数量的有机磷和矿物质磷的生态系统，因此，植物可能不得不使用不同的策略来获取它们在高二氧化碳条件下提高生长速度所需的额外磷。在全球碳循环的背景下，研究草原也是至关重要的，因为它们负责20%的陆地初级生产力。在英国，半自然草原的面积是落叶林的两倍，是仅次于泥炭地的最重要的土壤碳储存生态系统。此外，草原的可试验性及其植物获取磷的策略的多样性，使草原成为实验操作的理想模式系统。我们将从长期的N和P添加地块中收集完整的植物-土壤整体，并将它们暴露在附近的设施中，在其他环境条件几乎相同的情况下进行二氧化碳浓缩，并保持养分操作。这将使我们能够直接确定在二氧化碳升高的世界中，磷限制如何影响生态系统吸收额外碳的能力。我们将结合碳通量监测和植物和土壤采样，详细了解磷限制如何影响植物生产力，植物地上和地下的碳分配，以及最终土壤和总生态系统碳储量的变化。供应的额外二氧化碳的同位素特征将被用来确定如何控制土壤碳储量的变化，区分新有机质的形成和现有物质的损失。最后，更多的微观研究将被用来了解不同的植物物种如何缓解二氧化碳升高下的磷限制，以及这如何影响碳的动态。综上所述，我们的工作将首次直接评估在二氧化碳升高的世界中，磷限制对生态系统碳吸收速率的影响，并通过这样做，极大地提高我们对这一问题的理解，这一问题在很大程度上导致了对21世纪气候变化速率预测的不确定性。

Terrestrial ecosystems absorb nearly one-third of the carbon dioxide (CO2) released by man's activities. As the single most important factor limiting the increase in atmospheric CO2 concentrations, this uptake has slowed rates of global warming substantially. This restriction to global warming is thought to be due to rising atmospheric CO2 increasing plant photosynthesis, and ultimately resulting in ecosystems storing more carbon (C) in plant biomass and soil organic matter.However, to produce more biomass, plants also need nutrients. Nitrogen (N) and phosphorus (P) are the two nutrients that most commonly limit plant growth. It is already known that low availability of N can reduce the capacity of ecosystems to absorb C under elevated CO2, perhaps reducing uptake by half. Nutrient limitation therefore places a major restriction on how much ecosystems can limit global warming. However, we do not know how ecosystems in which P, rather than N, availability limits productivity will respond to elevated CO2. Critically, P-limited ecosystems are nearly as common as N-limited ecosystems, and ongoing N deposition from human activity is turning some previously N-limited ecosystems into P-limited ecosystems. Therefore, our current lack of understanding means that we are unable to predict how large areas of the biosphere will respond to elevated CO2. This uncertainty has been highlighted as a key gap in our understanding in the most recent IPCC report, making our study extremely important and timely.We will make use of a unique resource: contrasting P-limited acidic and limestone grasslands in the Peak District National Park, where N and P inputs have been manipulated for 20 years. Critically, the long-term nutrient additions have produced grasslands that differ in their degree of P limitation; P addition has alleviated P limitation while N additions have exacerbated it. The two grasslands also allow us to study ecosystems which contain different amounts of organic versus mineral P in their soils and, thus, plants may have to use contrasting strategies to acquire the additional P they need to increase growth rates under elevated CO2. Studying grasslands is also critical in the context of the global C cycle as they are responsible for 20% of terrestrial primary productivity. In the UK, semi-natural grasslands cover twice the area of deciduous forests and are the most important ecosystems for soil C storage after peatlands. Additionally, the experimental tractability of grasslands and the diversity of plant strategies for accessing P they contain, makes grasslands ideal model systems for experimental manipulation. We will collect intact plant-soil monoliths from the long-term N and P addition plots and expose them to CO2 enrichment at a nearby facility, with otherwise near identical environmental conditions, and maintain the nutrient manipulations. This will allow us to directly determine how P limitation influences ecosystem capacity to absorb extra C in an elevated CO2 world. We will use a combination of C flux monitoring, and plant and soil sampling to develop a detailed understanding of how P limitation affects plant productivity, plant C allocation above and below ground, and ultimately changes in soil and total ecosystem C storage. The isotopic signature of the extra CO2 supplied will be used to determine how changes in soil C storage are controlled, distinguishing between the formation of new organic matter and loss of existing material. Finally, additional microcosm studies will be used to understand how different plant species alleviate P limitation under elevated CO2, and how this impacts C dynamics.In summary, our work will provide the first direct assessment of the impacts of P limitation on the rates of ecosystem C uptake in an elevated CO2 world, and, in so doing, improve greatly our understanding of an issue that contributes substantially to uncertainty in predictions of rates of 21st century climate change.