Effects of nitrogen deposition on tree growth and soil nutrients in boreal Scots pine stands

The aim of the study is to analyse effects of nitrogen deposition on tree growth and soil nutrients in boreal Scots pine stands. We describe a model ‘Forest Element Fluxes’ (FEF) which is based on theories of carbon and nutrient metabolism of trees, decomposition of soil organic matter (SOM), and solution chemistry in forest soil. The main idea is that element fluxes and regularities in tree structure determine soil development and stand growth. Tree roots, microbes which immobilise nitrogen (N) or nitrify NH4+, and cation exchange sites compete for inorganic N. Hydrological balance and chemical reactions, for example cation-exchange reactions and bicarbonate equilibrium, determine the leaching of N, H+, Al, Ca, Mg and K to surface waters. Observed leaching of N to brooks after clear-cutting was used to calibrate immobilisation of nitrogen. The model behaviour was tested by comparing the model outputs to stand growth at sites of different fertility. The model generated the stemwood production at the different sites, and the increase in nitrogen leaching after clear-cut. The analysis suggests that increase in nitrogen deposition has increased tree growth by 12%, but it is evident that other nutrients will become crucial with the present nitrogen deposition. The present accumulation of nitrogen in forest soil will lead to major changes in forest growth, nutrient availability and nitrogen leaching.