Introduction
Paleosols that developed in Late Glacial eolian sediments like, e.g., loess,
dune sands or cover sands are valuable
archives to reconstruct paleovegetation, soil and land use history (e.g., Pye
and Sherwin, 1999; Van Mourik et al., 2012; Wallinga et al., 2013), with the
prerequisite that individual phases of soil formation and sedimentation are
distinguishable from each other. Traditionally, pedogenic processes are
assumed to impact the uppermost tens of centimeters of soil, as visible,
e.g., from soil models and soil classification systems commonly comprising at
maximum the uppermost meter (Schmidt et al., 2011; IUSS Working Group WRB,
2014). Although topsoils commonly show the highest root biomass and high
concentrations of soil organic carbon (SOC) and most nutrients (Jobággy
and Jackson, 2000, 2001), plants are able to generate deep roots which may
penetrate also deeper parts of soil and underlying soil parent material until
several meters (Canadell et al., 1996), depending on plant type and
environmental conditions. This demonstrates the important role of the deep
subsurface as a part of terrestrial ecosystems (Richter and Markewitz, 1995).
Root penetration may thus also affect paleosols as recently discussed
(Wiesenberg et al., 2015).
Gocke et al. (2014a) showed that ancient calcified roots of Holocene age
were present throughout a European Late Pleistocene loess–paleosol sequence,
penetrating several paleosols. This was recently confirmed by other
sequences in southeastern Europe and China (Újvári et al., 2014; Li et al.,
2015). Also living roots that were not connected to the vegetation on top of
the profile were found at 9 m depth due to the slope position of the
profile. Roots appeared despite comparatively low contents of most
nutritional elements in the loess deposits (Gocke et al., 2014a). Both
living and ancient roots were present preferentially in paleosols, which
leads to the question of how unique or common this phenomenon is, and whether it
applies, e.g., also to young (i.e., Holocene) soils with considerably higher
nutrient stocks compared to Pleistocene sediments and paleosols. One example
of young paleosols widely distributed in Central Europe are Plaggic
Anthrosols. Plaggic agriculture was a historical form of agricultural
land use which was applied on nutrient-poor sandy soils in northwestern Germany, the
Netherlands and northeastern Belgium, amongst others, during the Middle Ages until the
introduction of industrial fertilizers (Blume and Leinweber, 2004). Besides
straw, additional stable fillings like fermented forest litter, grass and
heath sods, obtained from the nearby landscape, were brought into stables
(Van Mourik and Jansen, 2016). After enrichment with animal excrements the
plaggic manure was brought back on the field, often together with compost
(Giani et al., 2014). Driessen and Dudal (1991) stated that several
properties of plaggic horizons like high porosity and high SOC contents
enable the unhampered penetration by roots. Evidence for modern root remains
in buried micro-Podzols was found by Wallinga et al. (2013). So far neither
intensity nor depth of rooting has been determined, especially for buried
plaggic horizons located in depths > 0.5 m.
Knowledge on potential rooting of paleosols by younger vegetation is,
however, crucial for studies aiming at the reconstruction of various sources
of soil organic matter (SOM) input. If only presence of roots is mentioned,
but ignored in data interpretation (Andreeva et al., 2013), this complicates
also interpretation of molecular markers that are proven to provide insights
into paleovegetation and paleoenvironmental conditions (Wiesenberg et al.,
2015). This is also crucial for Plaggic Anthrosols, where one might question
which type of stable filling was used in a certain area and what was the
composition of the applied manure (animal excrements vs. plant remains).
Additionally, Plaggic Anthrosols are thought to act as carbon (C) storage
(Giani et al., 2014) on the one hand, but on the other hand they tend to soil
degradation during self-restoration, i.e., recultivation of the ecosystem
without anthropogenic intervention. Soil degradation includes, among other
factors, a decrease of C stocks (Lal, 1994). Long-term C dynamics in deep
subsoil and in paleosols are still largely unknown and might be affected by
root penetration and associated rhizosphere processes (Rasse et al., 2005).
Hence, root penetration might significantly influence C storage and dynamics
even a long time after pedogenesis of buried soils, thus entailing problems
in terms of paleoenvironmental interpretations and soil functioning as C
source or sink.
The aims of this study were to elucidate (i) whether buried soils provide
beneficial growth conditions for deep roots, (ii) whether deep roots utilize the
soil volume below 0.5 m and in deeper parts on identical or different scales
in terms of utilized volume compared to topsoil and shallow subsoil, and
(iii) to which degree depth distribution of different root sizes is related
to given physical and chemical soil profile characteristics. These
investigations should give insights into the complexity of soil formation
phases, which can partially overlap in Late Glacial eolian deposits associated
with Holocene (paleo)soils. Therefore, a profile in the southeastern Netherlands that
comprises two distinct podzolization phases on sandy parent material,
interrupted by anthropogenic plaggen use, was investigated for a suite of
physical and geochemical parameters as well as root quantities, using a
non-traditional pseudo-three-dimensional approach for assessment of roots.
