The future impact of nitrogen in the acidification of surface waters : Modelling, empirical and experimental studies of changes in nitrogen leaching

Abstract

High nitrate (NO3) leaching rates have been observed in mountain and heathland areas in Europe at sites receiving high levels of nitrogen (N) deposition. These areas are characterised by sparse vegetation on thin, patchy acidic soils and steep slopes and with relatively short growing season; they are therefore highly susceptible to NO3 leaching. The main objective of this study was to contribute to increased knowledge about processes controlling N leaching in Norwegian mountain and heathland areas experiencing relatively high N load. To achieve these goals three research approaches have been used; empirical analysis, experimental studies and modelling. <br> The main objective of the paper I was to identify relationships between different vegetation cover (i.e. peat, heather and exposed bedrock) and N leaching on a seasonal and annual basis. As expected, highest NO3 leaching was found for the exposed bedrock dominated catchments. The fraction coverage of exposed bedrock explained 70% of the variation in NO3 concentrations during autumn and winter, but a much lower percent during the warmer seasons when biological processes exert higher control. The field experiment was designed to test a proposed causal inverse relationship in surface water between NO3 and dissolved organic carbon (DOC) concentrations. Paper II tested the hypothesis that increased availability of NO3 (or N+P) would stimulate bacterial oxidation of DOC in the waterbody. As intended, the NO3 concentrations increased from near zero up to 685 and 560 μg N l-1 at the N+P and N manipulated ponds. Correspondingly, PO4 concentration increased from below detection limit up to 32 μg P l-1 after the first addition and to 58 μg P l-1 after the second. The increase in N availability showed no effect on DOC, total organic carbon (TOC), bacteria or algae (measured as chlorophyll a) in the pond that received N alone. These results indicate that the inverse relationship between NO3 and DOC in surface waters is probably not due to processes occurring within the waterbody. A possible explanation for the inverse NO3-DOC relationship can be related to differences in N deposition and the characteristic of catchment soils.<br> Despite the extensive use of the dynamic model MAGIC on annual basis, there have been few attempts using monthly time steps to examine seasonal variations. NO3, in particular, shows a pronounced seasonal pattern in many lakes and streams. In this study the objective was to apply the MAGIC model to the 12-year time-series from Øygardsbekken using a monthly time step. The main goal was to test how well MAGIC could reproduce the observed long-term (1993-2004) and seasonal NO3 pattern. The MAGIC application to Øygardsbekken satisfactorily simulated the seasonal NO3 pattern over the 12-year record; the model explained almost 70% of the variation in monthly NO3 concentrations and nearly 90% of the variations in flux. <br> In study IV the main objective was to use the MAGIC model as was calibrated on Øygardbekken to simulate future NO3 leaching from the catchment, implementing a future N deposition scenario combined with four different scenarios for climate change. Both seasonal and annual changes in NO3 leaching were simulated. Implementing climate change scenarios for temperature and N deposition to the calibrated model at Øygardsbekken, resulted in increased future NO3 leaching to surface water. <br> Catchment N stores have increased during several decades of elevated N deposition and retention. The long-term stability of this N accumulation is largely unknown. Internal ecosystem cycling of N greatly exceeds system inputs and outputs, and any disturbance of this cycle has the potential to obscure the relationship between N deposition and runoff. <br> The uncertainty in future chemical recovery from acidification is mainly related to the effects of climate change and future behaviour of N in the ecosystem. More research is needed on the long-term fate of N stored in soil under changing N deposition and climate. Release of N from this pool has potentially large effects in freshwater and coastal marine ecosystems.