Expansion of invasive species on ombrotrophic bogs: desiccation or high N deposition?


  • Hilde B. M. Tomassen,

    Corresponding author
    1. Department of Aquatic Ecology and Environmental Biology, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, the Netherlands; and
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  • Alfons J. P. Smolders,

    1. Department of Aquatic Ecology and Environmental Biology, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, the Netherlands; and
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  • Juul Limpens,

    1. Nature Conservation and Plant Ecology Group, Wageningen University, Bornsesteeg 69, 6708 PD Wageningen, the Netherlands
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  • Leon P. M. Lamers,

    1. Department of Aquatic Ecology and Environmental Biology, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, the Netherlands; and
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  • Jan G. M. Roelofs

    1. Department of Aquatic Ecology and Environmental Biology, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, the Netherlands; and
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Hilde B. M. Tomassen, Department of Aquatic Ecology and Environmental Biology, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, the Netherlands (fax + 31 24 3652134; e-mail Hilde.Tomassen@sci.kun.nl).


  • 1In many ombrotrophic bog areas the invasion of grass (e.g. Molinia caerulea) and tree (e.g. Betula pubescens) species has become a major problem. We investigated whether the invasion of such species is due to high atmospheric nitrogen (N) deposition by conducting a fertilization experiment.
  • 2The effects of experimentally increased N input on Molinia, Betula and Eriophorum vaginatum were studied in desiccated bog vegetation in Ireland, where there is relatively low background N deposition. Four different N treatments were applied for 3 years: 0 (control), 2, 4 and 8 g m−2 year−1.
  • 3Ammonium and nitrate concentrations in the peat moisture increased at high N addition rates, leading to significantly higher carbon : nitrogen (C : N) and nitrogen : phosphorus (N : P) ratios in the top layer of the peat. The potential CO2 production rate of the peat was not stimulated at high N addition rates due to severe acidification of the peat.
  • 4Despite high tissue N : P ratios (above 40), above-ground biomass production by Molinia was stimulated at high N addition rates, and foliar nutrient concentrations were unaffected. In contrast to Molinia, Betula and Eriophorum were unable to increase their above-ground biomass, probably due to P limitation. Regrowth of the lichen Cladonia portentosa was suppressed at high N addition rates.
  • 5Synthesis and applications. We conclude that the invasion of bogs by Molinia and Betula is likely to be less affected by desiccation than by increased N availability. Apparently, Molinia is well adapted to P-limiting conditions, which may explain its success in regions with increased N deposition levels. The high availability of P in many Dutch bogs compared with Irish bogs, together with prolonged high N deposition levels, may explain the strong increase in both Molinia and Betula observed in the Netherlands. As long as N and P availabilities in Dutch bogs are too high to prevent invasion of Betula and/or Molinia, management measures stimulating growth of Sphagnum mosses could probably reduce the negative effects of high N deposition levels.


Levels of atmospheric nitrogen (N) deposition have greatly increased over large parts of north-west Europe during the past century, reaching levels of 1·5–6 g N m−2 year−1 (Bobbink & Heil 1993). There is clear evidence that this has forced ecological changes in oligotrophic N-limited ecosystems (Roelofs 1986; Aerts, Wallén & Malmer 1992; Bobbink, Hornung & Roelofs 1998). For dwarf shrub heathlands, increases in the long-term rate of N supply can cause a shift in the composition of the vegetation towards more competitive species (Heil & Diemont 1983; Berendse & Aerts 1984; Roelofs 1986; Aerts & Berendse 1988), a process accompanied by the loss of more sensitive species (Press, Woodin & Lee 1986; Pitcairn, Fowler & Grace 1995). At the end of the 1960s, a rapid loss of the lichen and bryophyte flora was observed in the Netherlands, due to high atmospheric N levels (Van Dobben, De Witt & Van Dam 1983; De Smidt & Van Ree 1991; Greven 1992). Among the ecosystems that are most sensitive to N enrichment are ombrotrophic bogs, which receive most of their nutrients from the atmosphere (Bobbink, Hornung & Roelofs 1998). The critical load for ombrotrophic bogs (the maximum amount of deposition that does not lead to changes in the composition of the vegetation) has been estimated to be between 0·5 and 1 g N m−2 year−1 (Bobbink & Roelofs 1995; Risager 1998; Tomassen et al. 2003).

Ombrotrophic bogs are unique wet ecosystems, supporting characteristic plant and animal communities. World-wide, the area of bogs has been drastically reduced by human activities such as peat cutting, and bogs often suffer the effects of drainage. As a response to the present-day rarity of typical bog vegetation, the conservation and restoration of bogs has become important (Joosten 1995). In areas with increased N deposition levels the invasion of ombrotrophic bogs by certain nitrophilous grass (e.g. Molinia caerulea) and tree (e.g. Betula pubescens) species has occurred, together with a decline of ombrotrophic species (Barkman 1992; Aaby 1994; Hogg, Squires & Fitter 1995; Risager 1998). Under wet conditions Sphagnum has high N uptake rates, resulting in low N concentrations in the peat moisture (Lee & Woodin 1988; Jauhiainen, Wallén & Malmer 1998; Lamers, Bobbink & Roelofs 2000), with vascular plants depending on N mobilized by mineralization processes in the underlying peat (Malmer, Svensson & Wallén 1994). At high N deposition rates (> 2 g N m−2 year−1), however, the Sphagnum layer reaches its maximum N content and N leaches from the Sphagnum layer to the roots of vascular plants (Lee & Woodin 1988; Aerts, Wallén & Malmer 1992; Lamers, Bobbink & Roelofs 2000).

