Abstract and Figures

Aim To further unravel P availability in mineral-rich fens, and test whether high Fe in the soil would lead to low P availability to the vegetation. Methods Mesotrophic fens were selected over gradients in Ca and Fe in central Sweden and the Netherlands, to study characteristics of vegetation, pore water and peat soil, including inorganic and organic forms of P, Fe and Al. Results Soil Fe was more important than region or soil Ca, and P availability to the vegetation increased from Fe-poor to Fe-rich fens. Contrary to expectations, precipitation of iron phosphates played a minor role in Fe-rich fens. Fe-rich fens were P-rich for three reasons: (1) high P sorption capacity, (2) relatively weak sorption to Fe-OM complexes and (3) high amounts of sorbed organic P, which probably consists of labile P. Also, nonmycorrhizal wetland plants probably especially take up weakly sorbed (organic) P. However, high P did not lead to high biomass or low plant diversity. Fe-rich fens were limited by other nutrients, and high P may help protect the vegetation against Fe-toxicity. Conclusions Fe-poor fens are P-poor, irrespective of Ca, and Fe-rich fens P-rich even under mesotrophic conditions. However, high P itself does not endanger Fe-rich fens.
Content may be subject to copyright.
REGULAR ARTICLE
Re-assessment of phosphorus availability in fens
with varying contents of iron and calcium
A. M. Kooijman &C. Cusell &L. Hedenäs &
L. P. M. Lamers &I. S. Mettrop &T. Neijmeijer
Received: 11 April 2019 /Accepted: 29 July 2019
#The Author(s) 2019
Abstract
Aim To further unravel P availability in mineral-rich
fens, and test whether high Fe in the soil would lead to
low P availability to the vegetation.
Methods Mesotrophic fens were selected over gradients
in Ca and Fe in central Sweden and the Netherlands, to
study characteristics of vegetation, pore water and peat
soil, including inorganic and organic forms of P, Fe and
Al.
Results Soil Fe was more important than region or soil
Ca, and P availability to the vegetation increased from
Fe-poor to Fe-rich fens. Contrary to expectations, pre-
cipitation of iron phosphates played a minor role in Fe-
rich fens. Fe-rich fens were P-rich for three reasons: (1)
high P sorption capacity, (2) relatively weak sorption to
Fe-OM complexes and (3) high amounts of sorbed
organic P, which probably consists of labile P. Also,
nonmycorrhizal wetland plants probably especially take
up weakly sorbed (organic) P. However, high P did not
lead to high biomass or low plant diversity. Fe-rich fens
were limited by other nutrients, and high P may help
protect the vegetation against Fe-toxicity.
https://doi.org/10.1007/s11104-019-04241-4
Highlights
This study provides further evidence that F-rich fens are not P-
poor but P-rich
Fe-rich fens have high total P, but especially a lot of weakly
sorbed P
i
and P
o
High P in Fe-rich fens does not lead to high biomass, but may
reduce Fe-toxicity
Ca-rich fens are P-poor, mainly due to low Fe and low P-sorption
capacity
Responsible Editor: N. Jim Barrow.
A. M. Kooijman (*):C. Cusell :I. S. Mettrop :
T. Ne i jme i j er
Department of Ecosystem and Landscape Dynamics, Institute for
Biodiversity and Ecosystem Dynamics (IBED), University of
Amsterdam, P.O. Box 94240, NL-1090 GEAmsterdam,
The Netherlands
e-mail: a.m.kooijman@uva.nl
C. Cusell :L. P. M. Lamers :I. S. Mettrop
Department of Aquatic Ecology & Environmental Biology,
Institute for Water and Wetland Research, Radboud University
Nijmegen, NL-6525 AJNijmegen, The Netherlands
C. Cusell
Witteveen+Bos, NL-7400 AEDeventer, The Netherlands
L. Hedenäs
Department of Botany, Swedish Museum of Natural History, Box
50007, 104 05 Stockholm, Sweden
I. S. Mettrop
Altenburg and Wymenga Ecological Consultants, Suderwei 2,
9269 TZVeenwouden, The Netherlands
Plant Soil (2020) 447:219239
/ Published online: 5 December 2019
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
Conclusions Fe-poor fens are P-poor, irrespective of
Ca, and Fe-rich fens P-rich even under mesotrophic
conditions. However, high P itself does not endanger
Fe-rich fens.
Keywords Mineral-rich fens .Plant diver sity .
Nutrients .Humic Fe-Al complexes .Central Sweden .
The Netherlands
Introduction
Mineral-rich fens are important habitats for biodiversity.
They are rich in spectacular and strictly restricted plant
species (European Union 2013), and home to the EU-
habitat directive species such as Hamatocaulis
vernicosus and Liparis loeselii. Mineral-rich fens be-
long to protected EU-habitat types such as Transition
mires (H7140), Alkaline fens (H7230) and Molinia
meadows on calcareous, peaty or clayey-silt laden soil
(H6410), but they have undergone a serious decline in
Europe. Apart from habitat destruction and drainage,
mineral-rich fens are threatened by eutrophication and
acidification (Lamers et al. 2015). They are often fed by
groundwater, and can be Ca-rich and/or Fe-rich, de-
pending on geological substrate, hydrology and water
chemistry.
Nutrient availability in mineral-rich fens has been
studied rather extensively in the past (e.g., Boyer and
Wheeler 1989; Verhoeven et al. 1990; Snowden and
Wheeler 1993,1995; Wassen et al. 2005; Kooijman
and Paulissen 2006). However, recent studies have pro-
vided controversial insights, especially related to avail-
ability of P in Fe-rich fens (Zak et al. 2008;Aggenbach
et al. 2013; Pawlikowski et al. 2013; Cusell et al. 2014;
van der Grift et al. 2016; Emsens et al. 2017).
Iron-rich soils are generally seen as P-limited, due to
proposed stable P binding at Fe compounds (Walker and
Syers 1976; Hamad et al. 1992). In aquatic ecosystems,
P-poor water and high plant diversity are associated with
high Fe:P ratios in water and soil (e.g., Geurts et al.
2008). The principle of P capture by Fe is even used in
phosphate removal from polluted surface water or agri-
cultural runoff (Thistleton et al. 2001; Zou et al. 2018).
In Fe-rich fens, sorption of P to, or precipitation with Fe
may thus be an important mechanism to reduce P-
availability in soil pore water (Zak et al. 2004). When
anoxic, Fe-rich groundwater is aerated, precipitation of
iron hydroxyphosphates may even precede formation of
iron (hydr)oxides as long as dissolved orthophosphate is
present (van der Grift et al. 2016).
However, in semi-terrestrial Fe-rich peat soils, P
availability may not be low at all. Recent studies showed
that P in vegetation and/or soil increased with Fe content
of the peat soil instead of decreased, which would
hamper restoration of formerly drained Fe-rich fens
(Aggenbach et al. 2013; Pawlikowski et al. 2013;
Emsens et al. 2017). In their studies, foliar N:P ratios
dropped below the critical level of 14.5 g g
1
,whichdo
not point to P limitation, but to excess of P (Olde
Venterink et al. 2003). High P availability in Fe-rich
fens may be caused by permanently high water levels,
which leads to anoxic conditions, reduction of Fe(III)
and mobilization of Fe-bound P (Patrick and Khalid
1974;Lamersetal.2012), However, high plant P con-
tents and foliar N:P ratios below 10 g g
1
were found in
Fe-rich soils even under well-drained conditions
(Emsens et al. 2017), so other factors play a role as well.
In Fe-rich fens, P availability to the vegetation could
also increase if a large part of the P is sorbed to com-
plexes of Fe and organic matter (OM), rather than to Fe
(hydr)oxides (Kooijman et al. 2009;Gerke2010;van
der Grift et al. 2016). Sorption of P to Fe-OM complexes
provides a weaker form of binding, but is often ignored
in studies about P-availability (Gerke 2010,2015).
Even though P availability to the vegetation may be
higher than expected in Fe-rich fens, it is unclear wheth-
er such a high P availability in Fe-rich fens actually
leads to higher biomass production and light limitation
to smaller fen species (Aggenbach et al. 2013). Emsens
et al. (2017) showed that the total amount of P in the
aboveground vegetation clearly increased with Fe con-
tent of the soil, but aboveground biomass itself seemed
less affected. In addition, Cusell et al. (2014)showed
that aboveground biomass production in Fe-rich fens
did not increase when fertilized with P, while
Pawlikowski et al. (2013) did not find differences in
aboveground biomass between Ca-rich and Fe-rich fens.
For Ca-rich fens, views on P-availability to the veg-
etation are less ambiguous. Ca-rich fens are generally
regarded as P limited (e.g., Boyer and Wheeler 1989;
Wassen et al. 2005; Pawlikowski et al. 2013;Cusell
et al. 2014;Kooijmanetal.2016), and eutrophication
with P is considered to be a major threat to plant diver-
sity (Kooijman and Paulissen 2006;Lamersetal.2015).
