Keywords:
barrier dunes
calcareous dune
non-calcareous dune
acidification
eutrophication
simulation
RESAM

Location:  The Netherlands

Dune ecosystems in The Netherlands
The Dutch coastal dunes form only 1% of the land area of The Netherlands, but contain 70% of species of the Dutch flora, including 15% not found outside the dune area. This landscape belongs to the so-called ‘ Younger Dunes’, developed since approximately 900 AD.Two mineralogically distinct dune regions with different soils and ecosystems can be distinguished: the dunes north of Bergen (the Wadden district) consist of sand with a very low content of calcium-carbonate (< 2%) as well as silicate minerals (usually between 6-8%) and the area south of Bergen (the Dune district) with dunes with relatively high carbonate (usually between 2-10%) and silicate mineral contents (generally between 13-17%). The boundary between these areas is rather sharp.
During recent decades dry dune ecosystems in The Netherlands have shown (i) a decrease in rare species and in lichen cover, (ii) an increase of nitrophilous species and of graminoids and (iii) an accelerated spread of shrubs and moss species. These changes are ascribed to an increased atmospheric deposition of SOx, NOx and NHx, furthermore to natural soil development and vegetation succession due to changed management practices (e.g. dune fixation; agricultural use)

Simulation with the RESAM
The study focuses on the long-term acidifying impact (during a 100-year period) of atmospheric deposition on both calcareous and non-calcareous soils and related plant cover. Estimates are given for changes in (i) natural man-induced acidification and (ii) soil and soil solution chemistry. Simulations were carried out on two representative dune ecosystems in the Dune and Wadden district. Average values for typical soil profiles and vegetation types were taken from the literature. A process-oriented Regional Soil Acidification Model (RESAM) was used. The model uses mechanistic descriptions for processes in the vegetation canopy, litter layer and mineral soil horizons which significantly influence the soil chemistry and the soil solution chemistry.

Results
The impact of atmospheric deposition on a calcareous dune soil will become very important when the calcium carbonate content has fallen below 0.3%. Then the pH in the topsoil will decline from about 6.5 to 3.0 over several decades, and acid deposition will become the dominating acidifying factor (more than 95%). In non-calcareous dune soil  present deposition levels cause a depletion of amorphous Al3+ hydroxides, which leads to a pH decline from about 3.5-4.5 to 3.0-3.5 over a depth of more than 50 cm during the simulation period. Evidence is given, that this will lead to a decrease in plant species. Soil acidification may have negative effects on the majority of species typical for dry dunes, especially when the soil pH becomes lower than 4.0. In addition to soil acidification, eutrophication will also affect plants.
To mitigate adverse effects of acid atmospheric deposition dune managers can choose between small-scale measures (aimed at the conservation of endangered species or communities, e.g. cutting of shrubs and trees, mowing, sod cutting) and large-scale measures (using natural landscape dynamics, e.g. extensive grazing, local sand blowing, formation of new outer dunes). Small-scale measures contribute to the removal of nitrogen by removal of biomass. However, this can lead to a greater output of cations than of anions, implying an increase in soil acidification. On the other hand, a decrease in taller vegetation leads to a less effective filtering of atmospheric components and therefore to a lower load of acid deposition. Excavation of dunes in order to remove both nutrients from organic horizons and decalcified soil layers can be a more effective measure, but is rather expensive. With respect to large-scale measures, grazing enhances the variation by e.g. redistribution of nutrients and triggering geomorphological and pedological rejuvenation by trampling, but cannot cope with the present high inputs from the atmosphere. It is therefore suggested to re-introduce natural dynamics like sand blowing in a large scale in areas, where coastal protection is less important, in order to decrease acidification rates by the input of calcareous sand. This is often a rather inexpensive measure and can be supported by small-scale measures.

References

• De Vries, W., J.A. Klijn & J. Kros (1994): Simulation of the long-term impact of atmospheric deposition on dune ecosystems in The Netherlands. Journal of Applied Ecology 31: 59-73. (33EiN94).