Acidification of Swedish forest soils
Abstract: Acidification of forest soils in Sweden has negatively affected nutrient status and caused toxic effects on biota. The aim of this thesis was to investigate potential impacts of two drivers of acidification of forest soils in Sweden: (1) tree growth with subsequent harvest and (2) deposition of sulphur and nitrogen compounds. The magnitude of acidification by tree growth was calculated using estimates of stem wood, logging residue and stump harvesting from a national forestry impact assessment and data of cations and anions in different tree parts. Acidification by tree growth was defined as the net cation uptake. The magnitude of acidification by deposition was estimated based on deposition data from four catchments throughout Sweden. Acidification by deposition was defined as the negative value of the acid neutralizing capacity in deposition. The acidifying effect of tree harvesting decreases in the order Norway spruce>birch>Scots pine. Harvesting of logging residues causes more soil acidification than harvesting of stumps, despite the fact that the biomass of stumps is larger, due to the higher contents of base cations in needles and branches than in stem wood. Harvesting of Scots pine and birch causes soil acidification at a magnitude equal to recent deposition levels while harvesting of Norway spruce causes more acidification than what is caused by deposition. To study the acidifying effects on the soil of tree growth during a forest generation and deposition, data from the Swedish Forest Soil Inventory – a large-scale inventory throughout Sweden – from the O (n=1990), B (n=1902) and C (n=1477) horizons were used. Four variables were chosen as indicators of soil acidity: pH-H₂O (pH), base saturation (BS), exchangeable calcium (Ca) and exchangeable aluminium (Al) from each horizon. The variables were analysed with respect to tree age and data of sulphur deposition, using linear and multiple regression. For tree age, a relative age concept was used to account for different tree growth rates in different parts of Sweden. Tree growth during a forest generation and deposition of acidifying substances both affect acidity related variables in Swedish forest soils. Tree growth causes decreased pH, BS and Ca and increased Al in the O and to some extent the B horizon in the beginning of the trees’ life cycle. Deposition causes decreased pH, BS and Ca and increased Al in the O, B and C horizons throughout the deposition gradient in Sweden. The effect of deposition is most evidently expressed in deep soil layers where biological acidification have less influence.
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