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- Materials and methods
Heathlands were once widespread in Western Europe, but their total distribution area and species composition have declined considerably. Therefore, heathlands are regarded as an internationally endangered habitat type of high conservation value (Pitcairn, Fowler & Grace 1991; Gimingham 1992; Webb 1998). In particular, the species composition of wet heath communities (Ericion tetralicis) and related wet matgrass swards (Nardo-Galion saxatilis) have changed drastically (e.g. Sansen & Koedam 1996). In the Netherlands, these species-rich ecosystems used to be common on sandy Pleistocene deposits. The soils were oligotrophic, slightly buffered (within the cation buffer range) and had a relatively low ammonium to nitrate ratio (Schaminée, Van’t Veer & Van Wirdum 1995; Schwertz, Schaminée & Dijk 1996). In many areas almost all of the endangered (target) species, such as Cirsium dissectum, Dactylorhiza maculata, Drosera intermedia, D. rotundifolia, Epipactis palustris, Gentiana pneumonanthe, Lycopodiella inundata, Narthecium ossifragum, Parnassia palustris, Pedicularis sylvatica, Rhynchospora alba, R. fusca and Succisa pratensis, have now disappeared from these ecosystems (Roelofs et al. 1996; Bobbink et al. 1998a). This decline in species diversity has been attributed to increased soil acidification as a consequence of atmospheric deposition and lowering of the groundwater table (Van Breemen et al. 1982; Houdijk et al. 1993; Roelofs et al. 1996; Bobbink, Hornung & Roelofs 1998b; Roem, Klees & Berendse 2002). Furthermore, enhanced soil nitrogen concentrations due to atmospheric deposition have resulted in the dominance of grasses over the characteristic wet heath species (Heil & Diemont 1983; Aerts & Berendse 1988; Pitcairn, Fowler & Grace 1991; Houdijk et al. 1993).
At the end of the 1980s, recognition of the deterioration of wet heaths and matgrass swards led to the introduction of restoration measures in the Netherlands. These measures aimed to restore oligotrophic and weakly buffered soil conditions to enable successful germination and establishment of the endangered wet heath and wet matgrass sward species. Examples of these restoration measures are sod cutting, which is the removal of vegetation and the topsoil layer to remove excess nutrients (Bakker 1989; Snow & Marrs 1997; Mitchell et al. 2000), and restoration of the former hydrology to increase the influence of groundwater (Roelofs et al. 1996).
In a few cases, these management practices have resulted in the restoration of species-rich wet heaths. Soil buffering capacity and pH were effectively restored, enabling endangered plant species to return to these sites or to increase their abundance (Jansen, De Graaf & Roelofs 1996; Roelofs et al. 1996). However, in many more instances these measures have resulted in species-poor dwarf shrub vegetation with few endangered plant species present. Field observations have led to the identification of possible causes. In areas where discharging groundwater had been acidified, as a result of acidification and cation depletion of the surrounding catchment areas, soil buffering capacity could not be restored. Suitable soil conditions could not be created through restoration measures (Roelofs et al. 1996). The removal of seeds from the soil seed bank by sod cutting might have been another constraint, because most seeds are concentrated in the upper layers of the soil (Putwain & Gillham 1990; Pywell, Putwain & Webb 1997). In this paper we consider a third constraint: increased soil ammonium concentrations after sod cutting, possibly up to toxic levels for endangered wet heath species. This phenomenon has been found in dry heaths (De Graaf et al. 1995, 1998b), but has not previously been studied in wet heath areas.
Several processes may cause the accumulation of ammonium after sod cutting. First, sod cutting usually does not remove all of the soil organic material. The remaining roots, in which retranslocation of nitrogen cannot take place, have a low carbon : nitrogen ratio and are readily decomposed. As a result, mineralization rates might initially be enhanced after sod cutting (Berendse 1990). Secondly, the uptake of ammonium by plants is eliminated through removal of the vegetation by sod cutting. Thirdly, atmospheric deposition of ammonium is high in the Netherlands (Bobbink & Heil 1993; Erisman, Bleeker & Van Jaarsveld 1998). Finally, it has been suggested that sod cutting removes nitrifying bacteria, which are mainly present in the topsoil layer (Troelstra, Wagenaar & De Boer 1990). The conversion of ammonium into nitrate by these micro-organisms might therefore be considerably reduced. Because of these processes, the ammonium input into the soil is increased, whereas the uptake of ammonium and conversion into nitrate are reduced, resulting in the accumulation of ammonium in the topsoil layer.