Methodology
Study site
The hamlet of Bedaf is located near Uden, in the Maashorst area, southeastern
Netherlands (51∘40.189′ N, 5∘34.660′ E; Fig. 1a).
Vegetation and land use history of the region were comprehensively
investigated by Van Mourik et al. (2012). Briefly, clearing of the natural
deciduous forest growing on Late Glacial to Preboreal eolian cover sand
deposits started in Late Bronze Age and caused transition into heathland.
Plaggen agriculture started during the Middle Ages ca. AD 1500 with
thicknesses of plaggic deposits of up to ca. 1 m. Intensification of land
use after AD 1600 and related heath degradation entailed sand drifting
(Driessen and Dudal, 1991), and sands were stabilized in the course of the
19th century under naturally regenerated and planted forests.
(a) Location of the study site in the Netherlands (source:
www.d-maps.com). (b) The nature reserve Bedafse Bergen and its
surroundings (source: http://maps.google.de, 17 July 2015). The profile
of the current study, “R2014“, is located approximately 50 m south of
the profile “R2012“ described by Van Mourik et al. (2012).
(c) The profile of the current study, R2014, containing several
sequences of soil development, reflecting the development of landscape and
land use from Late Glacial until today. Optically stimulated luminescence
(OSL) ages of drift sand and Plaggic Anthrosol were adopted from the nearby
profile R2012 (Van Mourik et al., 2012).
The soil profile of the current study was named “R2014” to account
for its vicinity to the profile “Rakt” described by Van Mourik et al. (2012), which is called “R2012” in the following. It was prepared in
an oak stand at the east side of the nature reserve “Bedafse Bergen” (Fig. 1b), a sand dune of > 10 m height that formed during the early
Middle Ages and started to deliver major portions of sand, which deposited
on the nearby plaggic deposits in the late 18th and early 19th century. The
recent vegetation comprises oak (Quercus ruber) with ages of up to
200 years, minor portions of birch (Betula alba) and mountain ash
(Sorbus aucuparia), as well as fern (Dryopteris carthusiana) and blackberry (Rubus fruticosus) understorey.
Profile preparation and field methods
The current soil profile has a thickness of ca. 2.4 m, whereas only the
mineral soil was subjected to detailed geochemical analysis. The ca. 10 cm
thick (ectorganic) moder layer was excluded from the current study as
predominantly different phases of soil formation and the significance of
roots in different soil horizons were targeted. A soil pit was prepared at
ca. 1 m distance of a dead (< 10 years) standing oak tree at the
left (north) side and a living oak tree at the right (south) side. In the pit,
material was removed layer by layer in 10 cm depth increments down to 0.6 m,
and in 15 cm increments from 0.75 to 2.25 m as described by Gocke et al. (2014a). This sampling resolution was chosen to obtain samples from at least
two different depths of each unit instead of pooled samples. To overcome the
low thickness of the drift sand, one planar horizontal level was created at
0.25 m instead of 0.3 m to include its top and base. The soil–sediment
sequence was investigated both at the profile wall and on each of the
18 horizontal areas, with dimensions of the latter accounting for 0.8 m × 1.4 m. Investigation of the material below the soil pit by auger revealed
that the base of the cover sand was not reached, and that the groundwater
level was in a depth > 3.5 m at the time of sampling.
Horizontal levels were carefully cleaned by a brush. A grid with a side
length of 0.5 m × 0.5 m subdivided into nine squares was applied to determine
quantities of roots, which were then extrapolated to 1 m2 (Gocke et
al., 2014a). To account for the various source plants and the significance
of root processes in the rhizosphere surrounding the roots of different
sizes, a rough size classification of roots was applied, distinguishing
between fine (diameter ≤ 2 mm), medium (2–5 mm) and coarse roots
(> 5 mm). Dead and decaying roots were quantified separately
from living roots. Double counting at one depth interval, i.e., on the right
and the left side of a horizontal area, was performed where either
distribution of living and dead roots was inhomogeneous (topsoil at 0 m), or
where sediment and paleosol material occurred in the same depth interval due
to morphology of plow furrows (transition between drift sand and Plaggic
Anthrosol at 0.4 m; Fig. 1c). Analogously, roots were quantified also at the
profile wall using the same grid (Gocke et al., 2014a). As dimensions of
horizontal and vertical levels did not necessarily take into account
identical depth intervals, interpolations were made for the data collected
at the profile wall. However, as profile wall data were collected at the
front of the horizontal levels, the left and the right side walls as well as
at the back of the profile wall, averaged data of profile walls were used
for the comparison with horizontal levels.
After root counting, at each horizontal level four replicates of soil or
sediment were collected as distant as possible from visible roots and root
remains. Material was collected as volumetric samples using an
Eijkelkamp soil corer with stainless steel cylinders of an inner volume of
100 cm3, each.
