Recovery from acidification in a strong deposition gradient in Sweden – the impact of nitrogen, sea salt episodes and sulphur adsorption

Cecilia Akselsson1, Gunilla Pihl Karlsson2, Per Erik Karlsson2, Hans Hultberg2, Sofie Hellsten2
1 Department of Physical Geography and Ecosystem Science, Lund University
2 IVL Swedish Environmental Research Institute

O 12.3 in Restoration and rehabilitation of ecosystems

17.07.2014, 11:40-12:00, H19

Deposition of sulphur has decreased substantially during the last decades, but surface water acidification is still a widespread problem in large parts of Sweden. Recovery from acidification in forest soils has proven to be slow and in some forests there is no recovery at all or even re-acidification.  As long as the soils are acidified, the problems with acidified lakes can be expected to persist. The progress of recovery in soil water below the root zone on 60 forests sites in Sweden has been studied. There was clear gradient related to the sulphur deposition gradient, with more recovery in areas where the sulphur deposition reduction has been large. However, there is a great variation between sites also on a local and a regional scale. Three important factors explaining this variation were identified: nitrogen status, sea salt episodes and sulphur adsorption/desorption processes. In the southwesternmost part of Sweden, the nitrogen status proved to be determinant for the recovery from acidification. On many sites in this region the nitrogen retention capacity is exceeded and nitrate is leaching from the root zone. This process is acidifying, which means that recovery is counteracted, and in some cases the soils are re-acidified. In large parts of southern Sweden, frequent sea salt episodes lead to acidification episodes, due to ion exchange where sodium ions are replacing hydrogen ions in the strongly acidified soils. This is a temporary effect, but frequent episodes mean that the soil water is acidified most of the time. The rate of desorption of adsorbed sulphur is another process that controls the recovery rate. Different soils have different adsorption capacities and some of the differences in recovery progress between nearby sites can be explained by differences in sulphur adsorption. The results highlight the importance to not exceed the nitrogen retention capacity, in order to prevent nitrogen leaching. This is important both from an acidification and eutrophication perspective. Moreover, the results indicate that the surface water acidification, when the sulphur deposition has decreased but the soils are still acidified, may be mainly an episode problem, i.e that surface water acidification episodes will occur mainly in periods of high sea salt inputs.

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Letzte Änderung 19.06.2014