However, the mechanisms for low P availability in Ca-
rich fens are not always clear. Calcium phosphates be-
comes coprecipitated with calcium carbonates in fens
Plant Soil (2020) 447:219239
220
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with Ca-rich groundwater (Boyer and Wheeler 1989),
which are sparingly soluble around pH 7 (Lindsay and
Moreno 1966;Hinsinger2001). However, in many Ca-
rich fens, Ca-concentrations are often too low for calcite
precipitation. In the view of Emsens et al. (2017), these
fens may be limited by P due to low Fe levels and low P-
sorption capacity, rather than high Ca contents.
The goal of the present study was to further unravel the
regulation of P availability to the vegetation in different
mineral-rich fens. We studied 25 mesotrophic and un-
drained mineral-rich fens over gradients from 0.2 to
8.1 mol kg
1
Ca in the soil and from 0.02 to 2.2 mol kg
1
Fe. In order to test whether relationships between P, Fe and
Ca differed between regions, 12 fens were selected in
undisturbed areas in central Sweden, and 13 in areas with
high human impact in the Netherlands. In each fen, we
studied plant species composition, aboveground biomass,
pore water and peat soil chemistry. We also measured
different soil fractions of P, Fe and Al, such as total P,
inorganic P, organic P, sorbed P and exchangeable P, and
total, inorganic and organic amorphous Fe and Al. We
included organic fractions, because organic P may be a
major source of P to the vegetation in these ecosystems
(Pérez Corona et al. 1996;Turneretal.2007;Cheesman
et al. 2014;Güsewell2017), and organic Fe and Al are part
of humic Fe-OM and Al-OM complexes, which show
weaker P sorption than Fe and Al (hydr)oxides (Gerke
2010). The research questions were (1): Does P availability
to the vegetation differ with respect to Fe and/or Ca content
of the peat soil, and between regions? (2) Does higher P
availability to the vegetation in particular fen types lead to
higher plant biomass and lower species richness?
Methods
Field survey
Vegetation, soil and pore water samples were collected
at the end of August and beginning of September in 13
fens with varying contents of iron and calcium in the
Netherlands and 12 in central Sweden (Table 1). Selec-
tion of the sites was based on the presence of bryophytes
such as Calliergon giganteum (Schimp.) Kindb.,
Scorpidium scorpioides (Hedw.) Limpr., S. cossonii
(Schimp.) Hedenäs and Hamatocaulis vernicosus
(Mitt.) Hedenäs, which are characteristic for mesotro-
phic fens, and occur over a range of Ca-rich to Fe-rich
habitats (Mettrop et al. 2018). In central Sweden, all fens
were characterized by groundwater seepage areas, but
Ca-rich fens were found in areas with limestone and
marl parent materials, while Fe-rich fens were found in
areas with granitic glacial deposits. In the Netherlands,
most Fe-rich fens also occurred in groundwater seepage
areas, although two were regularly flooded with Fe-rich
river water. However, many Dutch sampling localities
were located in areas without groundwater seepage.
Instead, these fens were fed by nutrient-poor and base-
rich surface water.
For each of the 25 sites, species composition and
cover percentages of vascular plants and bryophytes
were recorded in a 10 m
2
plot, with nomenclature ac-
cording to Van Tooren and Sparrius (2007) for the first,
and van der Meijden (2005) for the latter. In each plot,
three subplots were randomly selected. In each subplot,
height of the water level was measured relatively to the
soil surface just beneath the living moss layer, except in
two Dutch Fe-rich fens. In each subplot, living above-
ground biomass of vascular plants was harvested in
25 × 25 cm squares for further analysis. Pore water
samples were collected from the upper 10 cm of the soil
with Rhizon SMS soil moisture samplers with pore size
of 0.45 μm, connected to vacuumed syringes of 50 mL.
In addition, peat soil samples were collected from the
upper 10 cm of the peat soil, just below the living moss
layer. Samples for bulk density were collected with a
steel corer with a 100 ml volume. All samples were
collected in airtight plastic bags to avoid oxygen expo-
sure, and stored at 4 °C until further analysis.
Chemical composition
Pore water pH, electrical conductivity (EC) and alkalin-
ity were measured on the day of sampling with standard
electrodes and by titration down to pH 4.2 by using
0.01 mol L
1
HCl. Subsamples were acidified with 1%
HNO
3
to prevent metal precipitation. Concentrations of
o-PO
4
3
,NO
3
,NH
4
+
,SO
4
2
,Cl
and dissolved organic
matter (DOC) were measured in the laboratory with an
Skalar auto Analyzer (Westerman 1990). Unfortunately,
NH
4
+
and NO
3
concentrations could not be measured
in two of the Dutch Fe-rich fens. Also, in one of the
subsamples of one of the Ca-rich fens in Sweden, NO
3
concentrations were unrealistically high (>
250 μmol L
1
), and mean values for NO
3
in this site
were based on the other two subsamples. Total concen-
trations of P, Ca
2+
,Fe
2+
,S
2
,Mg
2+
,Al
3+
,Na
+
,K
+
,Zn
2+
and Mn
3+
were measured for all subsamples with Perkin
Plant Soil (2020) 447:219239 221
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Elmer Inductively Coupled Plasma - Optical Emission
Spectrometry (ICP-OES) spectroscopy (Westerman
1990). It is possible that Fe
2+
concentrations in pore
water samples of Fe-rich fens have been overestimated
to some extent, due to oxidation of this element during
the sampling process, and re-dissolution after the sam-
ples had been treated with 1% HNO
3
. Dissolved organic
P (DOP) was calculated as the difference between total
dissolved P and o-PO
4
.Inoneofthesubsamplesofone
of the Dutch Ca-rich fens, total P, PO
4
3
and DOP
concentrations were exceptionally high (> 10 μmol L
1
),
and mean values for P in pore water in this site were
based on the other two subsamples.
Vegetation samples were weighed after drying at
70 °C until constant weight, and ground. Total C and
N contents were measured in dried vegetation samples,
using a Elementar CHNS analyzer (Westerman 1990).
In addition, dry plant material was digested for 50 min in
a Perkin Elmer microwave with HNO
3
(65%) and HCl
(37%), after which total P, Ca, Fe, S, Mg, Al and K
contents in diluted digestates were measured by ICP
(Westerman 1990). For plant material, N, P and K
contents were expressed in g rather than mol, because
critical nutrient ratios, which are used as indicators of
which type of nutrient is limiting plant growth, are
generally given in g g
1
(Olde Venterink et al. 2003).
Critical levels are 14.5 g g
1
for N:P ratios, 2.1 g g
1
for
N:K ratios and 3.4 mg g
1
for K:P ratios.
Dry weights and gravimetric moisture contents of the
fresh peat soil samples were determined by drying at 70 °C
until constant weight. Total organic matter (OM) contents
were estimated for all subsamples by loss-on-ignition at
Tabl e 1 Sampling localities of mineral-rich fens with different total Fe and Ca content in the peat soil (010 cm) in the Netherlands (NL)
and central Sweden (S)
Site Region Coordinates Soil Fe mmol m
2
Soil Ca mmol m
2
Log Fe Log Ca
De Haeck NL 52 0859 N; 04 5036 E 91 1199 1.95 3.08
Stobbenribben NL 52 4709 N; 05 5903 E 102 2213 2.00 3.35
Kiersche Wiede NL 52 4148 N; 06 0757 E 166 1928 2.22 3.28
Blauwe Hel NL 52 0048 N; 05 3416 E 308 2135 2.49 3.33
Kikkerlanden NL 52 3945 N; 06 0227 E 529 2526 2.72 3.40
Geleenbeekdal NL 50 5534 N; 05 5403 E 609 3487 2.79 3.54
Ve l d we g N L 5 2 4 129 N; 06 0645 E 748 4865 2.87 3.69
Tienhoven South NL 52 1031 N; 05 5901 E 750 1508 2.87 3.18
Tienhoven North NL 52 1031 N; 05 5901 E 1085 1978 3.04 3.30
Bennekomse Meent NL 52 0022 N; 05 3537 E 1139 3315 3.06 3.52
Veerslootlanden NL 52 3709 N; 06 0815 E 3511 4528 3.54 3.66
Meppelerdieplanden NL 52 4005 N; 06 0737 E 3580 2102 3.55 3.32
Meppeler Diep NL 52 4100 N; 06 0851 E 8044 2992 3.91 3.48
Gulåstjårnen 2 S 63 2917 N; 14 5348 E 76 203,439 1.85 5.31
Gulåstjårnen 1 S 63 2917 N; 14 5348 E 167 124,325 2.22 5.09
Gulåstjårnen lake 1 S 63 2917 N; 14 5348 E 408 5497 2.62 3.74
Gulåstjårnen lake 2 S 63 2917 N; 14 5348 E 500 8425 2.70 3.93
Stormyran 3 S 63 1315 N; 16 0922 E 1503 1773 3.18 3.25
Stormyran 2 S 63 1315 N; 16 0922 E 1719 2513 3.23 3.40
Storflon 1 S 63 1333 N; 16 0045 E 4483 1338 3.65 3.13
Stormyran 1 S 63 1315 N; 16 0922 E 6427 2983 3.81 3.47
Flärkarna 1 S 63 0401 N; 16 1043 E 7652 4585 3.88 3.66
Flärkarna 2 S 63 0401 N; 16 1043 E 14,031 4063 4.15 3.61
Storflon 2 S 63 1333 N; 16 0045 E 15,346 2323 4.19 3.37
Kjällmyran S 63 2410 N; 14 3407 E 18,947 3858 4.28 3.59
Log Fe and log Ca in the soil are both based on the total amounts of Fe and Ca in the upper 10 cm of the peat soil in mmol m
2
. For each site,
total Fe and Ca contents are average values, based on three replicate samples
Plant Soil (2020) 447:219239
222
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
550 °C. Total C and N contents were measured in dried
peat soil samples with a CHNS analyzer. In addition, total
P, Ca, Fe, S, Mg, Al and K contents were measured with
microwave destruction (Westerman 1990). Samples of dry
soil were digested for 50 min in a microwave with 4.0 mL
HNO
3
(65%) and 1.0 mL HCl (37%).