Physical and geochemical analyses
Samples were oven-dried at 40 ∘C until their weight remained
constant, and the dry weight was normalized to 1 cm3 to obtain the dry
bulk density. Complete material of each dry sample was sieved and the
fraction < 2 mm was used for the following analyses. Grain size
distribution was measured in two replicates per depth. Therefore, an aliquot
of each sample was sieved, and 4.0 g of the fraction < 125 µm
was analyzed by X-ray sedimentometry (SediGraph 5100; Micromeritics, USA)
after dispersing the sample in 50 mL of 0.5 % sodium hexametaphosphate
solution (Müller et al., 2009). Color of soils and sediments was analyzed
in three replicates per depth using a spectrophotometer (Spectro-Color; Dr.
Lange, Germany) as described by Gocke et al. (2014a). Color indices are
presented according to CIELAB Color Space (L*a*b*; CIE, 1931). They
indicate lightness L* on a scale from 0 (absolute black) to 100 (absolute
white), as well as chromaticity coordinates on red-green (a*) and
blue-yellow (b*) scales. pH was determined via pH meter in three
replicates per depth after dispersing 10.0 g of sample in 50 mL of
0.01 M CaCl2 solution (Egli et al., 2013) and allowing the soil or
sediment to settle for 30 min.
For the following analyses, which were measured in triplicate per depth
interval, ca. 10 g of the fraction < 2 mm was milled in a ball mill
(Retsch MM400; Retsch, Germany). Aliquots between 15 and 40 mg were weighed
in tin capsules and analyzed for total C contents via a CO2 isotopic
analyzer (Picarro CRDS G2131-i; Costech Analytical Technologies Inc., US). Tests with
hydrochloric acid demonstrated the absence of carbonates throughout the
profile; therefore, total C refers to organic C (Corg). For
determination of bulk elemental composition, 5.0 g of milled sample was
poured into a plastic cup, the bottom of which consists of 4 µm
thin Prolene foil, and measured using an energy-dispersive X-ray fluorescence
spectrometer (Spectro X-Lab 2000, Spectro Analytical Instruments, Germany;
Egli et al., 2013).
Calculations and statistics
Organic carbon stocks STC were calculated as
STC=ρ×d×Corg ,
where ρ is dry bulk density, d the thickness of the respective depth
interval and Corg organic carbon concentration in mg g-1 in the
individual depth interval.
Depth diagrams show mean values and standard error of the mean. Differences
between depth intervals or between units were tested for significance by
one-way ANOVA with a significance level of 0.05 and level of high
significance 0.01, followed by post hoc Scheffé test using the software
STATISTICA 7.0 (StatSoft).
Crossplots show mean values and standard error of the mean, whereas
coefficients of determination were calculated by regression analysis, based
on values of individual field replicates.
Results and discussion
Characterization of the profile R2014
From bottom to top, the soil–sediment sequence R2014 comprised the
following horizons (FAO, 2006, IUSS Working Group WRB 2014; Fig. 1c): from at
least 3.5 to 1.8 m, light yellowish cover sand (3C, cs) dominated, with
sporadic orange-reddish mottles between 2.25 and 1.8 m marking the
transition towards the overlying dark reddish-brown 3Bshb horizon of a relict
Entic Podzol (Arenic) (1.8–1.5 m; rEP; Fig. 2a). The Podzol developed
during several millennia (Sauer et al., 2007) between Preboreal and Bronze
Age, likely under deciduous forest until anthropogenic conversion of forest
to heath vegetation started (Van Mourik et al., 2012). Podzolization
processes including redistribution of OM, iron (Fe) and aluminum (Al) oxides
led to the formation of consolidated crusts, which are continuous in its
upper part and more interrupted towards the underlying cover sand (cs;
Fig. 2b). The upper ca. 0.3 to 0.5 m of the Podzol, likely including the
eluvial and humic horizons, was reworked when plaggic agriculture began and
mixed with plaggic deposits, as visible by the presence of sporadic pick
marks within the Podzol, filled with plaggic material (Fig. 2c, d). A thick
gray Plaggic Anthrosol (Arenic) (PA) was accumulated in 1.5–0.4 m depth,
revealing the darkest color and the most dense, clayey material at the
bottom, whereas the material was lighter and more sandy towards the top
(Fig. 1c). Using optically stimulated luminescence (OSL) ages from the nearby
profile R2012 (Van Mourik et al., 2012), an average rate of plaggen
accumulation of 1.6 mm yr-1 was calculated, which is in agreement with
literature data (Giani et al., 2014). The top of the Plaggic Anthrosol is
characterized by an irregular boundary, showing the well-preserved morphology
of up to 0.2 m deep and 0.4 m wide plow furrows, which had been created in
lateral distances of ca. 0.9 m (Figs. 1c, 2e) during the last phases of
historic agricultural use at this site. The yellow drift sand (ds) overlying
the Plaggic Anthrosol (0.25–0.4 m) filled the irregular former surface
created by plowing. The profile is terminated by an
Epialbic Podzol (Arenic)
(0–0.25 m; EP) at its top. Due to its young age of ca. 200 years (Van
Mourik et al., 2012), eluvial and illuvial horizons are weakly developed but
well visible (Sauer et al., 2007). The complete succession of horizons is
shown in Fig. 1c.