Several authors, however, have ascribed the invasion of nitrophilous species to increased mineralization rates as a result of desiccation (Eigner 1995) rather than to increased inputs of air-borne N. Hayward & Clymo (1983) suggested that increased nutrient availability in a situation of desiccation is not only the result of increased mineralization rates, but is probably also caused by reduced growth of Sphagnum mosses at low water tables. In desiccated Sphagnum, gas exchange is suppressed, resulting in reduced photosynthetic rates and growth (Schipperges & Rydin 1998), reduced immobilization of nutrients by the Sphagnum layer (Aldous 2002) and increased availability of nutrients for vascular plants. In addition, during periods of surface water stress the productivity of Sphagnum mosses may be much lower than that of vascular plants (Malmer, Svensson & Wallén 1994). Saplings of Betula, for instance, are less rapidly overgrown by Sphagnum suffering from water stress.

In the Irish midlands no invasion of Molinia and Betula has yet occurred, even on (non-excavated) severely desiccated bog areas (H.B.M. Tomassen, personal observations). As N deposition levels in the Netherlands are much higher than in the Irish midlands, differences in peat moisture N availability could be responsible for the vegetation changes observed in the Netherlands. This idea is supported by the observed invasion of Molinia and Betula in ombrotrophic vegetation on floating rafts that are permanently wet but are subjected to high N loads (Lamers, Bobbink & Roelofs 2000). Because experimental evidence is lacking, it remains unclear whether the invasion of Dutch bogs by Betula and Molinia is due to desiccation, high N deposition levels or a combination of the two.

We hypothesized that the observed vegetation changes in the Netherlands are mainly the result of high N deposition levels, leading to high N concentrations in the peat moisture, thereby promoting the growth of nitrophilous species such as Molinia, Betula and Eriophorum vaginatum. To test this hypothesis, the growth of Molinia, Betula and E. vaginatum was investigated in a 3-year N fertilization experiment on a desiccated location in an Irish raised bog. Four different N addition rates were used, ranging from 0 to 8 g m−2 year−1. In addition, differences in the nutritional status of Dutch and Irish bogs were determined by comparing nutrient concentrations in peat moisture samples.

Materials and methods

field site

We studied the effects of high N deposition levels in a field experiment in Ireland (Clara bog) to avoid the high current background N input of 4 g m−2 year−1 in the Netherlands. The Irish bogs resemble the former Dutch bogs closely, but background N deposition levels are considerably lower. Clara bog (County Offaly; 53°19·504′N, 7°36·544′W) is situated in a depression adjacent to a complex of esker ridges, in the midlands of Ireland. Clara bog (665 ha) is 4·24 km long and 1·20 km wide and divided by a road into a western and an eastern part. In 1983, the eastern part of Clara bog was drained in preparation for peat extraction. Although the drains were blocked in 1989 and 1995–96, large parts of Clara East are still severely desiccated due to drainage and small-scale peat extraction. The experimental site was located at the edge of a cut-away, where the peat was severely desiccated. Sphagnum had completely disappeared from this site, which is currently dominated by Calluna vulgaris (L.) Hull (0·5–0·75 m height), Cladonia portentosa (Dufour) Coem. and some Eriophorum angustifolium Honck.

background deposition level

Between March 2000 and March 2001, background deposition levels of nutrients were measured at the experimental site by collecting bulk precipitation. Each of the rainwater collectors (n = 3) consisted of a black polyethylene bottle (2 l) connected to a funnel (74·5 cm2) containing 2 ml of a 200-mg l−1 HgCl2 solution to inhibit microbial activity. A plastic filter was placed in the funnel to avoid contamination of the rainwater by organic matter and insects. The collector was placed in a PVC pipe dug into the peat, and rainwater was collected 20 cm above ground level. Six times a year, samples were collected and their volume measured in the field. pH was determined within 4 h of collection and the samples were stored at −20 °C until further analysis.

experimental design

In July 1998, 16 plots measuring 150 × 150 cm were set out on the eastern part of Clara bog. Before the target species were introduced, the above-ground biomass of the standing vegetation was removed to avoid shading by the dense Calluna vegetation. In each plot, three Betula pubescens Ehrh. saplings, one Molinia caerulea (L.) Moench tussock and one Eriophorum vaginatum L. tussock were introduced, all of which had been collected from a nearby cut-away.

Three soil moisture samplers (Rhizon, 10 cm; Eijkelkamp Agrisearch Equipment, Giesbeek, the Netherlands), were placed in each of the plots, to a depth of 0–10 cm, to allow extraction of the peat moisture for the purpose of analysing its chemical composition. Four treatments were applied, differing in N concentrations and leading to N loads of 0, 2, 4 and 8 g N m−2 year−1 (0–0·57 mol N m−2 year−1). Each treatment consisted of four replicates, randomly assigned to the plots. N was partially supplied as ammonium sulphate, as co-deposition of sulphate and ammonium has frequently been observed in the Netherlands (Bobbink & Heil 1993). N was added as ammonium (70%) and nitrate (30%) using NH4NO3 and (NH4)2SO4, based on the current ratios in the Netherlands (1994 situation; Lamers 1995), which resulted in additional sulphur (S) loads of 0, 0·9, 1·8 and 3·7 g S m−2 year−1 (0–0·11 mol S m−2 year−1). N was dissolved in local bog water (5 l) and sprayed over the plots with a watering can. We were careful to add the nutrients during rainy weather, or the nutrients were applied in a smaller volume of bog water after which the remainder was used to wash away the nutrients and to avoid damage to the leaves. The experiment lasted from July 1998 to September 2001, N being supplied in 12 monthly applications during the first year. In the subsequent 2 years, due to practical limitations N was added in six applications during the growing season. The controls received only bog water in the same amount used in the N treatments.

sampling and measurements

Before every application of N, peat moisture was collected by connecting 50-ml syringes to each sampler. The three subsamples were pooled and pH was measured. Citric acid was added to a final concentration of 0·6 mmol l−1 to prevent metal precipitation; samples were then stored (for a maximum of 6 weeks) in iodated polyethylene bottles (50 ml) at −20 °C until further analysis.

