The dune water utilities face big challenges in the management and restoration of dry dune grasslands. As a result of a high atmospheric nitrogen deposition, and the associated extra accumulation of nitrogen in the soil, the ecological quality of these grasslands is deteriorating considerably. Lower levels of nitrogen deposition would allow for the restoration of biodiversity. But the question is whether, following many decades of ‘fertilisation’ with nitrogen, the soil has not been significantly altered, thus presenting an obstacle to restoration. This research project looked at the role of organic matter on the availability of nitrogen in the soil, and the relationship this has with the biodiversity of dry dune grasslands. This enables a better assessment of the restorability of this special biotope, and of the restoration strategy that needs to be developed.
Dilemma for nature management
Pursuant to national and EU regulations (Natura 2000), the dune water utilities have a significant responsibility in the management and restoration of dry dune grasslands. Over the last decades, the ecological quality of these grasslands has deteriorated considerably: they show grass encroachment and shrub formation. Dune grasslands develop on drift sand soils, in which, after stabilisation, there is a build-up of organic matter. Extra nitrogen can accumulate as a result of atmospheric nitrogen deposition, leading to the production of more biomass or greater build-up of organic matter in the soil and, thus, of nitrogen. If the nitrogen deposition falls, then the restoration into species-rich grasslands can occur. But the question is whether decades-long nitrogen fertilisation has not significantly altered the soil. In old dune soils this might have had a hysteresis (‘blockade’) effect because of the extra nitrogen accumulation, which could have consequences for restoration measures of dune grasslands. With nature restoration measures like the removal of the humus topsoil and the promotion of sand drift, the nitrogen legacy can be removed. However, these measures can be detrimental if a high biodiversity of dune grasslands is connected to older stages with a humus soil. To resolve this dilemma faced by nature managers, the research looked at the role of organic matter in the nitrogen availability, and the relationship this has with the biodiversity of dry dune grasslands. The project’s objective was to gain a better understanding of the restorability of dune grasslands and the development of an effective restoration strategy.
Looking back into the past
The research answered the question of whether a long-term high nitrogen deposition had resulted in an extra accumulation of organic matter and nitrogen in the soil of dune grasslands, and whether this is an obstacle to restoration. It also examined how the biodiversity and composition of the vegetation is connected to the build-up of organic matter and nitrogen during the soil development. The research consisted of field studies conducted in the Luchterduinen, near Amsterdam. A time series was reconstructed in the area by selecting sites where the soil varied in age (0 to 97 years). The results were compared with those of similar research conducted in Newborough in NW Wales by the Centre of Ecology and Hydrology, where the nitrogen deposition has always been low and the climate is comparable. Besides a description of the time series, dynamic modelling of the soil and vegetation was used to analyse how the increased nitrogen deposition had impacted the accumulation of carbon and nitrogen.
Effects of succession and nitrogen deposition
Dune soils accumulate most carbon and nitrogen between 10 and 40 years, after which the increase levels off. This increase is accompanied by an increase in the vegetation’s biodiversity. In soil with a high calcite content, the biodiversity remains high thereafter, while, in less calcareous soils, the biodiversity decreases again because of acidification of the top soil. A high nitrogen deposition here does not cause an accelerated accumulation of nitrogen. The absence of such an effect is closely connected to the suppression of biological nitrogen fixation in case of a strong input of mineral nitrogen. Because of the absence of a nitrogen legacy from a high deposition in calcareous dune grasslands, there is no need to remove the organic topsoil for restoration. On the contrary: a humus rich soil is actually beneficial for the biodiversity of such dune grasslands. Nature management should therefore restrain from engaging in large-scale removal of old soils. In the Luchterduinen significant soil acidification did occur in the topsoil – faster than in Wales. The acidifying action of nitrogen deposition contributed to this. Therefore, small-scale drift with deposition of calcareous sand on top of old soil is a suitable measure for the rapid restoration of species-rich dune grasslands. The soil remains intact and the biodiversity connected to base rich conditions is increased. Given the decades-long development time of species-rich dune grasslands, management planning for the restoration of dune grasslands needs to be on the same timescale. This also applies to mitigating measures against a high nitrogen deposition.