In one of the Dutch Ca-rich and one of the Dutch Fe-
rich fens, one of the three subsample showed 38times
higher amounts of amorphous than total Fe, which is
impossible. These subsamples were discarded for all
parameters, and mean values for these sites based on
the other two subsamples.
Fractionation of P, Fe and Al
Different mobile P, Fe and Al fractions were determined
using established chemical extraction procedures. It
must be noted that the interpretation of results must be
undertaken with caution since both the extraction pro-
cedure as well as used chemicals causes some limita-
tions (Kooijman et al. 1998,2009). The P, Fe and Al
fractions were measured on lyophilized peat soil sam-
ples in order to restrict redox sensitive reactions and to
keep soil moisture contents equal. For the 75 subsam-
ples of the 25 sites, we measured total P, inorganic P,
organic P and sorbed P. Total P content (after heating at
500 °C for 4 h) and inorganic P content (not heated)
were extracted with 0.5 M H
2
SO
4
, and measured color-
imetrically (Westerman 1990). With this method, total P
was on average 85% of the total P measured on dry soil
with microwave destruction, but the two values highly
correlated (R
2
= 0.97). Organic P was calculated as the
difference between total and inorganic P. Sorbed P was
measured with 0.073 M NH
4
-oxalate/0.05 M oxalic acid
extraction at pH 3.0 and subsequent element analysis by
ICP (Schwertmann 1964).
Total P consisted of inorganic and organic P, which
each consisted of solid or stable and sorbed P fractions.
These fractions could not be measured directly, but
minimum and maximum estimates could be calculated
for solid inorganic P such as calcium or iron phosphates,
sorbed inorganic P, stable organic P and sorbed organic
P, based on the overlap and differences between the
measured amounts of inorganic, organic and sorbed P
(Kooijman et al. 2009). In many fens, sorbed P was
larger than inorganic P, which means that at least part
of the sorbed P was organic. In these fens, the difference
between sorbed and inorganic P thus represented a
minimum estimate of sorbed organic P. The amount of
sorbed P itself was considered as a maximum estimate
of sorbed organic P. In three Ca-rich fens, however,
sorbed P was smaller than inorganic P, and larger than
organic P. In that case, the minimum estimate of sorbed
organic P was zero, and the maximum estimate the
amount of organic P itself. When the minimum and
maximum estimates of sorbed organic P were known,
it was possible to calculate maximum and minimum
estimates for sorbed inorganic P, by subtracting the
values for sorbed organic P from the total amount of
sorbed P. When minimum and maximum estimates of
sorbed organic and inorganic P were known, it was also
possible to calculate estimates for stable organic P and
solid inorganic P, by subtracting the values for the
sorbed fractions from the total amounts of organic or
inorganic P.
Exchangeable and EDTA-extractable P were mea-
sured with sequential extractions according to
Golterman (1996) on the three subsamples of 9 sites in
the Netherlands, and 12 sites in central Sweden. For this
analysis, the Dutch sites Blauwe Hel, Geleenbeekdal,
Tienhoven North and Bennekomse Meent were not
included. For exchangeable P, freeze dried and chopped
samples were shaken in 1 M NH
4
Cl. After this, Fe-
bound P was measured in extracts with 0.05 M Ca-
EDTA. After this, Ca-bound P, consisting of weakly
sorbed inorganic and organic P to Ca compounds was
measured in extracts with 0.1 M Na
2
-EDTA.
Total amorphous Fe and Al were measured for all 75
subsamples of all 25 sites with the oxalate extractions
for sorbed P described above. As already mentioned,
selective extractions give only a rough indication of
different forms. Also, even though we tried to restrict
oxidation by using lyophilized soil samples, the high
amounts of Fe(III) measured in Fe-rich fens could to
some extent have originated from Fe(II) compounds.
We also analyzed organic Fe and Al, which are part of
complexes with Fe, Al and organic matter (OM), and
lead to weaker P sorption than to Fe and Al oxides
(Kooijman et al. 2009;Gerke2010). Organic Fe and
Al were measured according to McKeague et al. (1971)
in 0.1 M Na
4
P
2
O
7
extracts with the Auto Analyzer.
Inorganic amorphous Fe + Al, present in amorphous
oxides and phosphates, were calculated as the difference
between total amorphous and organic Fe + Al. Part of
the organic Fe + Al may consist of small inorganic Fe +
Al particles, especially in Fe-rich soils with mobilization
and precipitation of Fe (Jeanroy and Guillet 1981).
These particles probably mostly consist of fine colloids
Plant Soil (2020) 447:219239 223
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
of iron hydroxides, associated with some organic matter,
which are important to P binding (Bol et al. 2016). In
any case, in Fe-rich dunes, the amount of plant-available
P positively correlated with the fraction of so-called
organic, pyrophosphate-extractable Fe, and negatively
with inorganic Fe (Kooijman et al. 2009).
Statistical analysis
The 25 vegetation relevés were clustered with Twinspan
(Hill 1979), with standard options. The statistical anal-
ysis of all other parameters was based on mean values
for each of the 25 sites, or 21 sites in the case of
exchangeable, Ca-EDTA and Na-EDTA extractable P.
For each parameter except number of plant species,
mean values were based on the three subsamples mea-
sured in each site. Potential effects of Ca and Fe content
of the soil, as well as region, were tested with stepwise
multiple linear regressions, with log Fe, log Ca and
region as explanatory variables (Cody and Smith
1987). Log Fe and log Ca in the soil were both based
on the total amounts of Fe and Ca in the upper 10 cm of
the peat soil, given in mmol m
2
. Univariate relation-
ships between individual parameters were further tested
with Pearson correlation tests. Most parameters showed
linear relationships with soil Fe and/or Ca, but for total
and inorganic amorphous Fe + Al, second order rela-
tionships were more appropriate.
Results
Plant diversity and biomass
We encountered 46 bryophyte and 151 vascular plant
species in the vegetation in total. In the first Twinspan
division, fens in the Netherlands were separated from
those in central Sweden (Table 2). This distinction was
primarily based on plant species with different distribu-
tion patterns. In the Netherlands, temperate species
prevailed, such as the EU-habitat directive species
Liparis loeselii, which does not occur in Sweden at all,
and the bryophyte Calliergonella cuspidate, which only
occurs in the southern part. In central Sweden, boreal
species were more common, such as Trichophorum
alpinum and Paludella squarrosa, which are absent
from the Netherlands. In both regions, the fens were
separated into relatively Fe-poor and Fe-rich ones.
Aboveground vascular plant biomass also differed
between the Netherlands and central Sweden, with
higher values for the first, possibly due to the more
temperate climate (Table 3, Fig. 1). However, plant
species richness did not differ between regions. The total
number of plant species ranged around 27 in both the
Netherlands and central Sweden, although the number
of bryophytes was significantly higher in the latter. Total
plant species number did also not change with Fe or Ca
in the soil, although the number of herb species was
significantly lower in Ca-rich fens. Even though cover
of the herb layer was significantly lower in Ca-rich fens,
aboveground vascular plant biomass did also not differ
between Fe-poor and Fe-rich fens, or Ca-poor and Ca-
rich fens.
Vascular plant nutrients
Plant nutrient contents strongly correlated with soil Fe,
but did not differ between regions or with Ca content of
the soil (Table 4). Plant N, P and K contents were all
significantly higher in Fe-rich than in Fe-poor fens. Iron
content of the soil especially correlated with plant P
contents, which more than quadrupled from Fe-poor to
Fe-rich fens (Fig. 2). Vascular plant N:P ratios also
clearly correlated with Fe content of the soil. Values
decreased from approximately 25 g g
1
in Fe-poor fens,
which point to P limitation, to 10 g g
1
in Fe-rich fens,
which indicate that P is not all a limiting factor for plant
growth. The plants showed balanced N:P ratios of
15 g g
1
around log Fe values of 3.7, or total Fe contents
of 5000 mmol m
2
.
However, although plant P contents clearly increased
with soil Fe, the total amount of P in aboveground
vascular plant biomass did not differ between Fe-poor
and Fe-rich fens, probably because vascular plant bio-
mass was not affected by soil Fe. For the entire dataset,
the correlation between soil Fe and total plant P showed a
R
2
value of 0.12. In the stepwise multiple regression for
total plant P, log Fe came out after region, and showed
significant additional effects only in central Sweden.
