Element cycling in forest soils -modelling the effects of a changing environment

Sammanfattning: Element cycling and nutrient supply in forest ecosystems are of vital importance for short-term productivity and for longer-term land management in terms of nutrient leaching and CO2 fixation. This thesis includes a series of studies with the objective of modelling some aspects of the effect of acidification and climate change on element cycling and nutrient supply in forest soil. A reconstruction model of atmospheric deposition and nutrient uptake and cycling, MAKEDEP, was developed. An existing model of soil chemistry, SAFE, was analyzed and applied. SAFE+MAKEDEP were then applied in combination with the RAINS model to perform scenario analyses of soil acidification/recovery for six European forest sites. A decomposition model intended to run in conjunction with the SAFE model was developed. Key elements were N, Ca, K, Mg, S and Al. In the decomposition model, only carbon release was included to date. The results show, that understanding the history of soil geochemistry is important for modelling the system and for projecting the future impact of acidification on nutrient supply in forest soils. The applied reconstruction models of acid deposition (MAKEDEP, RAINS) seem to generate reasonable and consistent estimates of historic acid deposition, so that present day conditions can be simulated starting from pre-acidification conditions. From applications of the SAFE model to large-scale forest manipulation experiments, we conclude that the geochemical processes and the degree of detail in process descriptions included in SAFE are adequate to capture the most important aspects of soil solution dynamics of forest soils in northern and central Europe. Therefore, SAFE is appropriate for the simulation of acidification and recovery scenarios for these soils. The precision in model prediction on a more general scale is often limited by factors other than model formulation, such as consistency and representativity of input data. It is shown that the physical parameters soil bulk density, volumetric moisture and mineral surface area are especially important for the calculation of weathering rates, and thus soil solution chemistry. Estimates of future non-marine base cation deposition are shown to be less important than previously assumed. A minimum data set required to apply the MAKEDEP+SAFE model package on a regional scale is suggested. Our results suggest that it in the future will become important to combine climate change modelling with the modelling of soil acidification and/or recovery to adequately reproduce and project nutrient dynamics in soils. The decomposition model presented in this thesis should provide a good platform for such integrated simulations.