De Levende Natuur nummer 1 van 2009 (English summary)

Quantitative foundation of top soil removal in The Netherlands

M.E. Sanders

The Dutch National Ecological Network is an important strategy of the Dutch government to restore and develop nature reserves and biodiversity. To complete this strategy over time, a total of 150.000 ha of agricultural land should be bought and developed into ‘new nature’. Due to heavy fertilization of the former agricultural land, the top soil of many of these new reserves is too rich and should be removed to be able to develop the desired biodiversity. Top soil removal is an expensive and radical measure. As all information on top soil removal is virtually unavailable, an attempt was made in the present study to obtain this quantitative information by using spatial information of plans and ownership.

Nature restoration by topsoil removal in Flanders (N-Belgium): a short overview

J. Van Uytvanck, K. Decleer & P. Adriaens

Data on nature restoration measures in Flanders are stocked in the Nature Restoration Database of the Research Institute for Nature and Forest (Brussels). Topsoil removal is a common tool in restoration projects. However, the total surface involved is relatively small: ± 1000 hectares (mean surface = 3,75 ha). Topsoil removal was mainly applied on former, intensively used agricultural land. A broad range of target habitats and species is set, but up till now the efficiency of the applied measures is not always well documented. Globally conservation managers evaluate developments in the field positively.

20 years practice of top soil removing on sandy soils

R.M. Bekker

Former agricultural fields in the Netherlands are being transformed into natural areas by removing the entire nutrient-rich top soil up to the mineral sand. These fields are mostly extensions of existing nature reserves and have often harbored a (semi-)natural divers community of plants and animals before its cultivation somewhere during the last century. The measure of stripping a vast layer of soil, often 20 - >100 cm, is expensive and of high impact to the landscape. It has been carried out in many projects all over the country since the 1990s. Now, after 20 years of practice, a comparative study was carried out on the effects and results of top soil removal. First of all, over 300 sites could be evaluated by reading through plans, databases and status reports of the nature management authorities, but this gave not much information about the colonization of these sites by plants and animals. A set of 42 sites was chosen to be included into a field survey during 2006 and 2007. Details about the plan, the soil removal and following management were gathered from the local managers. Vegetation structure and composition, soil chemical status and visible fauna aspects were recorded in the field. Target plant species were often found in low abundances, depending of the rate of closure of the canopy. The longer gaps are available in the sward after top soil removal, the more chance target species have to establish. This is also depending on the accuracy of top soil removal. The less nutrients (organic matter and phosphate) were left (well targeted depth of removal based on pre-research) the more successful the vegetation developed. Yet, these sites acquire a vast amount of follow-up management to prevent tree species and Common rush (Juncus effusus) from dominating the vegetation within a few years. Altogether this study reveals that top soil removal for the creation of more natural habitat is complex. It needs to be well considered against less invasive alternatives.

Restoring natural dune fringe-zones in The Netherlands

B.W.A.F.H. van den Boom, C.J.W. Bruin & H.G.J.M. van der Hagen

Because of their transitional position in between the nutrient poor, dry dunes and the nutrient rich, wet polder-area, dune fringe-zones are rich in natural gradients and subsequently they show a high variety in, often rare, plant species. Due to the intensive use of coastal sand dunes for drink water production, logging and pine planting severe desiccation occurred. Further deterioration of the dune fringe-zone was caused by sand extraction; often followed by flattening and manuring the area for flower bulb cultivation. Thus, natural dune fringe-zones have almost completely disappeared in The Netherlands. Parts of the inner dune fringes have been restored by sod cutting (followed by a grazing or mowing regime), other parts by opening up forests and where possible restoring old dune streams. These measures had mixed results, among others depending on removing the top soil. Plant and moss communities of dune streams and dune slacks mostly thrive fairly well, but in many cases these communities are a mix of the target species with species from the former agricultural use, indicating the ‘intermediate’ abiotic conditions realised. Further management as mowing is then crucial for the gradual development of meso- and oligotrophic conditions. Fauna is often badly monitored, but some cases show that an abundant array of birds and dragonflies occur in these ‘new’ habitats, favouring the wet conditions and abundant, but still nutrient rich vegetation that is realised.

