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更多的碳（C）。然而，为了产生更多的生物量，植物也需要营养。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，氮的低可用性会降低生态系统在CO2浓度升高时吸收C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限制全球变暖的程度产生了重大限制。然而，我们不知道生态系统中的P，而不是N，可用性限制生产力将如何应对CO2升高。关键的是，磷有限的生态系统几乎和氮有限的生态系统一样普遍，人类活动造成的持续的氮沉降正在使一些以前的氮有限的生态系统变成磷有限的生态系统。因此，我们目前缺乏了解，这意味着我们无法预测生物圈的大面积将如何应对二氧化碳浓度升高。这种不确定性已被强调为一个关键的差距，在我们的理解在最近的IPCC报告，使我们的研究非常重要和及时。我们将利用一个独特的资源：对比P有限的酸性和石灰岩草原在山顶区国家公园，在那里N和P输入已被操纵了20年。重要的是，长期的养分添加产生了不同程度的P限制的草地;磷的加入减轻了磷的限制，而氮的加入加剧了它。这两个草原也使我们能够研究生态系统，其中含有不同数量的有机磷与矿物磷在其土壤中，因此，植物可能不得不使用相反的策略来获得额外的P，它们需要在升高的CO2下增加生长速率。研究草原在全球碳循环的背景下也是至关重要的，因为它们负责20%的陆地初级生产力。在英国，半自然草原覆盖面积是落叶林的两倍，是泥炭地之后土壤碳储存最重要的生态系统。此外，草原的实验易处理性和植物策略的多样性，以获得它们所包含的P，使草原理想的模型系统进行实验操作。我们将从长期的N和P添加地块收集完整的植物土壤整料，并将其暴露于附近设施的CO2富集，否则接近相同的环境条件，并保持营养操作。这将使我们能够直接确定磷限制如何影响生态系统在CO2升高的世界中吸收额外C的能力。我们将结合使用碳通量监测，植物和土壤采样，详细了解磷限制如何影响植物生产力，地上和地下植物碳分配，并最终改变土壤和总生态系统碳储量。额外提供的二氧化碳的同位素特征将用于确定土壤碳储存的变化是如何控制的，区分新有机物质的形成和现有物质的损失。最后，额外的微观研究将被用来了解不同的植物物种如何缓解在CO2升高下的磷限制，以及这如何影响C动态。总之，我们的工作将提供第一个直接评估在CO2升高的世界中，磷限制对生态系统C吸收率的影响，并且，在这样做的过程中，大大提高了我们对这个问题的理解，这个问题在很大程度上导致了对21世纪世纪气候变化率的预测的不确定性。

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.