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

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.

Our ultimate aim is to improve understanding of how P limitation controls ecosystem responses to elevated CO2. Given the global extent of P limitation, and its potential to reduce rates of ecosystem C uptake under elevated CO2, this work will be of great interest to the IPCC, and the modellers and biogeochemists who aim to better understand the global carbon cycle and predict its responses to global change. Nationally, and more locally, the research will be of interest to stakeholders in conservation and farming who will be interested in the impacts of eCO2 on grassland productivity and biodiversity, and how this interacts with nutrient availability. Furthermore, given the importance of the science question, and the fact that it is a relatively accessible issue, the general public is also likely to be interested in our project.

The IPCC is placing a greater emphasis on Earth system modelling and in particular climate-carbon cycle feedbacks. In the latest IPCC assessment report, chapter 6 of the Working Group I report summaries current knowledge on 'Carbon and Other Biogeochemical Cycles' in the context of climate change feedbacks. Even within the Executive Summary, the uncertainty regarding the potential for P availability to limit C sequestration under elevated CO2 is emphasised. Therefore, we are investigating an extremely important and timely issue, and our findings will be directly relevant to the IPCC. It is important to note that, as emphasised in the academic beneficiaries, we enjoy excellent links with Met Office scientists. Also, the joint University of Exeter / Met Office Carbon Cycle research group that PI Hartley established has nearly 100 members and critically includes a number of current IPCC authors.

The two grassland ecosystems of our study are widespread and of considerable conservation and amenity value. While focused on carbon cycling, our work will also generate understanding of how eCO2 will alter the productivity and biodiversity of these grasslands, and will therefore be of direct interest to farming and conservation groups. We have long-established contacts with these, such as the Peak District Park Authority, Natural England, and the UK Farming and Wildlife Advisory Group. We will ensure our findings are disseminated to these groups through engagement from the start, including through presentations and writing for group magazines and web sites, and in seeking ways in which our data may be immediately beneficial to these groups.

The project should also be of great interest to the public, and be an excellent opportunity to promote NERC science. FACE installations, even miniFACE, are impressive experiments providing ideal opportunities for engaging the public. We will run site visits for school children and the public, including talks and demonstrations. We will also use this research topic in our ongoing collaborations with the Weston Park Museum (Sheffield) where we have had good success running demonstrations for school children. The LiCor 8100 chambers, automatically open and close and have proven very popular at exhibitions and with school children. The wireless data downloading and i-pad software allow us to show measurements in real time while the public are on site. Finally, the investigators are committed to, and have strong track records in, communicating science more broadly to the general public through various media outlets.

In summary, together with our plans for academic dissemination, our impact plan will ensure our results have the maximum possible impact in terms improving the representation of P dynamics in IPCC-facing modelling. Our plans also ensure that relevant stakeholders are engaged, and we will use the experiment itself to help educate school children and the public in how ecosystems are responding to, and feeding back to, climate change. Finally, we will use various media, to ensure that our research reaches the wider society.