Growth of Betula, Molinia and E. vaginatum was estimated non-destructively twice a year. Growth of Betula was estimated from the height and the number of leaves, that of Molinia from the number and length of leaves and inflorescences, that of E. vaginatum from the diameter, number of inflorescences and height. At final harvest, the above-ground biomass of Betula, Molinia and E. vaginatum was carefully cut and sorted into leaves, stems and inflorescences. Regrowth of Calluna and Cladonia portentosa was measured by estimating the cover. Next, samples were taken from Betula, Molinia, E. vaginatum, Calluna and Cladonia portentosa for foliar nutrient analysis. Peat samples were taken (five subsamples per plot; 0–5 cm depth) and stored at 4 °C in airtight polyethylene bags until further analysis.

nutritional status of dutch bogs vs. irish bogs

Between 1998 and 2001, nutrient concentrations in the peat moisture were measured at various locations in the Netherlands [ntotal = 12: Bargerveen (n = 3), Dwingelerveld (n = 3), Fochteloërveen (n = 1), Haaksbergerveen (n = 2), Korenburgerveen (n = 2) and Tuspeel (n = 1)] and in Ireland [ntotal = 7; Clara bog (n = 3), Raheenmore bog (n = 2) and Sharavogue bog (n = 2)]. The vegetation at the experimental sites was dominated by Sphagnum species. Peat moisture was collected four times a year at a depth of 0–10 cm using ceramic soil moisture samplers (Eijkelkamp Agrisearch Equipment) connected to 50-ml syringes. Samples were stored in iodated polyethylene bottles (50 ml) at −20 °C until further analysis.

chemical analysis

Peat moisture pH was determined with a combination pH electrode with an Ag/AgCl internal reference (Orion Research, Beverly, CA). Peat samples were thoroughly mixed and large roots were removed. To analyse nutrient concentrations in plant tissue and peat, dried samples (48 h at 70 °C) were ground in liquid N. N and carbon (C) concentrations were measured with a CNS analyser (type NA1500; Carlo Erba Instruments, Milan, Italy). Phosphorus (P), potassium (K) and S concentrations were determined by digesting 200 mg of dried material in sealed Teflon vessels in a Milestone microwave oven (type mls 1200 Mega, Milestone Inc., Sorisole, Italy) after addition of 4 ml HNO3 (65%) and 1 ml H2O2 (30%). After dilution, the digests were kept at 4 °C until analysis.

Water-extractable and exchangeable nutrients and minerals were determined by adding 200 ml bi-distilled water or 0·2 mol l−1 NaCl, respectively, to 40 g of thoroughly mixed peat. The mixtures were shaken for 1 h (100 movements min−1; VKS 75 Shaker, Edmund Bühler, Tübingen, Germany), after which the pH of the solution was measured. The suspension was then centrifuged (12 000 r.p.m. for 20 min; 23 500 g) and the supernatant stored in 50 ml polyethylene bottles at −20 °C. Potential C mineralization rates (CO2 production) were measured by aerobic incubation of 100 g peat in 250-ml infusion flasks, sealed with an airtight stopper. For each plot incubations were carried out in duplicate. The flasks were kept in the dark at 20 °C, and CO2 concentrations were measured in the headspace, using an infrared C analyser (model PIR-2000; Horiba Instruments, Irvine, CA, USA), for 6 weeks. CO2 production rates were calculated by linear regression and expressed on a dry weight (DW) basis.

The concentration of ortho-phosphate was measured colorimetrically with a Technicon AA (Auto Analyser) II system (Technicon Instruments Corp., Tarrytown, NY, USA), using ammonium molybdate (Henriksen 1965). Nitrate and ammonium were measured colorimetrically with a Traacs 800+ auto-analyser, using hydrazine sulphate (Technicon 1969) and salicylate (Grasshoff & Johannsen 1977), respectively. K was measured by flame photometry (FLM3 Flame Photometer; Radiometer, Copenhagen, Denmark). S and P were determined by inductively coupled plasma emission spectrophotometry (type FLAMEVML2-9032034; Spectro Analytical Instruments, Kleve, Germany).

statistical analysis

Prior to statistical analysis, data were log-transformed to make the variance less dependent on the means and to fit a normal distribution. All statistical analyses were carried out using the SPSS for Windows software package (version 10·0·7; SPSS Inc., Chicago, IL). Differences between treatments were tested with a one-way anova at the 0·05 confidence limit. Tukey post-hoc tests were used to identify differences between treatments. Differences in peat moisture concentrations during the experiment were tested with the general linear model (GLM) procedure for repeated measures. One experimental plot of the control treatment was found to be situated in a small depression and remained significantly wetter during the experiment. This wet plot produced more Molinia biomass; this replicate was therefore excluded from the data set (Dixon's test: P= 0·01; Sokal & Rohlf 1981). Differences in growth parameters of Molinia between treatments were identified using a univariate GLM procedure with a Tukey post-hoc test. Possible correlations between growth parameters and peat moisture N concentrations, and between peat characteristics and potential CO2 production rates, were assessed with a Pearson correlation. Differences in nutrient concentrations between Dutch and Irish bogs were tested with an independent samples t-test. For clarity of presentation, the means and standard errors (SE) presented in the figures represent the non-transformed data.


background deposition levels on clara bog

The bulk N deposition level at Clara bog was 0·46 g N m−2 year−1, comprising 30% oxidized N (NOx) and 70% reduced N (NHx) (Table 1). The deposition level of P was 0·01 g P m−2 year−1 and the bulk deposition level of S was 0·38 g S m−2 year−1. At the experimental site, N applications constituted an extra rainfall of 13 mm year−1. The amount of N supplied by the bog water used was negligible (Table 1).

