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The knowledge of tree species dependent turnover of soil organic matter (SOM) is limited, yet required to understand the carbon sequestration function of forest soil. We combined investigations of 13C and 15N and its relationship to elemental stoichiometry along soil depth gradients in 35-year old monocultural stands of Douglas fir (Pseudotsuga menziesii), black pine (Pinus nigra), European beech (Fagus sylvatica) and red oak (Quercus rubra) growing on a uniform post-mining soil. We investigated the natural abundance of 13C and 15N and the carbon:nitrogen (C:N) and oxygen:carbon (O:C) stoichiometry of litterfall and fine roots as well as SOM in the forest floor and mineral soil. Tree species had a significant effect on SOM d13C and d15N reflecting significantly different signatures of litterfall and root inputs. Throughout the soil profile, d13C and d15N were significantly related to the C:N and O:C ratio which indicates that isotope enrichment with soil depth is linked to the turnover of organic matter (OM). Significantly higher turnover of OM in soils under deciduous tree species depended to 46% on the quality of litterfall and root inputs (N content, C:N, O:C ratio), and the initial isotopic signatures of litterfall. Hence, SOM composition and turnover also depends on additional-presumably microbial driven-factors. The enrichment of 15N with soil depth was generally linked to 13C. In soils under pine, however, with limited N and C availability, the enrichment of 15N was decoupled from 13C. This suggests that transformation pathways depend on litter quality of tree species.
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The linkage of
C and
N soil depth gradients with C:N
and O:C stoichiometry reveals tree species effects on organic
matter turnover in soil
Marcel Lorenz .Delphine Derrien .Bernd Zeller .Thomas Udelhoven .
Willy Werner .So
¨ren Thiele-Bruhn
Received: 18 May 2020 / Accepted: 30 October 2020 / Published online: 12 November 2020
ÓThe Author(s) 2020
Abstract The knowledge of tree species dependent
turnover of soil organic matter (SOM) is limited, yet
required to understand the carbon sequestration func-
tion of forest soil. We combined investigations of
N and its relationship to elemental stoichiometry
along soil depth gradients in 35-year old monocultural
stands of Douglas fir (Pseudotsuga menziesii), black
pine (Pinus nigra), European beech (Fagus sylvatica)
and red oak (Quercus rubra) growing on a uniform
post-mining soil. We investigated the natural
abundance of
C and
N and the carbon:nitrogen
(C:N) and oxygen:carbon (O:C) stoichiometry of
litterfall and fine roots as well as SOM in the forest
floor and mineral soil. Tree species had a significant
effect on SOM d
C and d
N reflecting significantly
different signatures of litterfall and root inputs.
Throughout the soil profile, d
C and d
N were
significantly related to the C:N and O:C ratio which
indicates that isotope enrichment with soil depth is
linked to the turnover of organic matter (OM).
Significantly higher turnover of OM in soils under
deciduous tree species depended to 46% on the quality
of litterfall and root inputs (N content, C:N, O:C ratio),
and the initial isotopic signatures of litterfall. Hence,
SOM composition and turnover also depends on
additional—presumably microbial driven—factors.
The enrichment of
N with soil depth was generally
linked to
C. In soils under pine, however, with
limited N and C availability, the enrichment of
was decoupled from
C. This suggests that transfor-
mation pathways depend on litter quality of tree
Keywords Stable isotopes Microbial turnover
Litter Roots Common garden experiment
Recultivated forest soil
Responsible Editor: Edith Bai.
Electronic supplementary material The online version of
this article ( con-
tains supplementary material, which is available to authorized
M. Lorenz S. Thiele-Bruhn (&)
Soil Science Department, FB VI, University of Trier,
Behringstrasse 21, 54296 Trier, Germany
D. Derrien B. Zeller
INRAe Grand Est Nancy, UR 1138 Bioge
´ochimie des
`mes Forestiers, 54000 Nancy, France
T. Udelhoven
Environmental Remote Sensing & Geoinformatics
Department, FB VI, University of Trier, Behringstrasse
21, 54296 Trier, Germany
W. Werner
Geobotany Department, FB VI, University of Trier,
Behringstrasse 21, 54296 Trier, Germany
Biogeochemistry (2020) 151:203–220,-volV)(0123456789().,-volV)
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Soils in forest ecosystems bear a high potential as
carbon (C) sinks in the mitigation of climate change
(Pan et al. 2011). Tree species identity plays a crucial
role in the C cycle of these ecosystems, e.g. by fueling
soil with bio- and necromass, respectively (Augusto
et al. 2015). The total amount of C that is stored in the
forest floor and mineral soil is affected by the
dominating tree species (Mueller et al. 2015). Fur-
thermore, stoichiometric ratios like the carbon:nitro-
gen (C:N) ratio of soil organic matter (SOM) are
influenced by the forest stand (Cools et al. 2014;
Lorenz and Thiele-Bruhn 2019). Ecological stoi-
chiometry using elemental ratios is a suitable tool to
assess SOM and its turnover (Manzoni et al. 2010;
Zechmeister-Boltenstern et al. 2015). For this, the C:N
ratio is commonly used (Stevenson 1994). Also the
oxygen:carbon (O:C) ratio bears high potential to
characterize SOM because it reflects the state of
oxidation of SOM (Beyer et al. 1998). Furthermore,
the abundance of stable isotopes (
C and
N) in soils
also provide a powerful tool for investigating spatial
and temporal SOM dynamics (Ehleringer et al. 2000;
¨ggemann et al. 2011; Craine et al. 2015). Varia-
tions in the isotopic composition are useful for tracing
carbon sources and fluxes between plants, microor-
ganisms and soils, thus serving to elucidate the impact
of plant inputs on SOM formation (Balesdent et al.