Detailed pictures of the profile, with (a)
showing the profile wall and (b–e) displaying view on horizontal
levels. Note that the inner length of the square-shaped aluminum frame is
0.5 m. (a) Depth interval 1.5–2 m with clearly visible boundary
between the relict Entic Podzol (rEP) and cover sand (cs). (b)
Decomposed roots surrounded by dark accumulations of humic material and iron
and manganese oxides at 1.8 m depth. (c, d) Pick marks in the rEP
(reddish-brown) filled with gray plaggic material at 1.65 and 1.5 m depth,
respectively. (e) Top of the Plaggic Anthrosol (PA) with visible
plow furrow crossing the horizontal level diagonally, filled with yellow
drift sand (ds). The left half of the horizontal layer is located at 0.4 m
depth, the right half at 0.5 m depth.
Physical and geochemical characterization of the R2014 profile:
distinction of soil formation phases
Dry bulk density, with a range between 1.32 and 1.64 g cm-3 (Fig. 3)
and an average of 1.51 ± 0.02 g cm-3, was in agreement with
global values usually falling in the range of 1.1–1.6 g cm-3 (Hillel,
1980). Comparably low density between 1.43 and 1.50 g cm-3 agreed with
literature data on other plaggic soils (Giani et al., 2014). The highest
values ≥ 1.54 g cm-3 were restricted to cs, ds and the lowermost
part of the EP, thus confirming the general observation of high dry bulk
densities in sand-dominated soils and sediments.
Physical properties of the profile, including dry bulk
density, grain size distribution (left part of the diagram shows EP and ds
horizon, right part reveals PA, rEP and cs), as well as color indices
L* (lightness) and a* (redness). Symbol and column height
represent thickness of sampled interval (0.05 m). At 0.4 m depth, ds (white
symbols) and PA (black symbols) were analyzed separately.
Grain size distribution showed a general dominance of the sand fraction with
90.2–99.2 wt %, with values < 95 wt % being restricted to the
PA between 0.6 and 1.35 m (Fig. 3). Remarkable portions of silt > 4 wt % and of clay > 2 wt % occurred solely in the
mentioned depth interval. Similarities between grain size distribution of
the EP with that of cs and ds are likely due to the short time in which the
recent soil developed (Van Mourik et al., 2012).
Color indices largely confirmed field observations, with the highest
L* values (lightest color; > 57) occurring in the cs
and lowest values (darkest color; < 52) in the rEP and major parts
of the PA (Fig. 3). The dark color might be linked to high contents of OM,
which is however not necessarily valid for the sand fraction (Wiesenberg et
al., 2006). The continuous increase of L* towards the top of the PA
likely resulted from increasing incorporation of drift sand, and potentially
also from decreased heath biomass production during the Little Ice Age, which
peaked in the late 17th century (PAGES 2k Consortium, 2013), with heath sods
containing relatively less plant material and more mineral soil. The redness
of the profile was typically highest (highest a* values
> 1.8) in the lowermost parts of both Podzols as well as in
directly underlying sand, which indicates the effect of podzolization
processes like Fe accumulation (Sauer et al., 2007) extending into soil
parent material (see also Fig. 2a). In addition, b* (see Supplement Table S1) showed the highest values (yellow color; > 10) in cs
and constantly low values (blue color; mostly < 5) in the PA.
The complete soil–sediment sequence showed acidic conditions with pH values
between 3.1 and 4.7 (Fig. 4), and an average of 4.0 ± 0.1, explained
by the absence of CaCO3. Within both the EP and the rEP, the pH
increased consequently from the respective top towards underlying sand from
3.1 to 3.7 and from 4.1 to 4.7, respectively, whereas values in the PA were
in between both soil–sediment couples.
Geochemical properties of the profile, including pH,
Corg contents, and weight percentages of P, Ca and Fe. Symbol and
column height represent thickness of sampled interval (0.05 m). At 0.4 m
depth, drift sand (white symbols) and PA (black symbols) were analyzed
separately.