Table 1.  Amount of precipitation and bulk deposition levels of N (NH4 and NO3), P, K and S on Clara bog (mean ± 1 SE; n= 3; March 2000–March 2001) and in the ‘Pikmeeuwenwater’ (the Netherlands, 1994–95; L.P.M. Lamers & H.B.M. Tomassen, unpublished data), and the flux of nutrients deposited by added bog water (in 2001)
 Precipitation (mm)N-NH4 (g m−2 year−1)N-NO3 (g m−2 year−1)N (g m−2 year−1)P-inline image (g m−2 year−1)K (g m−2 year−1)S (g m−2 year−1)
Bulk deposition level Clara bog1003 ± 360·32 ± 0·050·13 ± 0·000·46 ± 0·050·01 ± 0·000·25 ± 0·060·38 ± 0·02
Bulk deposition level Pikmeeuwenwater 8741·260·701·960·030·320·99
Bog water  130·0010·0000·0020·0010·0090·017

water chemistry

Additions of N led to a significant increase in peat moisture ammonium and nitrate concentrations (P = 0·001; Fig. 1). The N concentrations showed a seasonal pattern, with high concentrations in summer and low concentrations in winter. Concentrations measured during the summer of 2001 were higher than those measured in the summer of 2000, especially in the highest N treatment (Fig. 1). Ammonium and nitrate concentrations remained low in the control and 2 g N m−2 year−1 treatments. Addition of 4 and 8 g N m−2 year−1 increased ammonium and nitrate concentrations significantly. Sulphate (measured as total S) concentrations were significantly increased at higher addition rates (P = 0·001; GLM for repeated measures; data not shown), as ammonium was partly added as ammonium sulphate. Peat moisture sulphate concentrations showed a similar seasonal pattern as the ammonium and nitrate concentrations, with high concentrations in summer.

Figure 1.

Peat moisture nitrate and ammonium concentrations (mean ± 1 SE; n= 4) from March 2000 until September 2001 at various experimental N addition rates: control (0), 2, 4 and 8 g N m−2 year−1. Amount of precipitation is indicated by the dotted line. N effect: P= 0·001 (GLM for repeated measures).

The pH in the peat moisture was very low during the experiment and ranged from 4·0 to 2·8, being significantly lower at the higher N addition rates (P = 0·05; GLM for repeated measures; data not shown). The pH decreased during the summer and increased again in winter. For all N treatments, phosphate and K concentrations were below 1 µmol l−1 and 20 µmol l−1, respectively, and did not differ significantly between the treatments (data not shown).

peat chemistry

After 3 years of experimental N addition, C : N, C : P and N : P ratios of the peat were significantly affected (Table 2). C : N ratios were significantly lower (P = 0·01), and C : P ratios were significantly higher (P = 0·05), than in the control treatment at an addition rate of 8 N g m−2 year−1. N : P ratios at addition rates of 4 g N m−2 year−1 and higher were significantly increased compared with the control treatment (P = 0·001; Table 2). There was no correlation between potential CO2 production rates of the peat and the various nutrient ratios, but higher pHNaCl of the peat significantly increased potential CO2 production rates (P = 0·01; Fig. 2). Water-extractable ammonium and sulphate concentrations were increased at extra N loads of 4 and 8 g m−2 year−1, respectively (Pammonium = 0·001 and Psulphate = 0·05; Table 3). Water-extractable ammonium concentrations did not differ between the control and 2 g m−2 year−1 treatments. Doubling the addition rate from 2 to 4 g N m−2 year−1 increased water-extractable ammonium concentrations threefold, and doubling input rates from 4 to 8 g N m−2 year−1 led to a fivefold increase. Ammonium concentrations measured in sodium extractions were two to six times as high as those in water extractions (Table 3).

Table 2.  C : N, C : P and N : P ratios and potential CO2 production rates of the peat (mean ± 1 SE; n= 4) at various N addition rates, after 3 years. Different letters indicate significant differences (P = 0·05) between N treatments (one-way anova)
N addition rate (g m−2 year−1)C : N ratio (g g−1)C : P ratio (g g−1)N : P ratio (g g−1)Potential CO2 production rate (µmol g−1 DW day−1)
Control (0)37 ± 1a1409 ± 56a38 ± 1a11·81 ± 1·17
238 ± 1a1561 ± 24ab41 ± 0ab 9·30 ± 0·55
435 ± 0ab1552 ± 23ab44 ± 1bc10·43 ± 0·77
834 ± 0b1618 ± 40b47 ± 1c10·94 ± 0·55
Figure 2.

Potential CO2 production rate and pHNaCl of the peat after 3 years of exposure to various experimental N addition rates. Pearson correlation: P= 0·01.

