Ecosystem quality and trends in soil pH, 2016

Soil pH in parts of grasslands and marshes unfavourable

The soils in natural areas can become more acidified as a result of the deposition of acidifying substances in the atmosphere, which can lead to the loss of certain plant species in these habitats. Soil pH can also be influenced by changes in the water regime, for example the cessation of the flow of base-rich groundwater seepage or the accumulation of organic matter in the humus layer.
The trend lines show that the pH of the soil is decreasing slightly in all ecosystems across the country. This acidification may be due to natural succession and not so much due to deposition of acidifying substances, because this has actual fallen in recent years. Acidification is therefore a relatively limited problem. Soil pH is classified moderate or bad in about a fifth of the total area of natural and semi-natural ecosystems. Locations where soil pH has been found moderate or bad are mostly situated in marshland. A possible explanation for this is the process of natural ageing in which the accumulation of organic matter (ageing) leads to a lower groundwater level and as a result the influence of rainwater is increased. The marsh becomes slightly drier, nutrient poorer and slightly more acid. In forests, and to a greater extent in dunes and heaths, the environmental conditions in terms of soil pH are generally good.

Results of policy to combat acidification

To prevent the effects of eutrophication and acidification, Dutch environmental policy focuses on reducing emissions of eutrophying and acidifying substances in the Netherlands and surrounding countries. The deposition of acidifying substances fell significantly in the 1980s due the effects of source control measures. Over the last few decades, both national and international environmental policies have led to cleaner air, resulting in lower acid and nitrogen deposition on ecosystems (Van Herk et al., 2002; Buijsman et al., 2010). Research shows that this reduction in deposition of acidifying substances has resulted in a much slower rate of acidification in sensitive sandy soils than before. The soils are still becoming more acid due to the deposition of ammonia and natural succession, but this is of a much lower order of magnitude.

Presence of plants indicative of environmental conditions

Direct measurements of soil and water pH can be used to determine the extent to which changes in environmental conditions are occurring and whether or not current conditions are suitable for the sustainable conservation of ecosystems. However, the availability of such measurement data is limited. Environmental conditions can also be estimated indirectly from the presence of certain plant species and changes in species composition, because plants have specific environmental requirements. The vegetation composition found in an area is therefore indicative of the prevailing environmental conditions at site level.
In the 'monitoring and evaluation methodology' (WMBN), the provinces and conservation management organisations have set out how environmental conditions can be determined. In anticipation of the first set of monitoring data from the provinces, these and the two indicators below illustrate current environmental conditions.

• Environmental quality of terrestrial ecosystems: nitrogen deposition 1994–2022
• Ecosystem quality and trends in groundwater-dependent ecosystems, 2018

Policy focuses on improving environmental conditions

The Netherlands has international commitments to meeting the goals of the Convention on Biological Diversity, the Birds and Habitats Directives (Natura 2000) and the EU Biodiversity Strategy. In the Nature Pact (Natuurpact) the national and provincial governments have agreed to improve ecological quality through the creation of the National Ecological Network and through more intensive restoration and management measures to improve water and environmental conditions.
To prevent the effects of eutrophication and acidification, Dutch environmental policy focuses on reducing emissions of eutrophying and acidifying substances in the Netherlands and surrounding countries. Over the last few decades, both national and international environmental policies have led to cleaner air, resulting in lower acid and nitrogen deposition on ecosystems (Buijsman et al., 2010). However, the results achieved are not yet sufficient to produce good conditions for ecosystems and species.

• Emissions of National Emission Ceiling (NEC) substances, 1990-2013

Restoration measures to combat eutrophication and acidification

In the Nitrogen Reduction Programme (PAS) a combination of generic and area-based policies are being applied to reduce nitrogen deposition on Natura 2000 sites. In addition, resources are being made available for restoration measures in existing natural areas to combat eutrophication and acidification. Many of these measures are also aimed at combating water table drawdown, and prevent the drying-out of areas. This is because current soil acidity levels depend not only on the current deposition of eutrophying substances, but also on the lowering of the water table and on the quality of surface water and groundwater.