Effect of top soil removal on nutrient reduction of N and P in loamy and sandy soils in the province Zeeland

F.P. Sival, W.J. Chardon, M. van Rooij & P.J. van der Reest

Successful nature development on former agricultural soils, necessary for realising the Main Ecological Structure, requires a new approach. Due to a nutrient-rich soil caused by long-term fertilization history, soil tillage and lowering of the groundwater table, only species will occur that are characteristic of a eutrophic environment. Nitrogen and phosphorus accumulation cause the eutrophication in the soil. This study has shown that the common management practice of top soil removal created oligotrophic soil conditions by reducing the amount of phosphorus and nitrogen in sandy soils. This method was less effective in loamy soils. Even though the conditions were oligotrophic, the target species and nutrient poor species were hardly found. Spatial isolation and also age of the nature reserves are probably the main causes of this.

Do butterflies benefit from nature development on sandy soils?

R.M. Bekker & M.F. Wallis de Vries

In The Netherlands former agricultural land on sandy soils has been transformed into nature development areas by removing the nutrient-rich top soil. This has already been carried out at hundreds of sites, although results have not been that positive all along. A comparative study evaluating information from over 60 sites was performed to gain insight into the factors that govern success or failure of restoration of these sites. Special attention was given to the response of butterflies to this measure. It became clear that 15 out of 16 selected butterfly species of heathland ecosystems managed to colonize these areas. Some of them, mostly Red List species, populated the stripped areas at much lower abundances than the surrounding area. However, never more than 75% of the species pool of butterflies in the surrounding areas was found in each of the top soil removal sites. Size, time after removal and vegetation development in terms of successful establishment of host plants are the main factors influencing the expansion of butterfly populations into these nature development sites on sandy soils. For rare species, the distance to source populations poses an additional constraint.

Removing the top soil of former agricultural land: good measure but is it enough?

A.J.P. Smolders, E.C.H.E.T. Lucassen, M. van Mullekom, H.B.M. Tomassen & E. Brouwer

More and more agricultural land in The Netherlands is becoming available for ecological restoration projects. However, nutrient levels in the top layer of the soils are high because the agricultural lands have been heavily overfertilized for decades. As a result, former agricultural land on moist or wet soils tends to develop extensive monotonous stands of J. effusus, which has very little value from an ecological point of view. Our findings show that the growth of J. effusus seems to be strongly determined by the Olsen-P concentration in the soil. The restoration of diverse, species-rich vegetation types on former agricultural lands will in most cases not be possible within a reasonable time-span without removing the topsoil. In most former agricultural lands, the recruitment potential for the target species from the seed bank is very poor, as many rare herbaceous species do not form a persistent seed bank. The establishment of many target species therefore depends on the presence of remaining nearby populations and their dispersal capacity. In many cases, additional measures such as the re-introduction of species deserve serious consideration. Experiences with restoration measures have shown that mowing and subsequent removal of the vegetation does not have a marked effect on nutrient availability in the medium term. Grazing may even enhance J. effusus development. Historically, the nutrient-deficient heathlands and biodiverse grasslands on sandy soils in The Netherlands have been turned into either agricultural lands or pine forest plantations. A win-win situation can be created by establishing new forest plantations on former agricultural land and re-creating heathlands and biodiverse grasslands on land dominated by low-quality pine-forest plantations. This new option should be considered seriously when the creation of oligotrophic ecosystems in former agricultural areas is an important goal.

Is it possible to remove enough phosphorus with vegetative mining instead of top soil removal?

W.J. Chardon, F.P. Sival, R.H. Kemmers, S.P.J. van Delft & G.F. Koopmans

A too high availability of phosphorus in former agricultural soils can create problems for successful nature development. In The Netherlands, removal of the top soil is common practice on these soils; however, this is not always possible. Vegetative mining by removing harvested crop can be an alternative. However, soil testing showed that phosphorus contents varied strongly on former agricultural soils, from values where removal or mining is not necessary, to values even at larger depths where both removal and mining do not make sense. Changing the aims of nature development will then be necessary. Thus, an unambiguous answer cannot be given if it is possible to remove enough phosphorus with vegetative mining. Soil testing is a prerequisite for making proper decisions about the usefulness of alternative options.