Table 3.  Concentrations of water-extractable nitrate, ammonium, phosphate and sulphate and exchangeable ammonium in the top layer of the peat (µmol kg−1 DW; mean ± 1 SE; n= 4) after 3 years at various N addition rates. Different letters indicate significant differences (P = 0·05) between N treatments (one-way anova)
N addition rate (g m−2 year−1)inline image(H2O) (µmol kg−1 DW)inline image(H2O) (µmol kg−1 DW)inline image(H2O) (µmol kg−1 DW)inline image(H2O) (µmol kg−1 DW)inline image(NaCl) (µmol kg−1 DW)
Control (0) 45 ± 9 140 ± 21a69 ± 9 791 ± 96a  257 ± 41a
2 74 ± 11 129 ± 16a48 ± 8 758 ± 56a  459 ± 216a
4 74 ± 25 423 ± 42b64 ± 21 997 ± 115ab 1706 ± 482b
8143 ± 192261 ± 264c71 ± 111360 ± 165b13189 ± 1832c

biomass and nutrient concentrations of the vegetation

Above-ground biomass production of Molinia was significantly stimulated by increased N addition rates over the 3-year period (P = 0·05; Fig. 3). At high N addition rates, numbers of leaves and inflorescences as well as height were slightly increased (data not shown). After 3 years of N addition, peat moisture N concentrations and above-ground biomass of Molinia were significantly correlated (P = 0·01; Fig. 4). Foliar nutrient concentrations of Molinia did not differ between the various N treatments, and N : P ratios ranged from 41 to 52 (Table 4).

Figure 3.

Above-ground biomass of Molinia caerulea after 3 years of exposure to various rates of experimental N addition (mean + 1 SE; n= 4). Different letters indicate significant differences (P = 0·05) between N treatments (univariate GLM).

Figure 4.

Correlation between above-ground biomass of Molinia caerulea and total inorganic N concentrations in the peat moisture (average concentrations during the 2001 growing season) at various rates of experimental N addition. Pearson correlation: P = 0·01.

Table 4.  Major nutrient and S concentrations, and N : P ratios (mean ± 1 SE; n= 4), in Molinia caerulea, Betula pubescens, Eriophorum vaginatum, Calluna vulgaris and Cladonia portentosa after 3 years at various N addition rates. Different letters indicate significant differences (P = 0·05) between N treatments (one-way anova)
 N addition rate (g m−2 year−1)N (mg g−1 DW)P (mg g−1 DW)K (mg g−1 DW)S (mg g−1 DW)N : P ratio (g g−1)
MoliniaControl (0)17·2 ± 1·30·38 ± 0·037·96 ± 0·501·56 ± 0·1246 ± 5
218·8 ± 0·90·45 ± 0·028·01 ± 0·561·83 ± 0·1341 ± 2
418·0 ± 1·70·41 ± 0·068·61 ± 0·562·05 ± 0·2546 ± 5
820·3 ± 0·90·39 ± 0·036·99 ± 0·492·07 ± 0·0552 ± 4
BetulaControl (0)13·3 ± 1·7a0·62 ± 0·143·84 ± 0·361·04 ± 0·1024 ± 5ab
218·6 ± 2·6ab1·00 ± 0·257·91 ± 1·872·15 ± 1·4220 ± 2a
420·9 ± 2·4ab0·57 ± 0·074·79 ± 1·051·34 ± 0·1037 ± 3b
827·4 ± 3·3b0·40 ± 0·084·88 ± 0·241·25 ± 0·4376 ± 14c
E. vaginatumControl (0) 9·4 ± 0·90·53 ± 0·065·17 ± 0·160·98 ± 0·0418 ± 1
2 9·5 ± 0·40·51 ± 0·085·47 ± 0·450·90 ± 0·0220 ± 3
4 9·2 ± 0·60·47 ± 0·055·20 ± 0·570·95 ± 0·0420 ± 1
810·4 ± 0·70·43 ± 0·104·33 ± 0·750·99 ± 0·0427 ± 4
CallunaControl (0)10·7 ± 0·4a0·37 ± 0·013·21 ± 0·311·25 ± 0·02a29 ± 1a
211·3 ± 0·3a0·36 ± 0·013·46 ± 0·341·30 ± 0·04a32 ± 1a
414·1 ± 1·2b0·35 ± 0·022·96 ± 0·311·45 ± 0·08ab40 ± 1b
819·7 ± 0·4c0·37 ± 0·023·03 ± 0·291·67 ± 0·04b54 ± 5c
CladoniaControl (0) 6·0 ± 0·4a0·26 ± 0·040·95 ± 0·060·68 ± 0·05a24 ± 2a
2 8·8 ± 0·2b0·24 ± 0·000·92 ± 0·030·95 ± 0·03b36 ± 1b
4 9·7 ± 0·6b0·26 ± 0·031·09 ± 0·111·09 ± 0·07b38 ± 2b
813·8 ± 0·0c0·24 ± 0·011·07 ± 0·051·46 ± 0·04c58 ± 2c

N addition had no significant effects on the growth of Betula and E. vaginatum (data not shown), nor on the foliar nutrient concentrations of E. vaginatum (Table 4). N : P ratios were above 18 for all treatments, suggesting that the growth of E. vaginatum was limited by P rather than N (Koerselman & Meuleman 1996). Growth conditions seemed unsuitable for the Betula saplings, and some of them even died, regardless of the N treatment level. Foliar N concentrations and N : P ratios in Betula increased significantly at addition rates of 8 g N m−2 year−1 (PN = 0·05 and PN : P ratio= 0·001; Table 4). N : P ratios in Betula leaves ranged from 20 to 76, suggesting growth limitation by P (Koerselman & Meuleman 1996).

