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

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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.