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

陆地生态系统吸收了人类活动释放的近三分之一的二氧化碳。作为限制大气中二氧化碳浓度增加的唯一最重要的因素，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)。然而，为了产生更多的生物量，植物也需要养分。氮(N)和磷(P)是最常限制植物生长的两种营养物质。众所周知，氮的低可利用性会降低生态系统在二氧化碳升高的情况下吸收碳的能力，可能会使吸收量减少一半。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的可用性限制生产力的生态系统将如何应对二氧化碳的升高。重要的是，限磷生态系统几乎和限氮生态系统一样普遍，人类活动造成的氮沉降正在将一些以前的限氮生态系统转变为限磷生态系统。因此，我们目前缺乏了解，这意味着我们无法预测生物圈的多大面积将对二氧化碳浓度升高做出反应。在最近的IPCC报告中，这种不确定性被强调为我们理解中的一个关键差距，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：对比峰区国家公园中限制磷的酸性草地和石灰岩草地，在那里，氮和磷的输入已经被控制了20年。重要的是，长期的养分添加产生了不同程度的磷限制草地；磷的添加减轻了磷的限制，氮的添加加重了磷的限制。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磷的生态系统，因此，植物可能必须使用不同的策略来获取额外的磷，以便在二氧化碳升高的情况下提高生长速度。在全球C循环的背景下，研究草原也至关重要，因为它们占陆地初级生产力的20%。在英国，半天然草地的面积是落叶林的两倍，是泥炭地之后最重要的土壤C储存生态系统。此外，草原的实验可耕性及其包含的植物获取磷策略的多样性使草原成为理想的实验操作模型系统。我们将从长期N和P添加地块收集完整的植物-土壤单体，并将其暴露在附近的设施中进行CO2富集，其他环境条件几乎相同，并保持养分操作。这将使我们能够直接确定在二氧化碳浓度升高的世界中，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、植物地上和地下碳分配，并最终影响土壤和生态系统总碳储量的变化。额外提供的二氧化碳的同位素特征将用于确定如何控制土壤C储存的变化，区分新有机质的形成和现有物质的损失。最后，我们将利用更多的微观研究来了解不同植物物种如何在二氧化碳浓度升高的情况下缓解磷限制，以及这如何影响碳动态。总之，我们的工作将首次直接评估在二氧化碳浓度升高的世界中，磷限制对生态系统碳吸收速率的影响，并通过这样做，大大提高我们对一个问题的理解，这个问题在很大程度上导致了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.