At the start of the experiment the above-ground biomass of the natural vegetation was removed. During the 3 years of experimental N addition, these species produced new biomass. At the control treatment, 31% of the plot was covered with the lichen Cladonia portentosa. N addition suppressed this regrowth dramatically (P = 0·001): in the plots treated with extra loads of 4 and 8 g N m−2 year−1, less than 1% of the surface was covered by Cladonia portentosa (Fig. 5). N and S concentrations in Cladonia portentosa were significantly increased at addition rates of 2 g N m−2 year−1 or higher (Table 4). During the first year, regrowth of Calluna was slightly greater at higher N loads, but differences were not significant after 3 years (data not shown). Foliar N and S concentrations of Calluna were significantly higher at N loads of 4 and 8 g m−2 year−1 (P = 0·001; Table 4). N : P ratios in Calluna were far above 16, suggesting its growth was greatly limited by P (Koerselman & Meuleman 1996).

Figure 5.

Treatment effects on regrowth of Cladonia portentosa after 3 years (mean + 1 SE; n = 4). Different letters indicate significant differences (P = 0·05) between N treatments (one-way anova).


background deposition levels of nutrients

Our data on both the quantity and composition of the bulk deposition (Table 1) differed from measurements at two nearby rainfall monitoring stations (Jordan 1997). Annual bulk deposition levels measured at the Kinnitty and Birr monitoring stations (at distances of 41 and 49 km, respectively) were two to three times lower than our measurements at Clara bog. The amount of oxidized N was comparable with the deposition levels that we measured, but the amount of reduced N was four- to fivefold higher at Clara bog. The elevated atmospheric ammonia deposition (NHx) levels we observed may have resulted from the extensive agricultural activities taking place in the immediate surroundings of Clara bog (Buijsman, Maas & Asman 1987). Compared with the bulk deposition levels of N in the Netherlands (2 g m−2 year−1; Table 1), levels in Ireland were four times lower. High N deposition levels in the Netherlands also resulted in significantly higher ammonium concentrations in the peat moisture compared with those measured in Irish bogs (Fig. 6).

Figure 6.

Concentrations (mean ± 1 SE) of ammonium and phosphate at a depth of 0–10 cm in bogs in the Netherlands (NL; n = 12) and Ireland (IR; n = 7). *P = 0·05; **P = 0·001 (t-test).

In areas with relatively low concentrations of atmospheric pollutants, bulk precipitation on Calluna-dominated vegetation amounts to approximately 60–75% of the total atmospheric input (R. Bobbink, personal communication). The calculated total atmospheric input on Clara bog must have been around 0·6–0·8 g N m−2 year−1, which is within the range of the empirically estimated critical load for ombrotrophic bogs (0·5–1·0 g N m−2 year−1; Bobbink & Roelofs 1995). This was as expected as no expansion of nitrophilous species in ombrotrophic vegetation has so far been observed. The total input levels of N on the experimental plots, including the current background deposition level, were approximately 0·7 (control), 2·7, 4·7 and 8·7 g N m−2 year−1. It must be noted that the annual N load was added in six applications, which is common in fertilization experiments, and this could have led to an imbalance between the supply and demand of nutrients in the vegetation. A laboratory experiment with weekly N applications, however, revealed comparable effects of high N addition rates on the growth of Molinia and Betula (Tomassen et al. 2003). Therefore we assume that the results of this experiment approached reality.

effect of n on rhizosphere chemistry

As N was applied only during the growing season, peat water N concentrations were higher during summer (Fig. 1). Although the vegetation takes up available N during this season, the applied N was completely taken up only at an addition rate of 2 g N m−2 year−1. At higher N addition rates, N was not completely taken up by the vegetation, partly because of the absence of a living Sphagnum layer. Incomplete N uptake by the vegetation was also indicated by the relatively higher fractions of ammonium adsorbed to the peat (Table 3) at addition rates of 4 g N m−2 year−1 and higher. In a similar experiment carried out in the same period at a location on Clara Bog dominated by Sphagnum mosses, addition of 4 g N m−2 year−1 for 3 years resulted in much lower peat moisture N concentrations (Limpens 2003). Sphagnum has a high N uptake rate, resulting in low N concentrations in the peat moisture (Lee & Woodin 1988; Jauhiainen, Wallén & Malmer 1998). In the Netherlands, with a total N deposition level of 4 g m−2 year−1, ammonium concentrations of up to 50 µmol l−1 have been measured. In our experiment, the availability of ammonium in the rhizosphere during winter was comparable with the concentrations measured in the Netherlands, but during the summer the availability of ammonium was much higher due to the N applications (Fig. 1) and the absence of a living Sphagnum layer. During the winter, when no nutrients were applied, the large amounts of precipitation (Fig. 1) probably flushed ammonium, nitrate and sulphate from the peat, resulting in lower concentrations.

The observed decrease in pH during the summer is probably due to increased (bio)geochemical oxidation processes in the dry peat. Oxidation processes generate protons and lead to acidification of the peat (Drever 1997). The addition of N further decreased the pH in the peat moisture. Uptake of ammonium by the vegetation (Raven 1985) and mobilization of protons from the peat cation exchange complex at high ammonium concentrations (Clymo & Hayward 1982), may have been responsible for the observed treatment effect.

