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Increased atmospheric N deposition and the restoration of formerly species-rich heathlands and grasslands: constraints and possibilities for the future by Roland Bobbink

Atmospheric nitrogen deposition, from both oxidised (NOy) and reduced (NHx) compounds, is nowadays one of the main threats for biodiversity in European semi-natural ecosystems of high conservational value. Long-term nitrogen input from the atmosphere may cause eutrophication, soil acidification and/or ammonium toxicity. The severity of these impact depends on the biogeochemistry of the particular ecosystem, but is especially severe under oligo- to mesotrophic, weakly buffered soil conditions.

Long-term field trials have been set up in deteriorated ecosystems such as grassland and heathland sites since the early 1990s to counteract the severe impacts of N pollutants. The first aim was to restore former soil conditions, as we feel that rehabilitation of ecosystems should start with recreating appropriate abiotic conditions. Removal of the vegetation and top soil (‘sod cutting’), liming, hydrological measures or a combination of them were used depending on the actual biogeochemical constraints after the degradation. The effectiveness was mostly evaluated by following the soil chemistry and plant composition. In this presentation an overview of the experimental restoration measures and the main factors of success or failure will be presented. In most cases a combination of measures proved to be successful in restoring appropriate soil conditions and a low productive sward. A full recovery of plant diversity was, however, seriously limited when the characteristic species had already disappeared, especially in dry conditions or when it was impossible to increase the soil buffer capacity after acidification. Additional measures to counteract the dispersal limitation of many endangered species may be needed. Finally, it is concluded that an decrease in atmospheric N deposition is still needed in several areas of Europe to protect sensitive ecosystems on the longer term.