Organic carbon (Corg) contents showed a wide range between 0.6 and
17.9 mg g-1 (Fig. 4), averaging 7.1 ± 1.3 mg g-1. In
contrast to extremely Corg-poor cs and ds horizons (< 0.3 mg g-1), high Corg contents (> 10 mg g-1)
were found in the topsoil as well as in lower PA and rEP (1.05–1.65 m
depth), matching the range of Corg reported for Plaggic Anthrosols
(Pape, 1970). This observed peak of Corg, together with decreasing
Corg contents towards the top of the PA, likely resulted from
increasing accumulation rates of mineral soil material over time (see
above). The negative correlation of Corg contents with L*
(R2=0.84; Supplement Fig. S1a) reinforced the assumption that high OM
contents caused the dark color (Schulze et al., 1993; Konen et al., 2003),
despite uncertainties due to its sandy nature. Total C stock of the
investigated profile (0–2.4 m) was 27.4 kg m-2. A total of 64 % of the total C
stock was located in the PA, whereas EP and rEP comprised solely 9 and
18 %, respectively, corresponding to 2.5 and 5 kg m-2 (Fig. 5a). The
latter two values were in the lower and medium range of C stocks reported
for European forest soils (Baritz et al., 2010), which is due to the young
age of the EP and incompleteness of the rEP. Minor portions comprising 4 and
5 % of the C stocks were found in ds and cs. Thus, major portions of C
stocks were located below 1 m depth (17.7 kg m-2; Fig. 5b). The current
study not only confirms the important role of deep forest soils for
terrestrial C pools (Harrison et al., 2011; Lorenz et al., 2011) but also
enforces the rising awareness of buried soils as important terrestrial C
pools, as recently discussed by Marin-Spiotta et al. (2014) and Johnson (2014). Implementation of deeper parts of soil and parent materials is thus
crucial for terrestrial C budgets but has been performed only scarcely so
far (e.g., Wiesmeier et al., 2012; Harper and Tibbett, 2013).
Corg stocks of the profile R2014, calculated
(a) for each unit and (b) per 10 cm depth increment.
Throughout major parts of the profile, the abundance of nutritional elements
phosphorous (P), calcium (Ca) and Fe was low, with highest values of 0.12,
0.11 and 0.79 wt %, respectively (Fig. 4; for further elemental
composition see Supplement Table S1). Due to the exposition of plaggen sods to
animal excrements and household garbage prior to spreading on the field, as
well as low P translocation in soils (Giani et al., 2014), relative P
contents were highest in the PA and especially in its lowermost part.
Relative enrichment of P proceeding into the rEP suggests limited depth
translocation of certain nutritional elements due to leaching processes,
significantly later than the time span of podzolization. A clear difference
occurred only between the uppermost 1.8 m and the cs, in which most elements
were depleted due to the high abundance of silicon (Si) from quartz grains.
Ca was slightly enriched (> 0.08 wt %) compared to other
profile layers only at the top of EP and in cs. The general Fe enrichment in
plaggen soils observed by Giani et al. (2014) could not be confirmed for the
profile investigated in the current study. Rather, Fe contents were similar
throughout the profile (0.2–0.4 wt %) with slightly increasing trend
towards deeper parts of the PA and only one distinct peak of 0.8 wt % at
the top of the rEP (Fig. 4). The latter is rather related to former
podzolization (Sauer et al., 2007) than subsequent plaggen accumulation. As
a consequence of this depth distribution, no correlation of Fe with
a* was found (Supplement Fig. S1b). Potentially, Fe coatings around sand
grains might themselves be coated with OM translocated to a later point in
time as discussed for Podzols (Anderson et al., 1982). This is supported by
a strong correlation of Fe with Corg within the PA (R2=0.94;
Fig. S1c). Thus, the most probable mechanism of OM immobilization in
the PA at Bedafse Bergen seems to be the adsorption on secondary Fe
precipitates, whereas stabilization by clay can be excluded due to low clay
contents, and similarly no correlation of Corg with Al was found
(Fig. S1d).
The following similarities and differences in physical and geochemical
properties between different layers and horizons supported distinction of
the respective (paleo)pedogenic phases. The PA showed two characteristic
features: on the one hand very high Corg content in the range of
surface soil, significantly higher than that of cs and ds, and on the other
hand significant enrichment (P, sulfur (S), copper (Cu), molybdenum (Mo);
Fig. 4 and Table S1) and depletion (sodium (Na), potassium (K), Ca;
Table S1) in certain nutritional elements compared to cs and ds. Both
features were highly significant (for Mo solely in the lower half of PA) and
partially proceeded into rEP. The latter indicates that downward transport
of these elements continued after transformation of the Podzol into a
Plaggic Anthrosol. Additionally, constant or scattering values of several
parameters (density, grain size, a*, L*, pH, Mg; Figs. 3, 4
and Table S1) instead of a depth gradient within PA confirmed the
accumulation of sods and their mixing by plowing, contrary to a natural soil
that would have developed systematically from top to bottom. Nevertheless,
several parameters, e.g., increasing bulk density, increasing sand fraction
as well as decreasing Corg, indicated the increasing portions of
drift sand incorporated into PA towards its top. Both Podzols, although
differing in absolute age by several millennia and in their developmental
stage, showed similar depth trends in terms of pH and color indices: the
strongest acidification occurred in the topsoil and the top of rEP
respectively, whereas a highly significant increase of pH was determined
from top to bottom of EP and rEP, and partially from the bottom of the
respective (paleo)soil into soil parent material (Table S2). A highly
significant increase both of a* and of L* with depth, the
latter exceeding into cs in the case of rEP, confirmed the effect of
podzolization processes including eluviation and illuviation of Fe and other
constituents. Similarly, cs and ds resembled each other and could be clearly
distinguished from respective overlying (paleo)soils due to higher dry bulk
density and pH and lower contents of nutritional elements (P, S) and
Corg. These differences were highly significant for the rEP-cs couple
and partially significant for the couple of EP and ds (Table S2).