It has been shown in hydroculture experiments that high ammonium concentrations have negative effects on the germination and survival of several endangered heath species (De Graaf et al. 1998a; Lucassen et al. 2003). However, the response of such species to high ammonium concentrations has never been investigated in soil column or field experiments. Research may provide insights into the constraints on the current restoration measures in wet heaths and matgrass swards.
This paper describes the results of a field study to determine whether ammonium accumulates in the soils of wet heaths after sod cutting. In addition, effects of increased ammonium concentrations on the germination and survival of seeds of wet heath species were examined using field and soil column experiments.
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- Materials and methods
Experimental sod cutting of two degraded Dutch wet heaths resulted in a strong accumulation of ammonium in the upper 10 cm of the soil. The soil ammonium concentrations increased for a period of 10–12 months and reached maximum values of about 570 µmol kg−1 dry soil in both Leemputten and the Molinia-dominated zone of Havelte-Oost. In the untreated vegetation the ammonium concentration did not exceed 150 µmol kg−1 dry soil. Such an accumulation of ammonium has previously been found in dry heaths (De Graaf et al. 1995, 1998b), but has never been measured in wet heathland ecosystems.
The accumulation of ammonium after sod cutting can be attributed to several factors, such as atmospheric deposition of ammonium and the absence of uptake by plants. The temporary decrease of the carbon : nitrogen ratio of the soil organic matter, as a result of high nitrogen contents of remaining roots after sod cutting, will also contribute to increased mineralization rates (Berendse 1990). Another important factor might be the decreased nitrification. Because nitrifying bacteria are mainly present in the topsoil layers (Troelstra, Wagenaar & De Boer 1990), they may be removed by sod cutting. Preliminary results of soil incubation experiments showed no nitrification in soil of sod-cut plots, in contrast to soil of untreated plots (our unpublished data). The decrease in ammonium concentrations of the sod-cut plots, which was found in both areas after about 10 months, might then be explained by the recovery of the populations of nitrifying bacteria. However, future experiments are necessary to elucidate the effect of sod cutting on the nitrification process.
In the Molinia-dominated zone of Havelte-Oost a sharp decline in the ammonium concentration of the sod-cut plots was found in November 2000. Meteorological data for September–November indicated that total precipitation during this period was much higher compared with long-term average values (260 and 222 mm, respectively, KNMI 2001). These high monthly precipitations were also reflected in the height of the groundwater table in Havelte-Oost. It is speculated that soil ammonium concentrations of the sod-cut plots were diluted, resulting in the sharp decrease. In Leemputten, the groundwater table was higher compared with Havelte-Oost and was hardly influenced by rainfall fluctuations. Soil ammonium concentrations were therefore not affected by changes in the groundwater table.
The difference in ammonium concentrations between the Molinia- and Erica-dominated zones of Havelte-Oost is interesting. In the latter, soil ammonium concentrations were much lower in the sod-cut plots as well as in the untreated vegetation. In this zone, sod cutting resulted in an increase in ammonium concentration, but to a much smaller extent. This can be explained by the difference in dominant plant species between these zones. Increasing dominance of Molinia is known to increase the flow of carbon and nutrients into the litter fraction of the soil (Aerts & Berendse 1988; Berendse 1990). The percentage of total carbon in the soil of the Molinia-dominated zone was indeed higher than that of the Erica-dominated zone (4·2 and 3·3%, respectively). In addition, the carbon : nitrogen ratios of the soil in the Molinia-dominated zone were lower than those of the Erica-dominated zone (on average 26 and 31, respectively). Because decomposition increases with decreasing carbon : nitrogen ratio, mineralization rates in the Molinia-dominated zone will be higher compared with the Erica-dominated zone (Berendse 1990; Van Vuuren et al. 1992). As a result, the ammonium accumulation after sod cutting was also higher in the zone dominated by Molinia.
