Impacts of nitrogen deposition on the forest carbon cycle: from ecosystem manipulations to national scale predictions

Combustion of fossil fuels and use of fertilisers in agriculture has increased the amount of nitrogen compounds present in the atmosphere and the biosphere. More atmospheric nitrogen is converted into reactive nitrogen by anthropogenic activities than by all natural processes combined. This phenomenal historical increase in nitrogen deposition is responsible for several serious environmental problems, such as soil acidification and nitrogen saturation, nitrate leaching in water courses and eutrophication and changes in the composition of natural ecosystems. In its most extreme forms, it can also cause plant mortality. The added nitrogen also contributes to forming active ozone in the troposphere (where it is damaging) and to producing the greenhouse gases methane and nitrous oxide. To partially counterbalance all these negative effects, nitrogen deposition is also known to increase the sequestration of carbon by forests, which helps to slow down climate change. However, the magnitude of this indirect positive service provided by a pollutant is uncertain, with many estimates suggesting that the effect if rather small. In a previous paper in Nature however, we reported new data showing the effect to be very large for forests of the boreal and temperate regions. This work raised several questions in the community and stimulated a debate which is still ongoing on the real magnitude of this effect. In this project we propose to test whether our findings stand to scientific scrutiny by using two complentary tests: a) an experimental manipulation which artificially elevates the levels of nitrogen deposition over one experimental forest, where we have been measuring carbon sequestration for the past 10 years. Many fertilisation experiments have been carried out in the past, but we contend that they did not realistically simulate the process of nitrogen deposition, as the many ways that nitrogen can interact with the canopy of trees are entirely eliminated when the fertiliser is applied directly on the ground (nitrogen deposition occurs from above the canopy either dissolved in rainfall or as dry deposition over the foliage). b) comparative observations of tree growth across known regional gradients in nitrogen deposition across the UK. Surprisingly few empirical observations have been carried out using this second method, on the assumption that changes in climate, soil fertility, etc. would mask any residual effect due to nitrogen deposition. However, the available literature strongly suggests that high levels of nitrogen deposition have increased tree growth during the 20th century across much of Europe, in support of our theory. Finally, we will use three models to compare our observations against the model predictions. This will also help to determine the mechanism(s) which may promote high rates of carbon sequestration in response to nitrogen deposition.