Pore water characteristics
At the time of sampling, the water table did not differ
with soil Ca or Fe content, or between regions, and was
close to the surface in all fens (Table 5). The pH of the
pore water increased with Ca content in the soil, with
values ranging from 6.2 in Ca-poor fens to 7.2 in Ca-
Plant Soil (2020) 447:219239
224
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Tabl e 2 Twinspan clustering of mineral-rich fens with different
soil Fe and Ca content in the Netherlands and central Sweden.
Eigenvalues were 0.567 at the first division, 0.464 at the second
division, and 0.420 at the third division. In the first division, the
two regions were completely separated. In the second and third
divisions, for both countries, Fe-rich fens were separated from Fe-
poor fens. Only species with frequency of 50% in at least one of
the groups are listed
Plant Soil (2020) 447:219239 225
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rich fens, but did not differ between regions or with soil
Fe. Alkalinity was not correlated with region, soil Fe or
Ca content, but ranged around 2.2 mol L
1
in all fens.
Electrical conductivity however clearly differed be-
tween regions, with average values of 209 μScm
1
in
central Sweden, and 374 μScm
1
in the Netherlands.
This was associated with higher Na
+
,Cl
,SO
4
2
and
DOC concentrations in the Netherlands.
At the time of sampling, pore water Ca
2+
concentra-
tions were also lower in the Netherlands, with average
Tabl e 3 Stepwise multiple linear regression of vegetation characteristics, with Fe and Ca content of the soil and region as explanatory
variables. The analysis is based on 25 samples from the Netherlands and central Sweden, all based on three subreplicates
Va r i ab l e R
2
log Fe soil R
2
log Ca soil R
2
region Total R
2
Vascular plant biomass 0.02 0.03 0.30 0.35**
Herb cover 0.36 0.36**
Moss cover 0.06 0.02 0.10 0.18
ns
Tot al sp e ci e s n um b er –– 0.10 0.10
ns
Number of herb species 0.37 0.37**
Number of moss species 0.03 0.03 0.19 0.25*
Log Fe and log Ca in the soil are both based on the total amounts of Fe and Ca in the upper 10 cm of the peat soil in mmol m
2
. Ns = not
significant; * = p< 0.05; ** = p<0.01;***=p< 0.001. Variables explaining more than 17% of the variance in a particular parameter are
given in bold
Fig. 1 Total number of plant species and vascular plant biomass
in mineral-rich fens in the Netherlands and central Sweden in
relation to the gradients in Fe and Ca in the soil. Log Fe and log
Ca are both based on the amounts of Fe and Ca in the upper 10 cm
of the peat soil, expressed in mmol m
2
. A = vascular plant
biomass in relation to soil Fe; B = vascular plant biomass in
relation to soil Ca; C = number of plant species in relation to soil
Fe; D = number of plant species in relation to soil Ca. None of the
correlations was significant (p < 0.05)
Plant Soil (2020) 447:219239
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Tabl e 4 Stepwise multiple linear regression of plant nutrients, with Fe and Ca content of the soil and region as explanatory variables. The
analysis is based on 25 samples from the Netherlands and central Sweden, all based on three subreplicates
Va r i ab l e R
2
log Fe soil R
2
log Ca soil R
2
region Total R
2
Plant N-content (mg g
1
)0.22 0.12 0.08 0.41*
Plant P-content (mg g
1
)0.36 0.04 0.02 0.43**
Plant K-content (mg g
1
)0.25 ––0.25*
Plant Fe-content (g g
1
)0.09 0.03 0.13
ns
Total plant P (g m
2
)0.24 0.15 0.40**
Total plant N (g m
2
) 0.04 0.36 0.40**
Total plant K (g m
2
)0.15 0.06 0.21
ns
Total plant Fe (g m
2
)0.10 0.10
ns
Plant N:P ratio (g g
1
)0.35 0.02 0.05 0.42**
Plant N:K ratio (g g
1
) 0.02 0.10 0.12
ns
Plant K:P ratio (g g
1
) 0.05 0.03 0.07 0.09
ns
Plant P:Fe ratio (g g
1
) 0.02 0.13 0.02 0.17
ns
Log Fe and log Ca in the soil are both based on the total amounts of Fe and Ca in the upper 10 cm of the peat soil in mmol m
2
. Ns = not
significant; * = p< 0.05; ** = p<0.01;***=p< 0.001. Variables explaining more than 17% of the variance in a particular parameter are
given in bold
Fig. 2 Aboveground vascular plant P-content and N:P ratio in
mineral-rich fens in the Netherlands and central Sweden in relation
to the gradients in Fe and Ca in the soil. Log Fe and log Ca are both
based on the total amounts of Fe and Ca in the upper 10 cm of the
peat soil, expressed in mmol m
2
. A = plant P content in relation to
soil Fe; B = plant P content in relation to soil Ca; C = plant N:P
ratio in relation to soil Fe; D = plant N:P ratio in relation to soil Ca.
Correlations were only significant (p< 0.05) for vascular plant P
content and N:P ratio with log Fe
Plant Soil (2020) 447:219239 227
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values of 879 μmol L
1
for central Sweden, and
1235 μmol L
1
in the Netherlands, but also increased
from Ca-poor to Ca-rich fens. Magnesium concentra-
tions mainly differed with Fe in the soil, with higher
values for Fe-poor fens, but K
+
was not correlated with
region or soil types. Iron concentrations mainly differed
with Fe in the soil, and increased from Fe-poor to Fe-
rich fens from 2 to 160 μmol L
1
,butAl
3+
concentra-
tions were not affected.
At the time of sampling, nitrate concentrations were
significantly higher in central Sweden than in the Nether-
lands, with average values of 2.8 and 1.5 μmol L
1
re-
spectively, but did not differ with Ca or Fe content in the
soil. Ammonium, phosphate and DOP concentrations did
not differ between regions or along soil gradients at all.
Soil characteristics
Soil organic matter content showed significantly lower
values in Ca-rich than in Ca-poor fens, due to precipitation
of secondary calcium carbonates in Ca-rich fens (Table 6).
However, most fens had high soil organic matter content,
and average values were 79% for the Netherlands, and
73% for central Sweden. Dry weight or bulk density of the
peat soil also increased in Ca-rich fens, because calcium
carbonates are heavier than organic matter. However, N
content and C:N ratio did not correlate with regions, soil
Fe or Ca. Potassium contents of the peat soil were ap-
proximately two times higher in the Netherlands, and also
increased from Fe-poor to Fe-rich soils.
Phosphorus contents in the soil, and C:P and N:P
ratios, mainly correlated with Fe content. Phosphorus
contents, measured by microwave destruction, increased
from Fe-poor to Fe-rich fens, and C:P and N:P ratios
decreased (Fig. 3). Soil C:P ratios decreased from ap-
proximately 900 g g
1
in Fe-poor fens to less than
300gg
1
in Fe-rich fens. Soil N:P ratios also showed
a more than threefold decrease over this gradient.
P fractions in the soil
In the selective extractions, total soil P mainly cor-
related with Fe content, and showed a fourfold in-
crease from Fe-poor to Fe-rich fens (Table 7). Iron
richness of the soil also affected organic P (Fig. 4).
However, for inorganic P, correlations with soil Fe
Tabl e 5 Stepwise multiple linear regression of pore water characteristics in the peat soil, with Fe and Ca content of the soil and region as
explanatory variables. The analysis is based on 25 samples from the Netherlands and central Sweden, all based on three subreplicates
Va r i ab l e R
2
log Fe soil R
2
log Ca soil R
2
region Total R
2
Wat e r t able –– 0.07 0.07
ns
pH 0.07 0.34 0.41**
Alkaninity (mmol L
1
)–– 0.09 0.18
ns
EC (μScm
1
)0.16 0.23 0.39*
Na (μmol L
1
)0.62 0.62***
Cl (μmol L
1
) 0.01 0.02 0.71 0.74***
SO
4
(μmol L
1
) 0.11 0.07 0.27 0.45*
DOC (μmol L
1
)0.04 0.34 0.38**
Ca (μmol L
1
)0.10 0.17 0.27*
Mg (μmol L
1
)0.21 0.09 0.03 0.33*
K(μmol L
1
)0.07 0.07
ns
Fe (μmol L
1
)0.17 0.10 0.26*
Al (μmol L
1
) 0.07 0.03 0.10 0.21
ns
NO
3
(μmol L
1
)–– 0.20 0.20*
NH
4
(μmol L
1
)–– 0.11 0.11
ns
PO
4
(μmol L
1
)0.03 0.08 0.11
ns
DOP (μmol L
1
)0.16 ––0.16
ns
Log Fe and log Ca in the soil are both based on the total amounts of Fe and Ca in the upper 10 cm of the peat soil in mmol m
2
.EC=
Electrical conductivity. Ns = not significant; * = p< 0.05; ** = p < 0.01; *** = p < 0.001. Variables explaining more than 17% of the
variance in a particular parameter are given in bold
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content were much lower, and not significant in the
multiple linear regression, although they were just
significant in the univariate analysis.