Root distribution: traditional vs. new approach
Depth distribution of living roots, quantified on
horizontal levels, and presence of decomposed roots (if abundant; < 10 m-2 not counted), marked by an arrow. Position of the symbols marks
the surface of the respective horizontal level. At 0.4 m depth, drift sand
(white symbols) and PA (black symbols) were analyzed separately.
Living roots were present throughout the uppermost 2 m of the profile, when
detected and quantified on horizontal levels (Fig. 6). Fine roots might
derive from trees or from understorey vegetation, whereas medium and coarse
roots are exclusively formed by the nearby mature shrubs and trees. Living
roots below ca. 0.5 m depth are likely attributed to oak trees, whereas
shallow roots may originate from various source plants of the present
vegetation. In agreement with previous observations (Millikin and Bledsoe,
1999; Moreno et al., 2005; Thomas, 2000), frequencies of fine roots decreased
continuously from topsoil (454 ± 65 m-2) towards bottom of ds
(16 ± 11 m-2). However, fine roots increased again within the PA,
slightly exceeding topsoil frequency for the first time at 0.75 m
(588 ± 81 m-2) and maximizing with > 4000 m-2 in the
upper part of the rEP. This factor ≈ 10 between topsoil root
quantities and values in 1.5 m depth strongly contradicts the general
assumption of rooting being largely restricted to the topsoil. Contrary,
solely 5 % of total fine roots, 2 % of total medium roots and none of the
coarse roots appeared in the topsoil, whereas 24, 74 and 78 % occurred
within the PA, respectively. Further, 78 % of fine roots, 42 % of medium
roots and 11 % of coarse roots grew below the uppermost 1 m, respectively.
The ability of plant roots to grow down to several meters has been known for
decades (Canadell et al., 1996) but has been largely ignored in soil science
so far (Maeght et al., 2013). Below 1.5 m, fine roots strongly decreased and
were absent below 2 m (Fig. 6). Medium roots showed a completely different
depth distribution, with very low abundances (< 60 m-2) in the
uppermost 0.4 m and between 1.65 and 2.25 m depth, whereas quantities were
mostly > 100 m-2 throughout the PA and uppermost rEP (Fig. 6).
Another distribution pattern was found for coarse roots, the occurrence of
which strongly varied within each depth interval and was restricted to a
depth between 0.25 m and 1.05 m (Fig. 6), confirming previous observations
(Millikin and Bledsoe, 1999). A dissimilar depth distribution of different
root sizes could be expected, as these perform different tasks like nutrient
acquisition and anchorage (e.g., Coutts et al., 1999; Hodge et al., 2009).
Nevertheless, similarities were found at three depth intervals. First,
frequencies of fine, medium and coarse roots were lowest in nutrient-poor cs.
Second, they showed a bimodal pattern at 0.4 m, with lower values in ds
compared to PA. The difference was highly significant for medium roots.
Third, fine and medium roots were most abundant at the transition between PA
and rEP. In the latter, roots occurred predominantly in former pick marks
filled with plaggic material (Fig. 2c, d). Only scarcely, the comparatively
hard sesquioxide crust of the rEP was directly penetrated by fine and medium roots.
These observations imply that roots exploit deeper parts of the soil and soil
parent material in search of preferable growth conditions, including, e.g.,
abundant nutrients (see Sect. 3.4; Sainju and Good, 1993).
Ancient roots with various degree of decomposition were observed at several
depths, but these occurrences did not match with abundances of living roots
(Fig. 6). For instance, decomposed fine roots occurred mainly in the ds and
cs horizons, whereas decomposed medium roots were found in several depth
intervals from topsoil down to cs. Decomposed coarse roots occurred at
similar depths as living coarse roots and additionally also at 2.25 m.
Concluding, ancient roots within the EP, ds and PA derived from recent
vegetation. In contrast, those at the bottom of the soil pit likely
originated from source vegetation other than the modern one, i.e., probably
from deciduous trees leading to the formation of the buried Podzol. This
demonstrates that deep roots, exceeding the actual soil depth and proceeding
into underlying soil parent material or paleosols, are not a special
phenomenon of recent vegetation but occurred also at earlier times.
