Simulating effects of S and N deposition on soil water chemistry by the nutrient cycling model NuCM

The nutrient cycling model (NuCM), developed during the Integrated Forest Study in the USA, is one of the most detailed models simulating processes in forest ecosystems. The present study is an approach to validate NuCM by testing the model against data from a highly controlled model lysimeter experiment. NuCM was calibrated against data from lysimeters receiving no S and N treatment. The calibrated model was then used to simulate observed changes in soil water and leachate chemistry due to the application of artificial acid rain of pH 3 and/or N-fertiliser (90 kg N/ha∗year). By adjusting key parameters like the soilʹs ability to adsorb SO42− and nitrification/denitrification rates, NuCM was able to simulate the observed changes in mean ion concentration for the experimental period fairly well. The conformity between observed and simulated seasonal/year-to-year variations was improved at elevated N-deposition. This study illustrates, however, the uncertainty which appears when complex models are applied to systems with less extensive parameter collection. Conditions for proper model testing are difficult to obtain when working with environmental systems.