Sorbed, oxalate-extractable P was more strongly cor-
related with Fe richness of the soil than organic or
inorganic P. When expressed as fraction of total P,
Fig. 3 Soil C:P and N:P ratio in mineral-rich fens in the Netherlands
and central Sweden in relation to the gradients in Fe and Ca in the
soil. Log Fe and log Ca are both based on the total amounts of Fe and
Ca in the upper 10 cm of the peat soil, expressed in mmol m
2
.A=
soil C:P ratio in relation to soil Fe; B = soil C:P ratio in relation to soil
Ca; C = soil N:P ratio in relation to soil Fe; D = soil N:P ratio in
relation to soil Ca. Correlations were significant (p< 0.05) for log Fe
(A and C), but not for log Ca (B and D)
Tabl e 6 Stepwise multiple linear regression of soil characteristics (010 cm), with Fe and Ca content of the soil and region explanatory
variables. The analysis is based on 25 samples from the Netherlands and central Sweden, all based on three subreplicates
Va r i ab l e R
2
log Fe soil R
2
log Ca soil R
2
region Total R
2
Organic matter (%) 0.56 0.56***
Dry weight (kg m
2
)0.72 0.72***
Total N (g m
2
)–– –
Tot al P (g m
2
)0.28 0.12 0.08 0.47**
Total K (g m
2
)0.13 0.08 0.21 0.42*
Soil C:N ratio (g g
1
)–– –
Soil C:P ratio (g g
1
)0.56 0.13 0.69***
Soil N:P ratio (g g
1
)0.38 0.11 0.49**
Log Fe and log Ca in the soil are both based on the total amounts of Fe and Ca in the upper 10 cm of the peat soil in mmol m
2
. Total P is
based on microwave destructionof the peat soil. Ns = not significant; * = p < 0.05; ** = p < 0.01; *** = p < 0.001. Variables explaining more
than 17% of the variance in a particular parameter are given in bold
Plant Soil (2020) 447:219239 229
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correlations between sorbed P and soil Fe were even
higher. Sorbed P accounted for approximately 5% of
total P in Fe-poor fens, but this value increased to 40%
in Fe-rich fens. The increase in sorbed P with soil Fe
was supported by Ca-EDTA extractable P, and to some
extent by Na-EDTA extractable P, which mainly consist
of sorbed P-fractions. However, exchangeable P was
significantly higher for the Netherlands than for central
Sweden, which may reflect the generally higher P
inputs.
In many fens, sorbed P was higher than inorganic P.
In fact, only three fens showed higher values for inor-
ganic than for sorbed P. All three fens were Fe-poor, and
two of them belonged to the most Ca-rich fens. With the
measurement of inorganic, organic and sorbed P, and the
overlap between them, minimum and maximum esti-
mates could be calculated for different inorganic and
organic P fractions (Fig. 5). For solid inorganic P,
expressed as percentage of total P, Fe in the soil was
unimportant, but the amount of soil Ca explained 79%
and 60% of its variance. Minimum estimates of solid
inorganic P were substantially higher than zero only in
the most extreme Ca-rich fens, which points to precip-
itation of calcium phosphates there.
Minimum and maximum estimates of stable organic
P suggested that this fraction predominated in all fens.
Table 7 Stepwise multiple linear regression of P, Fe and Al
fractions in the peat soil (010 cm), with Fe and Ca content of
the soil and region as explanatory variables. For most parameters,
the analysis is based on 25 samples from the Netherlands and
central Sweden, all based on three subreplicates. However, ex-
changeable P, Ca-EDTA and Na-EDTA P were determined for 21
sites only
Va r i ab l e R
2
log Fe soil R
2
log Ca soil R
2
region Total R
2
TotalP(mmolm
2
)0.24 0.15 0.13 0.53*
Organic P (mmol m
2
)0.18 0.09 0.12 0.40*
Organic P (% of total P) 0.07 0.16 0.23
ns
Inorganic P (mmol m
2
)0.150.080.23
ns
Inorganic P (% of total P) 0.07 0.16 0.23
ns
Adsorbed P (mmol m
2
)0.25 0.10 0.11 0.46*
Adsorbed P (% of total P) 0.38 0.03 0.05 0.46**
Ca-EDTA extractable P (mmol m
2
)0.57 0.04 0.09 0.70***
Na-EDTA extractable P (mmol m
2
)0.19 0.14 0.11 0.44*
Exchangeable P (mmol m
2
)0.050.32 0.38*
Exchangeable P (% of sorbed P) 0.49 0.21 0.69***
Minimum estimate solid inorganic P (% Total P) 0.03 0.79 0.82***
Maximum estimate solid inorganic P (% Total P) 0.60 0.60***
Minimum estimate stable organic P (% Total P) 0.20 0.20 0.02 0.42*
Maximum estimate stable organic P (% Total P) 0.24 0.14 0.03 0.42*
Minimum estimate sorbed inorganic P (% Total P) 0.09 0.09
ns
Maximum estimate sorbed inorganic P (% Total P) 0.04 0.04 0.08 0.16
ns
Minimum estimate sorbed organic P (% Total P) 0.35 0.02 0.09 0.46**
Maximum estimate sorbed organic P (% Total P) 0.42 0.11 0.03 0.55***
Total Fe and Al (mol m
2
)0.61 0.07 0.05 0.73***
Total amorphous Fe and Al (mol m
2
)0.77 0.02 0.79***
Organic Fe and Al (mol m
2
)0.71 0.03 0.04 0.77***
Organic Fe and Al (% of amorphous Fe and Al) 0.45 0.04 0.02 0.51***
Inorganic Fe and Al (mol m
2
)0.61 0.05 0.66***
Inorganic Fe and Al (% of amorphous Fe and Al) 0.45 0.04 0.02 0.51***
Log Fe and log Ca in the soil are both based on the total amounts of Fe and Ca in the upper 10 cm of the peat soil in mmol m
2
. Total P is
based on extraction with H
2
SO
4
. Ns = not significant; * = p < 0.05; ** = p < 0.01; *** = p < 0.001. Variables explaining more than 17% of
the variance in a particular parameter are given in bold
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230
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Both estimates correlated with Fe and Ca in the soil, and
increased to some extent from Ca-poor to Ca-rich fens,
but significantly decreased from Fe-poor to Fe-rich fens.
In most fens, sorbed P was higher than inorganic P,
which means that at least part of the sorbed P was
organic. The estimates of sorbed organic P, expressed as
percentage of total P, did not differ between regions, or
over gradients in soil Ca. However, both minimum and
maximum estimates of sorbed organic P showed a clear
increase with soil Fe. Estimates of sorbed organic P
ranged between 0 and 10% of total P in Fe-poor fens,
but 1830% in Fe-rich fens. As total P also increased over
this gradient, the actual increase in sorbed organic P from
Fe-poor to Fe-rich fens was even higher. Also, when
expressed as percentage of the amount of sorbed P, esti-
mates of the organic P fraction increased from 0 to 21%
of sorbed P in Fe-poor fens to 3863% in Fe-rich fens.
Estimates of sorbed inorganic P, expressed as per-
centage of total P, did not correlate with soil Ca, Fe or
regions. However, when expressed as percentage of the
amount of sorbed P, sorbed inorganic P ranged between
79 and 100% of sorbed P in Fe-poor fens, to 3762% in
Fe-rich fens.
Total, inorganic and organic Fe + Al
Total amorphous Fe + Al, which represents the total
P sorption capacity, highly correlated with Fe con-
tent of the peat soil (Fig. 6). Total P sorption capacity
increased at log Fe values higher than 2.7 (or
500 mmol Fe m
2
) in the topsoil. Organic and inor-
ganic Fe + Al also increased with soil Fe. However,
inorganic Fe + Al only increased when log Fe
values were 3.0 (or 1000 mmol Fe m
2
) or higher.
In fact, inorganic Fe + Al only substantially in-
creased in the six most Fe-rich Swedish fens, with
Fe contents of at least 4000 mmol m
2
. For organic
Fe + Al, which consists of Fe-OM complexes, the
Fig. 4 Organic, inorganic, sorbed and exchangeable soil P in
mineral-rich fens in the Netherlands and central Sweden in relation
to Fe in the soil. Log Fe is based on the total amounts of Fe in the
upper 10 cm of the peat soil, expressed in mmol m
2
.A=organic
P; B = inorganic P; C = oxalate-extractable sorbed P; D = NH
4
Cl
extractable exchangeable P. All correlations were significant
(p < 0.05), except for exchangeable P (D)
Plant Soil (2020) 447:219239 231
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increase with soil Fe started much earlier, around
500 mmol Fe m
2
. Also, in most fens, organic Fe +
Al was higher than inorganic Fe + Al, even at high
soil Fe. Inorganic Fe + Al was only substantially
higher than organic Fe + Al in the six most Fe-rich
Swedish fens. This may imply that organic Fe + Al
was more important to P sorption and plant P supply
than inorganic Fe + Al.
The importance of organic Fe + Al was further
supported by correlations with P-sorption and plant
P-supply (Fig. 7). Plant P content and sorbed P
were highly correlated, and showed R
2
values of
0.64 when sorbed P was expressed as log value.
