availability of both P and N (Kooijman and Besse 2002). At even lower pH, iron
concentrations in the soil solution increase, and P may precipitate in the form of
iron phosphates. However, this process only occurs in soils with low organic
matter content, where iron is mainly present in mineral form (Kooijman and
Hedenäs 2009). When soil organic matter content is high, iron is incorporated in
iron–organic matter complexes, which can still adsorb P, but in a much weaker
way. In acidic, iron-rich soils with high organic matter content, productivity will
still be high. Consequently, nitrogen and phosphorus availability increases during
succession along the Holland coast.
On the Wadden Sea islands, lime content is usually too low for substantial
calcium phosphate formation; thus, even in pioneer stages with low amounts of
organic matter the productivity of the dune ecosystems is limited only by nitrogen
(Lammerts et al. 1999). N availability may increase during succession, owing to soil
acidification and shift in microbial communities from bacteria to fungi with lower N
demand. However, before the 1980s, when high atmospheric N deposition became a
serious problem, N availability may have remained relatively low. Low microbial N
demand may lead to high N availability to the vegetation especially under high N
deposition, when N input has increased, but microbial uptake remains low. Before
the 1980s, decomposition rates were low, and N input was still mainly dependent on
the relatively low litter input, which further reduced gross N mineralization. However, in the last few decades, atmospheric N deposition and N input strongly
increased in northwestern Europe, in particular in The Netherlands, with their high
numbers of cows and pig stables (Bobbink et al. 1998).
15.1.3 Effects of Increased Nitrogen Deposition
The dry dunes in the Netherlands have been very negatively affected by atmospheric
deposition of nitrogen due to its stimulating effect on biomass production. Stuyfzand
(1993) showed that the atmospheric nitrogen deposition in the Holland dune areas
had increased from 5 to 36–55 kg N ha
-1 year
-1 between 1939 and 1981. In one
area with a large amount of industrial activity even 80 kg N ha
-1 year
-1 was
measured. Values on the Dutch Wadden Sea islands were slightly lower; 20–35 kg
(Sival and Strijkstra-Kalk 1999). Grass encroachment has reduced open, speciesrich dune grasslands to tall grass vegetation with low biodiversity. Grass
encroachment further increased with declining grazing activity, including rabbit
grazing. Rabbit populations have strongly declined because of viral diseases, such as
myxomatosis and viral hemorrhagic disease (VHD). Less than 50 years ago dry
dunes had a very high biodiversity in terms of both flora and vegetation, and harbored 70 % of the total flora of the Netherlands. Nowadays, almost 100 of them are
endangered and have been placed on the Red List.
As indicated above, calcareous and acid dunes may differ in sensitivity to high N
deposition. In the Netherlands, N deposition is higher along the Holland coast than
on the Wadden islands, but grass encroachment generally showed opposite patterns
15 Restoration of Dune Vegetation
239
concentrations in the soil solution increase, and P may precipitate in the form of
iron phosphates. However, this process only occurs in soils with low organic
matter content, where iron is mainly present in mineral form (Kooijman and
Hedenäs 2009). When soil organic matter content is high, iron is incorporated in
iron–organic matter complexes, which can still adsorb P, but in a much weaker
way. In acidic, iron-rich soils with high organic matter content, productivity will
still be high. Consequently, nitrogen and phosphorus availability increases during
succession along the Holland coast.
On the Wadden Sea islands, lime content is usually too low for substantial
calcium phosphate formation; thus, even in pioneer stages with low amounts of
organic matter the productivity of the dune ecosystems is limited only by nitrogen
(Lammerts et al. 1999). N availability may increase during succession, owing to soil
acidification and shift in microbial communities from bacteria to fungi with lower N
demand. However, before the 1980s, when high atmospheric N deposition became a
serious problem, N availability may have remained relatively low. Low microbial N
demand may lead to high N availability to the vegetation especially under high N
deposition, when N input has increased, but microbial uptake remains low. Before
the 1980s, decomposition rates were low, and N input was still mainly dependent on
the relatively low litter input, which further reduced gross N mineralization. However, in the last few decades, atmospheric N deposition and N input strongly
increased in northwestern Europe, in particular in The Netherlands, with their high
numbers of cows and pig stables (Bobbink et al. 1998).
15.1.3 Effects of Increased Nitrogen Deposition
The dry dunes in the Netherlands have been very negatively affected by atmospheric
deposition of nitrogen due to its stimulating effect on biomass production. Stuyfzand
(1993) showed that the atmospheric nitrogen deposition in the Holland dune areas
had increased from 5 to 36–55 kg N ha
-1 year
-1 between 1939 and 1981. In one
area with a large amount of industrial activity even 80 kg N ha
-1 year
-1 was
measured. Values on the Dutch Wadden Sea islands were slightly lower; 20–35 kg
(Sival and Strijkstra-Kalk 1999). Grass encroachment has reduced open, speciesrich dune grasslands to tall grass vegetation with low biodiversity. Grass
encroachment further increased with declining grazing activity, including rabbit
grazing. Rabbit populations have strongly declined because of viral diseases, such as
myxomatosis and viral hemorrhagic disease (VHD). Less than 50 years ago dry
dunes had a very high biodiversity in terms of both flora and vegetation, and harbored 70 % of the total flora of the Netherlands. Nowadays, almost 100 of them are
endangered and have been placed on the Red List.
As indicated above, calcareous and acid dunes may differ in sensitivity to high N
deposition. In the Netherlands, N deposition is higher along the Holland coast than
on the Wadden islands, but grass encroachment generally showed opposite patterns
15 Restoration of Dune Vegetation
239