Stable Carbon and Nitrogen Isotopes in a Peat Profile Are Influenced by Early Stage Diagenesis and Changes in Atmospheric CO2 and N Deposition

In this study, we test whether the δ13C and δ15N in a peat profile are, respectively, linked to the recent dilution of atmospheric δ13CO2 caused by increased fossil fuel combustion and changes in atmospheric δ15N deposition. We analysed bulk peat and Sphagnum fuscum branch C and N concentrations and bulk peat, S. fuscum branch and Andromeda polifolia leaf δ13C and δ15N from a 30-cm hummock-like peat profile from an Aapa mire in northern Finland. Statistically significant correlations were found between the dilution of atmospheric δ13CO2 and bulk peat δ13C, as well as between historically increasing wet N deposition and bulk peat δ15N. However, these correlations may be affected by early stage kinetic fractionation during decomposition and possibly other processes. We conclude that bulk peat stable carbon and nitrogen isotope ratios may reflect the dilution of atmospheric δ13CO2 and the changes in δ15N deposition, but probably also reflect the effects of early stage kinetic fractionation during diagenesis. This needs to be taken into account when interpreting palaeodata. There is a need for further studies of δ15N profiles in sufficiently old dated cores from sites with different rates of decomposition: These would facilitate more reliable separation of depositional δ15N from patterns caused by other processes

Differential effects of oxidised and reduced nitrogen on vegetation and soil chemistry of species-rich acidic grasslands

Emissions and deposition of ammonia and nitrogen oxides have strongly increased since the 1950s. This has led to significant changes in the nitrogen (N) cycle, vegetation composition and plant diversity in many ecosystems of high conservation value in Europe. As a consequence of different regional pollution levels and of the increased importance of reduced N in the near future, determining the effect of different forms of N is an important task for understanding the consequences of atmospheric N inputs. We have initiated three replicated N addition experiments in species-rich, acidic grasslands spanning a climatic gradient in the Atlantic biogeographic region of Europe in Norway, Wales and France at sites with low levels of pollution. N was added in two doses (0 and 70 kg N ha−1 year−1 above background) and in three forms (oxidised N, reduced N and a 50–50 combination). After 2.5 years of N additions, the effects of these treatments on plant biomass, plant nutritional status, soil pH and soil nutrient availability were determined. Impacts of the N additions were observed within the 2.5-year research period. In some cases, the first signs of differential effects of N form could also be demonstrated. In the French site, for example, grass biomass was significantly increased by the oxidised N treatments but decreased by the reduced N treatments. In the Norwegian site, the reduced N treatments significantly reduced soil pH, whereas oxidised N did not. Effects on nutrient availability were also observed. These experiments will be continued to elucidate the longer term impacts of N deposition on these grasslands