Sorbed P also showed high correlations with or-
ganic Fe + Al, but much lower with inorganic Fe +
Al. The latter correlation was not even significant
without the log-tranformation of sorbed P. In ad-
dition, plant P content showed a higher correlation
with organic than with inorganic Fe + Al.
Discussion
Differences between the Netherlands and Central
Sweden
The two regions mainly differed in vegetation charac-
teristics and pore water composition. The fens in central
Sweden typically contained plant species with a north-
ern distribution pattern, while Dutch fens contained
more temperate species. The higher vascular plant
aboveground biomass in the Netherlands may have been
due to the longer growing season, as the Dutch fens are
located around 52° instead of 63° northern latitude.
Increased nutrient supply may also play a role, as ex-
changeable P and total soil K were both higher in the
Netherlands than in central Sweden. Also, in the Neth-
erlands, atmospheric N-deposition is higher than in cen-
tral Sweden (Erisman et al. 2015), and higher than the
critical loads of 1517 kg ha
1
yr.
1
for Molinion
Fig. 5 Minimum and maximum estimates of different forms of
inorganic and organic P in mineral-rich fens in the Netherlands and
central Sweden, in relation to Fe in the soil. Log Fe isbased on the
total amounts of Fe in the upper 10 cm of the peat soil, expressed in
mmol m
2
. A = solid organic P; B = stable organic P; C = sorbed
inorganic P; D = sorbed organic P. Correlations were only signif-
icant (p < 0.05) for stable organic P (B) and sorbed organic P (D)
Plant Soil (2020) 447:219239
232
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meadows, Alkaline fens or Transition mires (van
Dobben and van Hinsberg 2008). Nevertheless, in both
regions, the fens mostly belonged to short fens, with low
biomass and high species richness, rather than tall fens
(Boyer and Wheeler 1989). Plant species richness was
high in both the Netherlands and central Sweden, and
most plant species were characteristic for mineral-rich
fens even under Fe-rich and P-rich conditions, such as
the EU-habitat directive species Hamatocaulis
vernicosus.
The two regions also differed in pore water chemis-
try, with higher values for EC, Na, Cl, SO4 and DOC in
the Netherlands had than in central Sweden. This may
reflect the more oceanic location of the Netherlands,
and/or pollution of surfacewater from the Rhine river
(Cioc 2002). However, these differences in water chem-
istry did not affect P availability to any large extent. As
already mentioned, only exchangeable P was higher in
the Netherlands than central Sweden, probably due to
the generally higher input of P from agriculture in the
surface water, which affects many fens there (e.g.,
Cusell et al. 2014).
Differences between ca-poor and ca-rich fens
Differences between Ca-poor and Ca-rich fens in vege-
tation, pore water and soil characteristics were smaller
than expected, possibly because the gradient in soil Ca
could only partly be separated from the gradient in soil
Fe. However, soil organic matter content was lower in
the most extreme Ca-rich fens, due to precipitation of
secondary calcium carbonates (Boyer and Wheeler
1989) and the associated dilution effect. Also, these fens
were the only ones in which at least part of the inorganic
P consisted of solid forms, which points to co-
precipitation of calcium phosphates. The fens with cal-
cite precipitation were also characterized by low P
availability and even P limitation, low herb cover and
Fig. 6 Total amorphous, inorganic and organic Fe and in mineral-
rich fens in the Netherlands and central Sweden in relation to the
gradients in Fe and Ca in the soil. Log Fe and log Ca are both
based on the total amounts of Fe and Ca in the upper 10 cm of the
peat soil, expressed in mmol m
2
. A = total amorphous Fe + Al in
relation to soil Fe; B = total amorphous Fe + Al in relation to soil
Ca; C = inorganic Fe + Al in relation to soil Fe; D = organic Fe +
Al in relation to soil Fe. Correlations were significant (p < 0.05) for
soil Fe (A,C and D), but not for soil Ca (B)
Plant Soil (2020) 447:219239 233
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lownumberofherbspecies.However,inmanyfens,
precipitation of calcium phosphate is unlikely, due to
undersaturated conditions (Boyer and Wheeler 1989).
For many fens with low P availability, it was more
important to be Fe poor than Ca rich. In Fe-poor fens,
P sorption capacity may be low, because sorption of P to
organic matter is very low (Daly et al. 2001). This
suggests, in accord with Emsens et al. (2017), that Fe
is more important for P availability than Ca.
High P-availability in Fe-rich fens
In our study, P availability to the vegetation clearly
increased from Fe-poor to Fe-rich fens. Almost all soil
P fractions increased from Fe-poor to Fe-rich fens, while
soil C:P ratios decreased, and vascular plant P contents
more than quadrupled. The decrease in vascular plant
N:Pratiosfrom25to10gg
1
clearly showed that Fe-
rich fens were not P limited (Olde Venterink et al. 2003),
but instead characterized by excess P. In the common
view, Fe-rich soils have low P availability (Walker and
Syers 1976), and high Fe concentrations reduce P loss to
overlying surface water or downstream aquatic systems
(Zak et al. 2004; Geurts et al. 2008; van der Grift et al.
2016). However, our findings support earlier reports that
P availability within the soil may actually be higher in
Fe-rich fens than in Fe-poor fens (Aggenbach et al.
2013; Pawlikowski et al. 2013; Emsens et al. 2017).
High Fe levels may thus reduce orthophosphate concen-
trations in the water, but nevertheless increase P avail-
ability to the vegetation in the peat soil.
Inorganic, organic and sorbed P in Fe-rich fens
In Fe-rich fens, a large part of the P flowing in with
groundwater and surface water is probably captured by
Fe (Emsens et al. 2017), which may explain the higher
amounts of total and sorbed P than in Fe-poor fens. In
Fig. 7 Relationships between plant and soil characteristics in
mineral-rich fens in the Netherlands and central Sweden. A = plant
P content in relation to sorbed P in the soil; B = sorbed P in relation
to inorganic Fe + Al in the soil; C = sorbed P in relation to organic
Fe + Al in the soil; D = plant P content in relation to inorganic
Fe + Al in the soil. All correlations were significant (p < 0.05)
Plant Soil (2020) 447:219239
234
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theory, capture of P by Fe may lead to precipitation of Fe
hydroxyphosphates (van der Grift et al. 2016). Howev-
er, high amounts of inorganic P were only found in Ca-
rich fens with calcite precipitation, Also, the presence of
stable inorganic P could only be established with some
certainty for these Ca-rich fens, which implies that pre-
cipitation of iron hydroxyphosphates in Fe-rich fens is
less important than expected.
In all fens, a large part of the P was organic. We did
not study the composition of organic P, but in Cheesman
et al. (2014), monoesters, which mainly consist of ino-
sitol hexakiphosphates (IHP) or phytates, accounted for
6265% of organic P in organic wetland soils. Such
numbers are found in many soil types (Turner et al.
2007). Phytates are the principal storage form of P in
vegetation and seeds, and consist of cyclic acids, satu-
rated with up to six phosphate groups. With so many
phosphate groups, phytates are generally more strongly
bound to the soil solid phase than labile organic P
(McKercher and Anderson 1989;Gerke2015). In Ca-
rich soils, phytates may even precipitate as Ca3-IHP,
which is insoluble at high pH (Prietzel et al. 2016), and
may explain the high proportion of stable organic P
there. In Fe-rich fens, phytates may precipitate with
Fe, but especially with Al (Shang et al. 1992).
In most fens, at least part of the organic P consisted of
sorbed organic P. Even though only minimum and max-
imum estimates could be calculated, sorbed organic P
most likely increased from Fe-poor to Fe-rich fens.
Sorbed organic P probably mostly consists of labile
forms, as phytates are more strongly bound to the soil
solid phase (McKercher and Anderson 1989; Gerke
2015). Sorbed organic P is probably an important P
source for wetland plants. In terrestrial Ca-rich or Fe-
rich habitats, arbuscular mycorrhizal plants predomi-
nate, because they efficiently take up inorganic calcium
or iron phosphates with their mycorrhizal network
(Hoeksema et al. 2010; Smith and Smith 2011). How-
ever, in wetland habitats, nonmycorrhizal plants pre-
dominate, because mycorrhizal fungi are restricted by
low oxygen (Read and Perez-Moreno 2003).
Nonmycorrhizal plants have different strategies to take
up P, such as root exudation of phosphatase enzymes,
which disintegrate organic P (Raven et al. 2018). Such
enzymes probably mainly attack labile organic P. For
nonmycorrhizal wetland species such as Carex spp.,
excretion of phosphatase enzymes was almost ten times
higher for labile forms of organic P than for phytate
(Güsewell 2017). Also, Carex spp. were able to grow on
labile organic P, but not on phytate (Pérez Corona et al.
1996;Güsewell2017).
Weak P sorption to Fe-OM complexes in Fe-rich fens
In Fe-rich fens, availability of P to the vegetation prob-
ably further increased due to relatively weak P sorption.
In fens, P sorption may be relatively weak, because wet
conditions lead to reduction of Fe(III) to Fe(II) and
lower P-binding capacity (Patrick and Khalid 1974;
Emsens et al. 2017). However, as Fe-rich fens also
showed high P availability under drained and oxygen-
rich conditions (Emsens et al. 2017), this cannot be the
only explanation.
