Modelling of Nitrogen Leaching from Forest Ecosystem: case study using data from SW Sweden.

Modelling nitrogen leaching from forest ecosystems: case study using data from SW Sweden SUMMARY The role of nitrogen as a pollutant but also an important nutrient in all of the environments of the world has gained increased attention today in both the scientific and policy arenas. The marine waters between Sweden and Denmark have, since the mid-70s, experienced algal blooming interpreted as a response to an elevated inflow of nitrogen from the surrounding land (Rosenberg et al. 1990). It is believed that most of the nitrogen comes from agricultural land, but for some regions in Sweden the total transport from forest areas may actually be higher than from agricultural areas. Joelsson and Fleischer recently wrote summaries of historical series of water chemistry analysis for the County Board of Halland (South West Sweden). These data would suggest that while the contribution in nitrate leaching is more or less steady for arable land in the last 30 years in the south of Sweden, in the same region the contribution of forested land is increasing. This work was articulated as a study of the different processes which can bring to such increased nitrogen concentrations. The environmental conditions and processes associated with elevated NH 4 + and N0 3 - in drainage water can be a great number but we have focused on the following different scenarios: • Changes in forest management practices • Climate variability • Nitrogen deposition By using the numerical simulating tool CoupModel (developed in Sweden during many years and made available on internet at the LWR in KTH, Stockholm) as main tool of the study, this work underlined the processes which are basically involved in the nitrogen cycle within the forest ecosystem and aimed to point out which processes could be more likely involved in the increased nitrogen leaching from forested lands. The study implied the use and treatment of data about today state of forests, climate data and nitrogen deposition. The treatment and construction of these data was an important part of the work and brought different scenarios, whose simulated results were compared among themselves and with experimental data in terms of plant growth and N and C content in the different pools of the system. The general aging of forests in Halland resulted to have small effect on leaching and also the increasing intensity of treatments as clearcuts, thinning and cleaning didn’t prove as important in terms of nitrogen balances as they involve relatively small amounts of nitrogen when compared to the total storage in the system. Climate variability in Halland resulted to be instead a major reason for nitrogen release changes: data confirm that the exceptionality of last decades when compared with the whole 20th century lays in combination of high temperature and high precipitation. High precipitation proved to have the greatest effects on the simulated systems: the consequences are higher runoff values (which were observed and already studied in many works) and higher nitrogen leaching, as result of an increased water flow and enhanced turnover processes in the soil. Halland is subdued to the highest levels of nitrogen deposition of the whole Sweden and this proved a likely explanation for the investigated phenomenon: with higher inputs, the system stores more nitrogen within its pools and releases such nitrogen after the cut. The model did not account for diseases that were observed in forests subdued to high levels of deposition: so the model could not describe the problem of nitrogen saturation, which has been frequently observed and discussed. By a deepened review on literature on the issue, we build up two different conceptual models to account of these processes. v