How to prevent mass development of Soft Rush after the transformation of abandoned agricultural lands into wetlands

L.P.M. Lamers, E.C.H.E.T. Lucassen, H.B.M. Tomassen, A.J.P. Smolders & J.G.M. Roelofs

After the rewetting of abandoned agricultural lands in order to recreate wetland nature, these areas often become quickly invaded by Soft rush (Common rush; Juncus effusus). This species forms dense, monotonous stands and the biodiversity of vegetation and wildlife is generally low. Shallow flooding results in blooms of algae and cyanobacteria in the surface water. This unwanted development is caused by high nutrient levels, particularly of phosphate, as a result of over-fertilization in the past. In addition, soils become more acidic after the cessation of liming for agriculture. Although Soft Rush seems to prefer acidic soil, it can also prevail under less acidic conditions. Re-liming reduces both soil acidity and the availability of phosphate, but Olsen-P levels (as an estimate for the plant available fraction) are still much higher than the threshold value for biodiverse grasslands on sand and peat, set at 300 µmol.L-1. The mobilization of phosphate to the surface water and groundwater is, however, strongly reduced, which is a significant effect. It is argued that the solution lies in removing the nutrient rich top soil, which contains hardly any seeds of target species. In order to be successful, it is vital to assess Olsen-P depth profiles for the soil in advance.

Practical aspects of the removal of top soil of former agricultural land

A. Stoker

As a consequence of a very high availability of phosphorus in former agricultural soils removal of the top soil is frequently performed to increase biodiversity. Removal of the top soil is very expensive. However, several funds are available to participate in the costs, in The Netherlands but also in the EU (LIFE). Since the removal of top soil is a typical Dutch phenomenon, explanation is needed when asking for financial support from Brussels. One of the main problems in these projects is what to do with the removed soil. Creative solutions are frequently needed; examples have been given. Projects with removal of top soil need a long preparation time, among other things because municipalities and provinces must grant licenses, which takes a long time.

Top soil removal and archaeology: problem or opportunity?

D.M. Purmer

Knowledge about archaeology and historical development can be important in the preparation of removing the top soil within nature development. This knowledge enables to incorporate historical and archaeological structures in the planning process. This article shows an opportunity to preserve and even develop the historical aspects of the site.

Top soil removal and soil organisms: not a good combination

P. Kardol, A. van der Wal, T.M. Bezemer, W. de Boer & W.H. van der Putten

Transition from agricultural land towards more natural ecosystems is a slow and often problematical process. Soil fertility can be reduced by top soil removal; however, it may have adverse effects on soil organisms. From earlier greenhouse studies we know that soil organisms are important for nutrient cycling and can affect rate and direction of plant community development. In the present paper, we describe the results of two field experiments in which we studied effects of different management strategies on vegetation development and soil organisms on ex-arable land. The first experiment involved top soil removal, carbon addition (to immobilize nutrients) and sowing mid-successional plant species. In the second experiment, which was carried out on top soil-removed land, we tested effects of spreading hay (for seeds), spreading soil (for soil organisms), and transplanting monoliths. Material was collected from a nearby site with ‘target’ vegetation. We found that introducing plants (by hay, or by seeds) was the primary driver of vegetation development and establishment of introduced plants was not negatively influenced by high concentrations of N and P. We also found that top soil removal drastically reduced the amount of soil organisms. Re-introduction of soil organisms turned out to be challenging. Future studies should attempt to enhance establishment and survival conditions of soil organisms on ex-arable land in order to make use of their contribution to grassland restoration.

Lessons learned from 20 years top soil removal for nature development

R.M. Bekker, I.C. Knevel, E.C.H.E.T. Lucassen, B.F. van Tooren & H.L. Schimmel-ten Kate

During the evaluation of the results of top soil removal projects on former agricultural land the biggest problem turned out to be the lack of information and registration of valuable details. This was mostly due to organisational problems. Suggestions have been made to overcome these problems and to organise good monitoring of the projects in the future. This is not only important from a nature point of view, but helps to learn from the experiences and to prevent obvious mistakes when developing new plans.
In The Netherlands former agricultural lands are nutrient rich. This means that in most nature development projects conservation management aims for nutrient reduction, to a level where plant growth is limited by the amounts of available N and P. Top soil removal is a fast measure to attain this, however it has a very dramatic impact on the landscape. The success of nature development after top soil removal depends highly on the soil type and the depth of top soil removal. In all cases one should measure the phosphate concentration in the soil at several depths before one decides to start digging. It is important that available phosphate in the soil is reduced to a maximum of 300 µmol P-Olsen per liter of fresh soil in combination with a maximum of 2,5 mmol total-P per liter fresh soil. The critical value of Pw is less than 0,5 µmol per liter.
Not always will top soil removal be the answer to a proper reduction of the phosphate concentration in the soil. Sometimes one will create open water by having to remove too much soil or effective top soil removal will just be too expensive. In that case other possibilities should be considered such as growing nursery crops or reduction of phosphate by mowing.