The effect of nitrogen deposition on nature quality in the Netherlands

“The CDV is one value, above which negative effects of nitrogen deposition on nature can occur,” explains ecologist Wieger Wamelink. “But this does not yet make it clear what happens with a different amount of deposition: does the quality of nature directly above the CDV already deteriorate sharply, or does it gradually deteriorate?

Exceedance Degree

The study looked at the relationship between nitrogen deposition and the presence of plant species and vegetation types. “Some plant species benefit from nitrogen, such as grasses,” explains Wamelink. “Heathlands for example, deal with high levels of nitrogen. Here grasses can gain the upper hand, while species that are less resistant to nitrogen, such as creeping gorse and characteristic lichens, disappear. In this way, nitrogen changes the species composition. If species that are characteristic of a specific habitat type disappear, this means that the quality of that type of nature decreases. Until now, we did not know exactly how this process relates to the extent of CDV exceedance.”

European field studies

The researchers used two methods to determine the relationship between nitrogen deposition and the effects on nature. The first is based on field studies in areas with the same habitat type, but different nitrogen deposition. In this method, the quality of the habitat type was compared to a gradient of nitrogen deposition: from natural deposition to very high deposition values. The data comes from research conducted in several European countries. The number of available studies was limited, but did concern different types of landscapes, such as raised moors, grassland and forest. For 8 of the 61 nitrogen-sensitive habitat types that occur in the Netherlands, there was sufficient information to be able to say something about the effects of increasing nitrogen deposition on nature quality. “The advantage of this method is that measurements were taken directly in the habitat types themselves under controlled conditions,” says Wamelink.

Exploring a statistical method

Because the field studies provide information on a limited number of habitat types, it was also explored to what extent dose-effect relationships can be determined for all habitat types. This was done on the basis of statistical relationships between the probability that a particular plant species occurs somewhere and the amount of nitrogen that precipitates on that spot. This resulted in a so-called response curve for almost all 61 nitrogen-sensitive habitat types. For more than half, the curve provided a plausible picture of the effects of nitrogen on plant species.

The data for this method is based on the presence and absence of species over the past 70 years in much of Europe. Using the European EMEP model, the average nitrogen deposition per field location was estimated for the year that the location was visited and the four years before that. In this way an attempt was made to establish a relationship between the amount of deposition and the presence or absence of species that are characteristic of a habitat type. In addition, the coverage of so-called displacement species was examined: species that benefit from nitrogen and can therefore dominate, such as the aforementioned grasses in heathlands.

Application and follow-up

In general, both methods show that as soon as the nitrogen deposition exceeds the CDV, the quality of nature decreases. The degree of decline appears to differ per habitat type. Both methods also show that the quality of nature can already decrease in some of the habitat types before the CDW is reached, but according to Wamelink more research is needed to draw firm conclusions. The results of the gradient studies, which therefore look at the effects of different levels of nitrogen deposition per habitat type, are promising, he says. “Unfortunately, however, this type of study is still rarely performed, while this is important information for management and policy.”