KCl-extractable soil ammonium concentrations as high as 570 µmol kg−1 dry soil, found after sod cutting in both degraded wet heaths, are likely to hamper the restoration of formerly species-rich wet heathlands and matgrass swards (Gigon & Rorison 1972; De Graaf et al. 1998a; Lucassen et al. 2003). To compare the field ammonium concentrations with those of the glasshouse experiments, we used the concentrations of the extractions with demineralized water instead of those of the 0·2 m KCl extractions. The patterns of the water-extractable ammonium concentrations in both wet heaths areas were the same as in Figs 1–3, because a strong positive correlation between the ammonium concentrations of these methods existed (NH4–demi = 0·468 × NH4–KCl + 8·481; R2 = 0·789, P = 0·000, n = 493). It is clear that the highest water-extractable ammonium concentrations reached in the field (namely 275 µmol kg−1 dry soil) were above the 251 µmol kg−1 dry soil of the glasshouse 1000 µm treatment, at which significant negative effects on the germination and survival of Cirsium dissectum and Succisa pratensis were found. Furthermore, Lucassen et al. (2003) showed in hydroculture experiments that Cirsium dissectum could tolerate ammonium concentrations up to 250 µmol L−1 when the soil pH was relatively high (pH 6). However, at pH 4, detrimental effects of ammonium on growth and survival of this species were found at concentrations as low as 100 µmol L−1 (De Graaf et al. 1998a; Lucassen et al. 2003). Because soil pH values in wet heaths and matgrass swards are usually between 4 and 5 (Table 1; Roelofs et al. 1996; De Graaf et al. 1998b), the negative effects of high ammonium concentrations on germination and establishment of the endangered species might be substantial. This is also reflected by the poor germination of these species found in the sod-cut plots in both areas. Low germination and poor establishment of target species after sod cutting were found in the restoration of dry heaths (De Graaf et al. 1998b) and also in Cirsio dissecti–Molinietum grasslands (Jansen & Roelofs 1996). In the sod-cut wet heath plots, target species, except for Erica tetralix, were rarely found. By contrast, Molinia caerulea germinated in all sod-cut plots and had the highest cover percentage after two growing seasons. This fast-growing grass species has a broad tolerance of soil conditions and it can become dominant over the endangered wet heath species because it can respond more efficiently to the current high nutrient availability (Berendse & Aerts 1984; Aerts & Berendse 1988; Sansen & Koedam 1996). The poor soil conditions after sod cutting were further illustrated by small-scale growth experiments with seedlings of Cirsium dissectum (data not shown). One year after the start of these experiments, the survival percentage of the seedlings was only 3% in Leemputten (n = 72) and 8% in Havelte-Oost (n = 120).
The lack of viable seeds of wet heath target species, as a consequence of the removal of seeds by sod cutting or poor seed dispersal possibilities in the fragmented Dutch landscape, has also been proposed as a bottleneck in the restoration of species-rich wet heaths and matgrass swards (Jansen, De Graaf & Roelofs 1996; Britton et al. 2000). Because most seeds of wet heath species are concentrated in the topsoil layers (Putwain & Gillham 1990; Pywell, Webb & Putwain 1995), sod cutting is likely to remove many of these seeds. However, this will have played only a minor role in the degraded parts of the two wet heath study areas. These degraded parts are already dominated by Molinia caerulea, the endangered species having disappeared several years ago. Because seeds of most of the endangered wet heath target species are short-lived (Thompson, Bakker & Bekker 1997), the presence of viable seeds of these species in the upper soil layers will probably be low.
It is also possible that poor seed dispersal of the target species may have contributed to the low germination and establishment in the field plots. Although populations of the target species are still present in the less degraded areas within 10–50 m of the sod-cut plots, seed dispersal from these plants to the sod-cut plots might be low (Bakker & Berendse 1999). Analysis of the soil seed banks of these two areas will elucidate this matter. However, even if seed dispersal is abundant, our results show that successful germination and establishment of the target species will be hampered by the inappropriate soil conditions after sod cutting.
In conclusion, our results indicate that sod cutting alone may not be an effective measure to restore formerly species-rich wet heaths and matgrass swards. The accumulation of ammonium after sod cutting hampers the germination and establishment of target species. Therefore, additional measures are necessary to prevent or reduce this build-up of ammonium and to overcome a possible scarcity of seeds. Experiments are in progress to investigate if the application of lime after sod-cutting might be an effective additional restoration measure.