In Fe-rich fens, sorption of P is probably also weak
due to the high amount of organic Fe + Al, which is part
of Fe-OM complexes. Sorption to Fe-OM complexes is
weaker than to Fe oxides (Kooijman et al. 2009; Gerke
2010;Gerke2015). In Fe-OM complexes, organic mat-
ter reduces the P-binding capacity, and P is probably
mostly bound in monodentate fashion, with only one
phosphate oxygen atom bound to an adjacent Fe surface
site rather than two (Kim et al. 2011). Possibly, part of
the organic Fe + Al consist of fine colloids of iron hy-
droxide, associated with some OM (Jeanroy and Guillet
1981). However, in soils with high amounts of DOC,
such as peat soils, sorption of organic anions to Fe
oxides may be rather high, and transform them into
Fe-OM complexes (Gu et al. 1994). Also,
nonmycorrhizal plants exudate large amounts of carbox-
ylates such as citrate and oxalate, as strategy to improve
P uptake (Lambers et al. 2008; Raven et al. 2018). These
small organic anions can release organic and inorganic P
from the P-sorption complex by ligand exchange (Gerke
et al. 2000; Johnson and Loeppert 2006;Gerke2015),
but also weaken future P sorption by transforming Fe
oxides into Fe-OM complexes (Gu et al. 1994). In any
case, organic Fe + Al showed higher correlations with
sorbed P and plant P content than inorganic Fe + Al.
Also, inorganic Fe + Al only substantially increased in
fens with Fe content of 4000 mmol m
2
in the topsoil or
more, while organic Fe + Al already increased at Fe
contents of 500 mmol m
2
.
High P in Fe-rich fens does not lead to high biomass
As already mentioned, high availability of weakly
sorbed P in Fe-rich fens may especially favour
nonmycorrhizal wetland plants. Nonmycorrhizal plants
Plant Soil (2020) 447:219239 235
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
can mobilize weakly sorbed organic and inorganic P
through root exudation of carboxylic anions and ligand
exchange (Gerke et al. 2000, Johnson and Loeppert
2006; Gerke 2015), and further increase P-availability
through root exudation of phosphatase enzymes, which
especially disintegrate labile organic P (Güsewell 2017).
Nonmycorrhizal Carex spp. may even produce
dauciform roots, especially under P-poor conditions,
which excrete large amounts of carboxylic anions such
as citrate and oxalate (Bakker et al. 2005; Güsewell
2017). High P-uptake capacity by the vegetation was
supported by the clear increase in plant P content over
the gradient from Fe-poor to Fe-rich fens, and the strong
correlation with weakly sorbed (organic) P.
However, while high P-availability in Fe-rich fens clear-
ly resulted in higher plant P uptake, this did not lead to
higher aboveground biomass production than in Fe-poor
fens. Even in the Netherlands, Fe-rich fens were still
mesotrophic and species-rich. Such patterns were also
found by Pawlikowski et al. (2013) in pristine fens in
Poland, with aboveground biomass around 200 g m
2
in
both Ca-rich and Fe-rich fens. Emsens et al. (2017)didnot
report biomass values, but the underlying data revealed that
aboveground biomass was 450 g m
2
or lower when the
two rewetted fens on former agricultural land were exclud-
ed, and did not correlate with Fe-content in the soil at all.
Part of the explanation is that Fe-rich fens may be
limited by other nutrients than P. In Fe-rich fens, vascu-
lar plant N:P ratios ranged around 10 g g
1
, which points
to N as limiting factor (Olde Venterink et al. 2003). Fe-
rich fens may have low N availability due to high
microbial respiration and N immobilization even under
oxygen-poor conditions, with low net N mineralization
as a result (Mettrop et al. 2014). However, in the Neth-
erlands, high atmospheric N deposition probably re-
duced N limitation to some extent.
Aboveground biomass production in Fe-rich fens may
also be reduced by Fe-toxicity. In wetland soils, concen-
trations of reduced Fe may reach high and toxic levels
under oxygen-poor conditions (Patrick and Khalid, 1974;
Lucassen et al. 2000), although Fe-rich fens are protected
to some extent by the high pH and alkalinity. Also, high
Fe tolerance in plant species is associated with low rela-
tive growth rates (Snowden and Wheeler 1993).
High P may protect vegetation against Fe-toxicity
High P availability in Fe-rich fens may actually protect
the vegetation against Fe-toxicity. High P supply
reduced Fe-toxicity in mesocosm experiments of
Wheeler et al. (1985), and may lead to iron phosphate
precipitation on the roots, although Fe-tolerant species
also reduce Fe toxicity with precipitation of iron oxides
by radial oxygen loss (Snowden and Wheeler 1995;
Fageria 2001). Also, Fe concentrations in the plant
clearly decreased with increasing P concentrations in
the nutrient solution (Elliot and Lauchli 1985). High P
supply may even lead to Fe deficiency when the latter is
in low supply (DeKock and Wallace 1965;Fageria
2001). High P concentrations in the plant may decrease
the concentration of active Fe, probably by internal
formation of insoluble iron phosphate complexes in
the plant cells (Greipsson 1995;Fageria2001).
Although critical levels for foliar P:Fe ratios have not
been established and probably depend on plant species,
they indicate to some extent whether plants are affected
by Fe deficiency or toxicity. Deficiency of Fe was found
for Mustard plants at P:Fe ratios around 140 g g
1
(DeKock et al. 1960). In contrast, toxicity of Fe oc-
curred for Epilobium hirsutum at P:Fe ratios around
0.1 g g
1
(Wheeler et al. 1985). In Lucassen et al.
(2000), foliar P:Fe ratios for Glyceria fluitans were
0.31.8 g g
1
in sites with Fe toxicity. In our study,
P:Fe ratios ranged around 3.3 g g
1
in most fens, but
between 0.050.1 in two Fe-rich fens, which may thus
be close to Fe toxicity. Nevertheless, the plants looked
healthy, and diversity was very high. It is however
possible that less Fe-tolerant species, or species with
less efficient P uptake do not survive.
Ecological implications
This study shows that Fe-poor fens are P limited, due to
low Fe concentrations and low amounts of weakly
sorbed (organic) P, independent of Ca richness of the
peat soil and potential precipitation of calcium phos-
phates. This is an advantage, as many characteristic and
endangered fen species occur in P limited habitats
(Wassen et al. 2005). However, Fe-poor fens are threat-
ened by external P eutrophication, because high P avail-
ability to the vegetation is not used to combat Fe toxic-
ity, but for increased plant growth. Nevertheless, low P-
sorption capacity in Fe-poor fens may help to reduce P
availability once the water quality improves. In the Fe-
poor fens of Kooijman et al. (2016), aboveground bio-
mass decreased from 1000 to 250 g m
2
within 25 years,
and vascular plant N:P ratios increased from 16 to
23 g g
1
, which clearly points to lower P availability
Plant Soil (2020) 447:219239
236
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
(Olde Venterink et al. 2003). Also, eutrophic bryophytes
were replaced by more characteristic species such as
Scorpidium scorpioides.
For Fe-rich fens, high P availability is an inherent
characteristic, associated with high amounts of weakly
sorbed (organic) P, even in mesotrophic fens with high
plant diversity. High P availability in Fe-rich fens does
not necessarily lead to high aboveground biomass pro-
duction, but may instead protect the vegetation against
Fe toxicity. However, Fe-rich fens are sensitive to other
nutrients, such as N and/or K (Cusell et al. 2014), which
may be a problem for fen restoration in former agricul-
tural areas (Emsens et al. 2017). Also, like Fe-poor fens,
Fe-rich fens are sensitive to lowered water levels and
acidification. However, in undrained situations, Fe-rich
fens with high plant diversity and EU-habitat directive
species such as Hamatocaulis vernicosus may persist
even in the Netherlands, as long as buffer capacity and
pH remain high enough.
Acknowledgements The authors wish to thank Leen de Lange,
Bert de Leeuw, and Leo Hoitinga for analytical assistance and
Geert Kooijman for assistance in selecting field sites and
conducting the fieldwork. Per-Olof Nystrand at the County Ad-
ministrative Board of Jämtland assisted with obtaining collecting
permits (Dnr 522-2464-12) and indicating suitable sampling loca-
tions. This research was financially supported by Kennisnetwerk
Ontwikkeling en Beheer Natuurkwaliteit (O + BN) of the Dutch
Ministry of Economic Affairs, Agriculture and Innovation.
Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://
creativecommons.org/licenses/by/4.0/), which permits unrestrict-
ed use, distribution, and reproduction in any medium, provided
you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons license, and indicate if
changes were made.
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... Studies indicate that there is usually an increase in P availability after drainage in peatlands (Becher et al., 2020;Munir et al., 2017;Wang et al., 2018), due to the intensification of mineralization and release of P fractions associated with SOM (Saurich et al., 2019a,b). On the other hand, the oxidation condition created after drainage allows inorganic fractions of P to be immobilized by the hydrolyzed Fe 3+ , reducing its availability and mobility in the soil (Bhadha et al., 2020;Herndon et al., 2019;Kooijman et al., 2020), increasing the potential for P adsorption by soil. Globally, studies involving P dynamics in peatlands subjected to drainage for long periods are scarce. ...