Comparison of (a) fine- and (b) medium-root quantities determined at the profile wall at the right side of the soil
pit, and those determined on horizontal levels.
The reason why deep roots have been mostly neglected and their abundance has
been strongly underestimated so far is that root quantification – if at all –
was commonly performed at the profile wall for fast and convenient
investigation (Thomas, 2000), as root length density in collected soil
samples (Moreno et al., 2005) or weighed biomass after excavation (Millikin
and Bledsoe, 1999). Detailed root investigation on horizontal levels of
respective dimensions, as presented in the current study, is scarce. Figure 7
shows the limited informative value of studies using either one or the other
approach, especially in the case of fine roots (Fig. 7a). For these,
quantification at the profile wall yielded considerably lower values than
quantification on horizontal levels in depths with generally low and
intermediate frequency (< 1000 m-2), whereas very high
abundances in the rEP were strongly underestimated at the profile wall.
These considerable differences between both approaches likely are attributed
to contrasting living conditions within the current soil profile. That is,
compactness at the top of the rEP forces fine roots to grow more vertically
and beneficial nutritional conditions in the PA rather entail spreading of
fine roots and thus a higher portion of diagonally and horizontally growing
roots, which are registered by profile wall counting better than by
horizontal counting. For medium roots, quantities obtained by the two
approaches matched better (R2=0.14; Fig. 7b), with deviations from
the 1:1 line usually < 20 %. Larger divergences occurred,
however, in those depths where frequencies determined on horizontal levels
are either very low like in ds or very high like in rEP. Medium root
frequencies were further largely underestimated by wall quantification
especially in the EP including the topsoil, which is most commonly studied
for root abundances, whereas deeper parts (> 1 m) are scarcely
considered in terms of roots (Perkons et al., 2014). In summary,
quantification of roots exclusively at the profile wall can underestimate
fine roots, whereas combination with counting on horizontal levels might
give a more comprehensive insight into root distribution, as well as
improved understanding of the possible reasons. In addition to depth
distribution and preferred rooting direction, the approach chosen for the
current study revealed the high variability of root abundances even within
short distances of up to 1.4 m, the side length of the soil pit (Fig. 8a,
b).
Frequencies of (a) fine and (b) medium
roots determined at different places within the soil pit.
Beneficial and unfavorable rooting conditions in the soil–sediment sequence
R2014
The following parameters pointed to preferable or non-preferable rooting
conditions in respective units of the soil–sediment sequence like especially
the PA.
Negative correlation of fine roots with dry bulk density in the buried soils
and sediments, excluding EP (R2=0.29; see Fig. S1e), confirmed
the previous suggestion of preferable rooting conditions provided by low-density substrates with high porosity (Driessen and Dudal, 1991). In
addition, plaggic horizons commonly contain enhanced contents of both total
and plant-available P (Eckelmann, 1980); therefore, high total P in
R2014 suggests a similar situation in the profile of the current study.
Here, very likely roots exploited the buried PA for P (and S), whereas other
nutritional elements like K or Ca are less relevant at that site. However,
only a positive trend instead of a clear correlation was found between fine
roots and P (R2=0.20; Fig. S1f) as well as medium roots and P
(R2=0.32; Fig. S1g) in the buried soils and sediments,
excluding EP. Due to low mobility of P, we suggest that the original P
contents in PA have been considerably higher prior to penetration by the
recent vegetation, and that major parts of P were transported towards the
topsoil via vertical nutrient uplift within root systems (Jobbàgy and
Jackson, 2001; Kautz et al., 2013). This is further enforced by a high
variation of P contents between replicates in upper PA (Fig. 4), which
coincides with spatially highly varying root abundances (Fig. 8).
Contrary to these positive growth conditions for roots, rEP provided a
special situation with very abundant fine and medium roots on its top. This
is most likely caused by a combination of two factors. First, high Al
contents in the rEP (Table S1) potentially lead to Al toxicity for
plant roots; thus, fine roots penetrate rEP only a few centimeters and preferentially
grow in pick marks filled with plaggic material, preventing the contact with
rEP. The high Al concentrations can lead to inhibition of further root
growth (Ryan et al., 1992; Poschenrieder et al., 2008) and might further
promote the generation of abundant fine roots (R2=0.62; Fig. S1h)
by the plant to “search” for other, more beneficial regions in the
soil. Second, the enrichment of Fe and manganese (Mn) oxides which coat the
sand particles and form encrustations in the rEP leads to a higher
compactness and thus inhibits further root penetration in major areas of the
horizon. These encrustations also lead to a reduction in the pore size,
leading to enhanced water and air permeability, but limited root
penetration. The latter effect is well known for cemented soil horizons with
spodic or other properties (Bockheim, 2011). Strongly decreasing root
abundances below the top of rEP result from minor possibility for the plant
roots to penetrate the sesquioxide accumulations.
