Is plant biodiversity loss and recovery in N polluted ecosystems regulated by phosphorus acquisition?

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Increased emissions of nitrogen (N) to the atmosphere from fossil fuel burning and agriculture are increasing the rates of deposition of this N to ecosystems. These increased rates of N deposition are one of the most important causes of biodiversity loss in ecosystems at a global scale. Within northern Europe, for example, many ecosystems first to receive these high N inputs (such as species rich grasslands), have lost much biodiversity, including loss of rare or protected species. It is most important, therefore, that we understand the mechanisms that drive this species change, both to allow us to predict impacts on ecosystems, and better manage and restore already polluted systems. In ecosystems, phosphorus (P) is generally the most or second most important nutrient for determining the growth of plants. An impact of N pollution on plant P uptake could therefore significantly impact on plant growth and therefore also alter the biodiversity of plant communities. Recent evidence suggests that this may be happening. In particular, the major plant types of species rich calcareous grasslands show considerable change in abundance under N pollution (sedges and grasses increase in abundance which reduces the diversity of herbaceous plants, including many attractive flowers that give these grasslands their conservation importance). Importantly, these major plant groups have contrasting methods of P uptake. Sedges (and to a lesser extent grasses) have special root adaptations while forbs are reliant on mycorrhizal symbioses to acquire P. It is therefore apparent that N pollution impacts on these contrasting P uptake mechanisms could drive species change. Using a floristically rich calcareous grassland (Peak District National Park, Derbyshire, UK), this programme of research will be the first study to determine and compare the impacts of N deposition on the distinctive mechanisms of P acquisition within the major plant types. It will determine the impacts of N deposition on plant P supply and determine whether these impacts can drive species change. It will also determine whether vegetation can recover from these impacts on P supply when N deposition declines as a result of effective control measures. The work is of considerable importance to those concerned with the conservation of ecosystems and preservation of biodiversity. It will inform policy makers of the impacts of pollutant N loading and allow us to better predict pollutant N impact on ecosystems and better establish 'critical loads'. The mechanistic understanding will also provide important insight into how other related systems around the globe may respond to pollutant N loading, particularly other systems where P supply is the most important factor in determining plant growth.