As we expected, N addition significantly affected C : N and N : P ratios of the peat (Table 2). Peat C : P ratios, however, were also influenced by N addition. Increased N availability probably stimulated uptake of P, resulting in slightly lower peat P concentrations and significantly higher C : P ratios. The differences in C : N, C : P and N : P ratios of the peat had no significant effects on the mineralization rate (i.e. potential CO2 production). Several studies have found increased mineralization rates at low C : N and C : P ratios (Aerts & Chapin 2000) due to increased nutrient availability for micro-organisms. As the low pH of the peat probably caused the lack of stimulation of the mineralization rate by high N inputs, the potential CO2 production rate was significantly influenced by the pHNaCl of the peat (Fig. 2). An increased pH is known to enhance mineralization by stimulating microbial activity (Smolders et al. 2002). From the above we conclude that desiccation of the peat does not necessary lead to increased mineralization rates, because acidification of the peat as a result of oxidation processes, ammonium uptake by plants and proton exchange processes can inhibit mineralization.

effect of n addition on vascular plants

The above-ground biomass of Molinia increased with the N addition rate (Fig. 3), indicating limitation by N, as has also been found in earlier studies of heathlands (Berendse & Aerts 1984; Roelofs 1986; Heil & Bruggink 1987; Aerts & Berendse 1988). Growth limitation by N was also indicated by the constant foliar N concentrations in Molinia (Table 4), showing that extra available N was used for biomass production. However, foliar N : P ratios ranged from 41 to 52, suggesting that growth was severely limited by P (Koerselman & Meuleman 1996). Kirkham (2001) reported average N concentrations and N : P ratios of 18·5 mg g−1 DW and 21, respectively, in Molinia from upland regions in England and Wales. Foliar N concentrations of Molinia in our experiment were comparable to those measured by Kirkham (2001), but P concentrations were much lower, resulting in higher N : P ratios (Table 4). Despite these very high N : P ratios, the growth of Molinia was still stimulated, indicating that Molinia is a species well adapted to low P availability, as has been found elsewhere (Kirkham 2001; Tomassen et al. 2003).

In contrast, we found no N effect on Betula and E. vaginatum. It is likely that nutrients other than N were limiting to their growth. On drained mires in northern Finland, NPK and PK fertilization has been found to stimulate the growth of Betula pubescens, while N alone had no effect (Penttila & Moilanen 1997). N addition in our experiment significantly increased the foliar N concentration in Betula (Table 4), suggesting that its growth was not limited by N. The N : P ratios show that the growth of Betula was limited by P in all N treatments (Table 4; Koerselman & Meuleman 1996). Optimum nutrition levels for Betula pendula are known to be achieved at N : P ratios between 10 and 12 (Ericsson & Ingestad 1988). Hytönen & Kaunisto (1999) found growth limitation by P at foliar P concentrations below 2 mg g−1 DW. This indicates that the growth of Betula in our experiment was probably constrained by P. In a N addition experiment in the laboratory, growth of Betula in a wet Sphagnum fallax vegetation also seemed to be limited by P (Tomassen et al. 2003).

In our experiment, the growth of E. vaginatum was not stimulated by N addition. Redbo-Torstensson (1994), however, found increased densities of E. vaginatum at addition rates of 1 g N m−2 year−1 and more in central south Sweden, after 4 years. In contrast, fertilization experiments on drained peat sites in southern Norway and Sweden showed stimulated growth of E. vaginatum after NPK and PK fertilization, indicating P or K limitation (Finér & Brække 1991). As in the cases of Molinia and Betula, the N : P ratios suggested that E. vaginatum was limited by P (Table 4). P limitation was also observed in a study by Leith et al. (1999), in which tissue N concentrations of E. vaginatum increased at higher N input levels and were twice those measured in our experiment (19 mg N g−1 DW), while P concentrations decreased at high N deposition levels and were comparable with the concentrations we measured (0·5 mg P g−1 DW; Leith et al. 1999).

In our experiment, regrowth of Calluna was not significantly stimulated by N addition after 3 years, which has been found for Calluna growing in heathland vegetation (Roem, Klees & Berendse 2002). Foliar N concentrations of Calluna increased upon N addition (Table 4) and were in the same range as those measured in the UK, where N concentrations in shoots reached levels of 9 mg g−1 DW at additional N loads of 1·5 g m−2 year−1 (Power et al. 1995; Uren et al. 1997). In a large-scale study by Pitcairn, Fowler & Grace (1995), tissue N concentrations in mature Calluna growing on heathland soil were significantly correlated with the total deposition level of N, whereas Risager (1998) found no effect of N deposition levels on tissue N concentrations in Calluna growing on ombrotrophic bogs. This discrepancy could be explained by suppression of the growth by water shortage, which can only occur in heathland soils, but also by the presence of a Sphagnum layer on bogs, which limits the availability of N for Calluna (Risager 1998). The correlation between N deposition and foliar N concentrations of Calluna in our experiment is probably due to both the absence of a Sphagnum layer and the extremely high N inputs. In our experiment, tissue N : P ratios in Calluna indicated P limitation (29–54), which has been found for Calluna in heathlands (Roem, Klees & Berendse 2002).

Peat moisture P concentrations in the rhizosphere of Irish bogs were very low compared with those measured in Dutch bogs (Fig. 6). Beltman et al. (1996) also found P shortage in Irish blanket bogs. Molinia and Betula were both able to grow around wooden poles on Clara bog, where the availability of both N and P was high as a result of increased input by bird droppings (H.B.M. Tomassen & A.J.P. Smolders, unpublished data). The high P concentrations in Dutch bogs probably enabled the expansion of Molinia, Betula and E. vaginatum at high N deposition levels. Why most of the Dutch bogs are rich in P remains unclear. There is not much information about the input of atmospheric P, but it probably ranges from 0·01 to 0·1 g P m−2 year−1, the main sources being fine soil particles, pollen and the burning of plant material, coal and oil (Newman 1995). We did indeed measure higher P deposition levels in the Netherlands than in Ireland (Table 1). Increased P availability in Dutch bogs often results from former agricultural use, such as buckwheat fire cultivation (Joosten 1995) or from the inflow of nutrients from surrounding pastures, which are extensively fertilized (Koerselman, Bakker & Blom 1990). Several bogs are located in former heathland pools, which could have been enriched with P in the past by large numbers of waterfowl, as bird droppings contains high concentrations of P (H.B.M. Tomassen & A.J.P. Smolders, unpublished data).