Implications for pedology, paleoenvironmental records and carbon
sequestration potential
The intensive penetration of the soil–sediment sequence R2014 by
various generations of plants, including ancient deciduous trees associated
with rEP (van Mourik et al., 2012), agricultural plants associated with PA
and recent mixed forest associated with EP (Fig. 9), might have considerable
consequences for pedology, the recorded paleoenvironmental, landscape
developmental and cultural signal on the one hand, and for the potential to
sequester or release C on the other hand.
Schematic figure of the successive phases of
sedimentation and soil formation that led to the development of the
investigated profile.
Young (Holocene) buried soils have a high value for reconstruction of
regional and local vegetation, land use and history. Therefore,
traditionally the record is investigated via pollen records (e.g., Van Mourik
et al., 2012) and archeological artifacts (e.g., Wells, 2004), which are not
influenced by later root penetration. However, in the last decades molecular
approaches including plant- and microorganism-derived lipids, as well as
isotopic analyses (δ13C, δ2H, Δ14C)
on OM and fractions thereof, became of increasing interest to get improved
insights into paleovegetation composition, paleohydrological conditions and
time span of OM incorporation in soil (e.g., Sachse et al., 2009;
Mendez-Millan et al., 2014; Wiesenberg et al., 2015). These proxies can be
considerably overprinted by incorporation of younger root- and
microorganism-derived OM in surrounding sediment up to several centimeters distant
from roots, entailing uncertainties in terms of reliability of the proxies
(Huguet et al., 2012; Gocke et al., 2014b). Based on pyrolysis-GC/MS, Van
Mourik et al. (2010) reported the incorporation of presumably both above-
and belowground biomass from heath vegetation in buried plaggic soils of
the Maashorst area, thus emphasizing the meaning of root biomass, however
without discussing the potential subsequent disturbance of the chronological
context within the sequence. Also, 14C dating of humic acids from the
PA in the nearby profile Nabbegat (Van Mourik et al., 2012) yielded
consistently increasing ages with depth and did not indicate any overprint
by recent roots. However, detection of incorporated root-derived OM of
considerably younger age, assumed based on the here shown root distribution,
has not been tried yet and may largely affect the paleoenvironmental record on a
molecular level. The different phases and sources of molecular proxies in
recent and buried soils can be deciphered based on modeling including simple
end-member models (Buggle et al., 2010). In contrast, the complexity of
multiple sources of different root and aboveground biomass requires more
sophisticated approaches combining various sets of biomarkers of potential
source vegetation, as successfully applied in the VERHIB model (Jansen et
al., 2010). Such an approach might enable not only the assessment of
different sources of OM in soils of different ages but potentially also the
quantification of the recent root overprint of the paleoenvironmental record, and
the correction of the paleoenvironmental signal biased by the root overprint. However,
such an unraveling depends strongly on accurate information about root input
and rooting depth (Van Mourik and Jansen, 2013). Therefore, a combined
application with the root estimating methodology described in the present
study would appear very promising.
It is known that plaggic soils may act as carbon storage (Giani et al.,
2014). However, long-term stability of incorporated OM depends not only on
chemical composition and thus portions of above- and belowground biomass
(Rasse et al., 2005) but also on physical and chemical properties of the
soil and soil parent material as well as environmental conditions (Schmidt
et al., 2011). Commonly, roots are thought to introduce additional OM to
soil. Therefore, Kell (2012) suggested the application of deep-rooting crops
to sequester C. However, potential priming effects have to be considered,
leading to mineralization not only of fresh root-derived OM but also of
stabilized OM that is commonly not attracting microorganisms in the absence
of fresh root-derived OM (Fontaine et al., 2007). Likely, priming led to the
net loss of C over long time spans of several millennia at a southwestern German
loess–paleosol sequence containing ancient calcified roots (Gocke et al.,
2014b). At the site of the current study, C within the PA and rEP might not
be stabilized very well due to low clay, high sand and at the same time low
Fe contents, thus facilitating these large C stocks to mineralization by
microorganisms associated with the recent tree vegetation that penetrates
the profile until > 1.5 m depth. Likely, C loss is still low
after solely 200 years of oak growth and could still be counteracted by
introduction of high amounts of root-derived OM but might increase in the
future.
Although root distribution patterns and distribution of elemental
composition within the investigated profile point to a strong root-derived
overprint of the buried soils and OM therein, the difficulty of general
conclusions and extrapolation of the results is related to the fact that
quantitative assessment of root effects remains difficult. Molecular and
isotope analyses together with modeling approaches, which are not targeted
in the current study, might substantially improve the conclusions based on
the currently available data. Nevertheless, the strong enrichment of roots
together with the heterogeneity of the spatial root distribution demands for
quantitative assessment of root abundances in paleopedological studies that
are based on chemical properties. Otherwise, the obtained data are likely
prone to misinterpretations due to root penetration and related modification
of geochemical proxies as only recently highlighted (Gocke et al., 2014b).