SLU news

New evaluation of acidification of Swedish forest soils

Published: 14 November 2016

In a recent licentiate thesis, Johan Iwald has investigated the impact of tree growth with subsequent harvest as well as deposition of sulphur and nitrogen compounds on acidification of forest soils in Sweden.

Acidification of forest soils is a natural process as well as one of the major environmental problems in Europe and several other parts of the world. Acidification of forest soils occurs in two ways. The first is by trees as they grow. The trees take up an excess of cations and release hydrogen ions that acidify the soil. The second is by anthropogenic emissions of sulphuric and nitric oxides that are converted to acids in the atmosphere and brought to forests as either dry or wet deposition. In Sweden, as well as large areas in Europe and North America, acidification of forest soils has led to nutrient depletion and aluminium toxicity for primarily aquatic organisms.

In the licentiate thesis “Acidification of Swedish Forest Soils - Evaluation of Data from the Swedish Forest Soil Inventory”, Johan Iwald has investigated potential impacts of the two drivers of acidification on forest soils in Sweden, the first one being tree growth with subsequent harvest, and the second one deposition of sulphur and nitrogen compounds.

Harvesting Norway spruce leads to most acidifying effects

The magnitude of acidification by tree growth and harvest was defined as the net cation uptake in harvested tree parts. The magnitude of acidification by deposition was estimated based on deposition data from four catchments throughout Sweden. The tree species that causes most acidifying effects is Norway spruce, followed by birch. Harvesting Scots pine causes the least acidification of the tree species in this study.

Harvesting of logging residues causes more soil acidification than harvesting of stumps, despite the fact that the biomass of stumps is larger. This is 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 level equal to recent deposition levels. However, harvesting of Norway spruce causes more acidification than deposition.

Johan Iwald nails his licentiate thesis to the wall. Photo: Gertrud Nordlander.

Using data from a large-scale inventory

In order to study the acidifying effects on the soil of tree growth during a forest generation and deposition, Johan used data from the Swedish Forest Soil Inventory – a large-scale inventory of productive forest land throughout Sweden.

Tree growth during a forest generation and deposition of acidifying substances both affect variables that are related to acidity in Swedish forest soils. Tree growth causes decreased pH, base saturation and calcium levels and increased aluminium levels in the O and to some extent the B horizon of the soil in the beginning of the trees’ life cycle. Deposition causes decreased pH, base saturation and calcium and increased aluminium in the O, B and C horizons of the soil throughout the deposition gradient in Sweden. The effect of deposition is most evidently expressed in deep soil layers where biological acidification has less influence.

Important to continue monitoring acidification of forest soils

As depositions continue to decline and biomass growth and harvest increase, the biological acidification may become relatively more important, compared to acidification by deposition, in the future. It is likely, however, that the effects of historic deposition and biomass harvest will remain in Swedish forest soils for a long time to come.

– Recovery may be delayed due to continued high nitrogen deposition and intensive tree harvesting. Therefore, it is important that we continue to monitor forest soils to determine the effects of increased tree growth and harvest, and examine to what extent previous high deposition levels will continue to affect the soil, says Johan Iwald.

Johan Iwald will defend his thesis on November 24th at 9 am.



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