effect of n on regrowth of lichens

Lichens are very sensitive to atmospheric pollution and have almost completely disappeared from ombrotrophic bogs in the Netherlands (H.B.M. Tomassen, personal observations), and Cladonia portentosa has declined under Dutch forest canopies since 1960 (Van Dobben, De Witt & Van Dam 1983). In our experiment, regrowth of Cladonia portentosa was suppressed at high N addition rates (Fig. 5). We cannot exclude the possibility that the low application frequency could have stimulated the detrimental effects of N on Cladonia portentosa, although the disappearance of lichens at high nutrient availability has been confirmed by various other studies (Press et al. 1998; Gordon, Wynn & Woodin 2001). Discoloration and die-back of Cladonia portentosa in Danish heathlands has been associated with atmospheric N pollution (Søchting & Johnsen 1987). Vagts & Kinder (1999) found that NPK fertilization stimulated the growth of Cladonia portentosa, but P or N alone had a negative effect. In addition to the effects of high N deposition levels, S pollution may also have had deleterious effects (Farmer, Bates & Bell 1992). Tissue N and S concentrations in Cladonia portentosa were significantly elevated at addition rates above 2 g N m−2 year−1 (Table 4). Several other studies have found an effect of N deposition levels on the tissue N concentrations in Cladonia portentosa (Søchting 1990; Hyvärinen & Crittenden 1998a). Cladonia portentosa transplanted to sites subjected to high N deposition levels had increased N concentrations in both apices and bases due to retarded growth at these locations (Hyvärinen & Crittenden 1998b). Because lichens are very sensitive to S as well as N pollution (Poikolainen et al. 1998), we think that both the high S and high N loads hampered the regrowth of Cladonia in our experiment. Fortunately, this seems to be a reversible process as a systematic mapping of epiphytic lichens on conifers in Finland in 1985–86 and 1995 showed an increase in lichen abundance due to decreasing S deposition levels (Poikolainen et al. 1998). The coverage of Cladonia in our experiment, however, had not yet changed a year after the experimental N additions had ended (data not shown).

invasion of bogs: desiccation or n effects?

Is the invasion of bogs in the Netherlands by species such as Betula and Molinia a result of desiccation or long-term high atmospheric N loads? The results of our experiment show that experimental elevation of N deposition to Dutch levels did indeed stimulate the growth of Molinia under desiccated conditions. However, this was not the case for Betula and E. vaginatum, most probably because their growth was limited by P instead of N. Foliar N : P ratios indicated that the growth of Molinia, Betula and E. vaginatum was limited by P. Despite this presumed P limitation, however, Molinia was able to increase its biomass at increased N addition rates, although Betula and Eriophorum clearly were not. The N : P ratio tool developed by Koerselman & Meuleman (1996) is based on results obtained at the vegetation level and not for individual plant species. Our results indicate that the N : P ratio is not a suitable tool for detecting the absence of N limitation in Molinia. This is in agreement with findings by Güsewell & Koerselman (2002), who found that the N : P ratios cannot predict the responses of individual plant species to fertilization.

The effects of desiccation on the invasion of bogs by Molinia and Betula are likely to be smaller than those of increased N availability. Experimental N fertilization of Molinia and Betula in a wet vegetation dominated by S. fallax revealed consistent effects of high N addition rates (Tomassen et al. 2003). Under wet conditions, growth of Molinia was also stimulated by N (4 g m−2 year−1), whereas that of Betula was probably limited by P instead of N. It is plausible that the drastic changes in vegetation composition in Dutch bogs can be ascribed to the combined effects of high N deposition levels and relatively high P availability. We propose that invasion of Betula in bogs can only take place in conditions of simultaneously high N and relatively high P availability. If N deposition levels increase while P availability remains extremely low, only Molinia is likely to be able to outcompete other species.

prospects for ombrotrophic bogs at high n loads

As long as the critical N loads for ombrotrophic bogs are greatly exceeded in the Netherlands, invasion of bogs by species such as Molinia and Betula is likely to remain a problem. The effects of high N loads, however, will be more pronounced under desiccated conditions due to reduced growth or even absence of Sphagnum mosses. A living Sphagnum layer can immobilize large amounts of N and make it unavailable for vascular plants (Lamers, Bobbink & Roelofs 2000; Limpens 2003; Tomassen et al. 2003). Additional management measures to optimize growing conditions for Sphagnum, such as rewetting and mowing (reduction of shading), could therefore probably decline the negative effects of high atmospheric N input.


The authors would like to thank Jelle Eygensteyn, Jan Dobbelman and Liesbeth Pierson for their help with the chemical analyses. Roy Peters, Jeanine Berk, José Roelofs-Hendriks, Marcel Isken, Esther Lucassen, Roos Loeb, Janneke Roelofs and Michelle McKeon provided useful practical assistance. We are grateful to Dúchas the Heritage Service, National Parks & Wildlife, Colin Malone and Jim Ryan for their permission to perform the experiment at Clara bog. Jan Klerkx provided linguistic advice. This study is part of the National Research Programme ‘Overlevingsplan Bos en Natuur’, funded by the Dutch Ministry of Agriculture, Nature Management and Fisheries.