... Studies conducted in drained areas of peatlands have demonstrated the occurrence of two fundamental processes associated with soil P dynamics: i) mineralization of the organic fraction of P associated with SOM, due to the increased biological activity in the aerobic environment, increasing the availability of labile inorganic P (Munir et al., 2017;Wang et al., 2018;Saurich et al., 2019a,b); and ii) adsorption of a portion of inorganic P, reducing its availability as a result of oxidation of the environment (Herndon et al., 2019;Kooijman et al., 2020). In the present study, however, we observed the occurrence of the two processes together, notably in the IC system. ...
... In addition to the lower P extraction in MC system, probably, the longer drainage time (80 years), maintaining the oxidation condition for a long period, it may have made it possible to block the P adsorption sites, either by the high P content already adsorbed or by the recalcitrant fractions of the highly decomposed SOM (Kooijman et al., 2020;Schmieder et al., 2020), acted in the maintenance of P readily available in solution, which explains the maintenance of the availability of labile inorganic P in the MC drained for 80 years. ...
Article
Extensive areas of peatlands are drained around the world to create a favorable environment for food production, altering the natural anaerobic environment and nutrient dynamics in the soil. In this study, we used tropical peatlands subjected to monocultive (MC) of Manihot esculenta for 80 years and intercropped (IC) of Manihot esculenta and Cocos nucifera for 20 years and an area of natural vegetation (Forest), to estimate the effects of MC and IC systems maintained under drainage on the dynamics and accumulation of organic and inorganic fractions of P and on the potential of P adsorption by the soil. Our results showed that drainage predominantly altered inorganic fractions of P, having little influence on its organic fractions. In the short-term, the IC system reduced the total soil P content in subsurface by 35.6 and 37.9 % when compared with MC system and Forest, respectively , as a result of the reduction of all inorganic fractions of P. In all areas, regardless of the position in the profile, we observed the predominance of the highly recalcitrant residual P fraction (> 70 %) in the total P content of the soil. The adsorption process controlled P availability in peatlands subjected to drainage, reducing P availability only in the IC system. Based on our results, we indicate the MC of Manihot esculenta as the best cultivation system and we do not recommend of the IC system in tropical peatlands, as it presents serious risks of total depletion of the P content in soil. Our results provide new evidence on P dynamics in tropical peatlands subjected to drainage for long periods.
... The availability of P depends strongly on pH because it is often bound to Fe at pH <5 and to Ca at pH >6.5 (Bourbonniere, 2009). Yet, Kooijman et al. (2020) showed that Fe-rich fens are also P-rich with a high P-availability, presumably due to weak P sorption to Fe-OM complexes. The Fe-poor fens, however, had small P-contents and low P-availability. ...
Chapter
Organic soils (Histosols) are formed by accumulation of incompletely decomposed organic matter. This chapter introduces the classification, genesis and properties of organic soils and potential management. Organic soils comprise histosols with folic horizons formed in cold climates as well as fens and bogs with or without contact with minerotrophic groundwater (mainly Rheic and Ombric Histosols, respectively). The soils have low bulk density and high water storage capacity, but may be limited by nitrogen, phosphorus, and other inorganic trace elements, particularly when plants lack contact with groundwater. Histosols cover 3% of the global land surface, but store 21% of terrestrial soil carbon.
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Soil organic matter (SOM) and pH are key ecosystem drivers, influencing resilience to environmental change. We tested the separate effects of pH and SOM on nutrient availability, plant strategies, and soil community composition in calcareous and acidic Grey dunes (H2130) with low, intermediate, and/or high SOM, which differ in sensitivity to high atmospheric N deposition. Soil organic matter was mainly important for biomass parameters of plants, microbes, and soil animals, and for microarthropod diversity and network complexity. However, differences in pH led to fundamental differences in P availability and plant strategies, which overruled the normal soil community patterns, and influenced resilience to N deposition. In calcareous dunes with low grass‐encroachment, P availability was low despite high amounts of inorganic P, due to low solubility of calcium phosphates and strong P sorption to Fe oxides at high pH. Calcareous dunes were dominated by low‐competitive arbuscular mycorrhizal (AM) plants, which profit from mycorrhiza especially at low P. In acidic dunes with high grass‐encroachment, P availability increased as calcium phosphates dissolved and P sorption weakened with the shift from Fe oxides to Fe‐OM complexes. Weakly sorbed and colloidal P increased, and at least part of the sorbed P was organic. Acidic dunes were dominated by nonmycorrhizal (NM) plants, which increase P uptake through exudation of carboxylates and phosphatase enzymes, which release weakly sorbed P, and disintegrate labile organic P. The shifts in P availability and plant strategies also changed the soil community. Contrary to expectations, the bacterial pathway was more important in acidic than in calcareous dunes, possibly due to exudation of carboxylates and phosphatases by NM plants, which serve as bacterial food resource. Also, the fungal AM pathway was enhanced in calcareous dunes, and fungal feeders more abundant, due to the presence of AM fungi. The changes in soil communities in turn reduced expected differences in N cycling between calcareous and acidic dunes. Our results show that SOM and pH are important, but separate ecosystem drivers in Grey dunes. Differences in resilience to N deposition are mainly due to pH effects on P availability and plant strategies, which in turn overruled soil community patterns.
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Utilizing natural wetlands to remove phosphorus (P) from agricultural drainage is a feasible approach of protecting receiving waterways from eutrophication. However, few studies have been carried out about how these wetlands, which act as buffer zones of pollutant sinks, can be operated to achieve optimal pollutant removal and cost efficiency. In this study, cores of sediments and water were collected from a lacustrine wetland of Lake Xiaoxingkai region in Northeastern China, to produce a number of lab-scale wetland columns. Ex situ experiments, in a controlled environment, were conducted to study the effects of aeration, vegetation, and iron (Fe) input on the removal of total P (TP) and values of dissolved oxygen (DO) and pH of the water in these columns. The results demonstrated the links between Fe, P and DO levels. The planting of Glyceria spiculosa in the wetland columns was found to increase DO and pH values, whereas the Fe:P ratio was found to inversely correlate to the pH values. The TP removal was the highest in aerobic and planted columns. The pattern of temporal variation of TP removals matched first-order exponential growth model, except for under aerobic condition and with Fe:P ratio of 10:1. It was concluded that Fe introduced into a wetland by either surface runoff or agricultural drainage is beneficial for TP removal from the overlying water, especially during the growth season of wetland vegetation.
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We compare carbon (and hence energy) costs of the different modes of phosphorus (P) acquisition by vascular land plants. Phosphorus-acquisition modes are considered to be mechanisms of plants together with their root symbionts and structures such as cluster roots involved in mobilising or absorbing P. Phosphorus sources considered are soluble and insoluble inorganic and organic pools. Costs include operating the P-acquisition mechanisms, and resource requirements to construct and maintain them. For most modes, costs increase as the relevant soil P concentration declines. Costs can thus be divided into a component incurred irrespective of soil P concentration, and a component describing how quickly costs increase as the soil P concentration declines. Differences in sensitivity of costs to soil P concentration arise mainly from how economically mycorrhizal fungal hyphae or roots that explore the soil volume are constructed, and from costs of exudates that hydrolyse or mobilise insoluble P forms. In general, modes of acquisition requiring least carbon at high soil P concentrations experience a steeper increase in costs as soil P concentrations decline. The relationships between costs and concentrations suggest some reasons why different modes coexist, and why the mixture of acquisition modes differs between sites.
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Phosphorus availability in terrestrial ecosystems is strongly dependent on soil P speciation. Here we present information on the P speciation of 10 forest soils in Germany developed from different parent materials as assessed by combined wet-chemical P fractionation and synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy. Soil P speciation showed clear differences among different parent materials and changed systematically with soil depth. In soils formed from silicate bedrock or loess, Fe-bound P species (FePO4, organic and inorganic phosphate adsorbed to Fe oxyhydroxides) and Al-bound P species (AlPO4, organic and inorganic phosphate adsorbed to Al oxyhydroxides, Al-saturated clay minerals and Al-saturated soil organic matter) were most dominant. In contrast, the P speciation of soils formed from calcareous bedrock was dominated (40–70% of total P) by Ca-bound organic P, which most likely primarily is inositol hexakisphosphate (IHP) precipitated as Ca3-IHP. The second largest portion of total P in all calcareous soils was organic P not bound to Ca, Al, or Fe. The relevance of this P form decreased with soil depth. Additionally, apatite (relevance increasing with depth) and Al-bound P were present. The most relevant soil properties governing the P speciation of the investigated soils were soil stocks of Fe oxyhydroxides, organic matter, and carbonate. Different types of P speciation in soils on silicate and calcareous parent material suggest different ecosystem P nutrition strategies and biogeochemical P cycling patterns in the respective ecosystems. Our study demonstrates that combined wet-chemical soil P fractionation and synchrotron-based XANES spectroscopy provides substantial novel information on the P speciation of forest soils.