1987). A combination of both approaches, ecological
stoichiometry and stable isotopes, in soil depth
gradients promises to get deeper insights into the
turnover of SOM. To our knowledge, this has not been
used before to characterize the turnover of tree species
dependent organic matter (OM) in the soil.
C and
N show trends of enrichment
with increasing soil depth that were related to aging
and turnover of OM (Nadelhoffer and Fry 1988;
Billings and Richter 2006; Trumbore 2009). Several
SOM turnover and stabilization mechanisms were
identified that can lead to a variation of the natural
abundance of
C. Litter with lower d
C values from
aboveground plant materials triggers the topsoil, while
the contribution of
C-enriched root inputs to SOM
C increases with increasing soil depth (Bird et al.
2003). Root inputs encompass C release from plant
roots to soil including: (1) root cap and border cell
loss, (2) necromass from root cells and tissues, (3) C
flow to root-associated, soil living symbionts (e.g.
mycorrhiza), (4) gaseous losses, (5) root exudates, and
(6) mucilage (Jones et al. 2009). During the microbial
metabolism of C, preferentially
C-depleted mole-
cules will be respired by microorganisms and the
remaining SOM will be
C-enriched (Lerch et al.
2011). In general, microorganisms are
compared to plant material or bulk SOM (Dijkstra
et al. 2006) and the contribution of microbial derived
C increases with the extent of OM turnover (Bostro
et al. 2007). Additionally, OM associated with soil
minerals is characterized by increased d
C values
compared to free or occluded light OM fractions
(Schrumpf et al. 2013). The association of OM with
minerals is an important mechanism for its stabiliza-
tion in soil (von Lu
¨tzow et al. 2007). The prevalence of
SOC decrease and d
C increase with depth in well-
drained forest soils has prompted the use of the
gradient of SOC plotted against d
C as a proxy for
SOM turnover (Acton et al. 2013). Consequently,
depth-related interconnection of d
C and SOC
describes the rate of change in
C natural abundance
along a decay continuum from fresh litter inputs to
more decomposed SOM (Garten et al. 2000).
The absolute enrichment of
N over soil depth can
be determined as the difference between the maximum
enrichment of
N in the mineral soil and the litter
bearing OL horizon (Hobbie and Ouimette 2009). The
development of
N with soil depth is related to N
cycling processes that are coupled to SOM turnover
(Emmett et al. 1998). Similar to d
C values, organo-
mineral associations (Kramer et al. 2017) and the
accumulation of
N enriched microbial biomass in
more transformed SOM (Wallander et al. 2009) can
drive the d
N patterns within soil. Furthermore, the
type and degree of mycorrhizal associations (Hobbie
and Ho
¨gberg 2012), enzymatic hydrolysis (Silfer et al.
1992), N losses after ammonification, nitrification and
denitrification (Ho
¨gberg 1997;Po
¨rtl et al. 2007),
atmospheric depositions (Vallano and Sparks 2013)
and mixing of soil N through bioturbation (Wilske
et al. 2015) contributes to the
N enrichments along
the soil profile. Both d
C and d
N are mechanisti-
cally linked through the decomposition and microbial
processing of SOM (Nel et al. 2018), thus highlighting
the suitability of both parameters to determine the
degree of organic matter turnover in soil.
In natural mixed forest ecosystems it is difficult to
track down a tree species effect on SOM status;
therefore common garden experiments, where
204 Biogeochemistry (2020) 151:203–220
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different tree species were planted in adjacent blocks
at the same time on similar soil, were established to
study tree species effects (Reich et al. 2005; Vesterdal
et al. 2013). Important insights into the relationship
between tree species and the cycling of soil C and
other nutrients in forest ecosystems were gained from
common garden experiments (e.g. Mueller et al. 2012;
Gurmesa et al. 2013). Nevertheless, such experiments
are often handicapped by a previous land-use conver-
sion, e.g. from arable land or from clear felled forests
(Vesterdal et al. 2008). Old SOM from former land-
use types often makes the interpretation of the effects
of tested species and their SOM on SOM dynamics
rather difficult (Balesdent et al. 2018). Callesen et al.
(2013) revealed in a common garden experiment that
the patterns of d
N in soil profiles reflected the former
arable land-use type. Comparable with common
garden experiments, differently afforested soils at
post-mining sites provide a unique opportunity for
understanding mechanisms in SOM formation (Frouz
et al. 2009). In particular, sites that are free from old C
sources can be suitable but investigations on such sites
are rare. Due to this, further research is required to
clarify if d
C and d
N, and thus the SOM status in
organic forest floor horizons (litter—OL, frag-
mented—OF, humified—OH) and mineral soil differs
between tree species.
This research was conducted on a post-mining site,
where previous accumulation of plant or coal material
are negligible (Lorenz & Thiele-Bruhn 2019). We
studied monocultural stands of Douglas fir (Pseudot-
suga menziesii), black pine (Pinus nigra), European
beech (Fagus sylvatica) and red oak (Quercus rubra)
that were grown for 35 years under identical soil and
geomorphological conditions to assess tree species
effects on the SOM status. In more detail, we
investigated the natural abundance of
C and
combination with C:N and O:C stoichiometry of
litterfall and root inputs (determined as belowground
phytomass) as well as SOM in depth gradients of
forest soils to answer the following questions:
(1) Do litterfall and root inputs differ in their
isotopic signatures of d
C and d
N between
tree species?
(2) Is there a tree species effect on d
C and d
the depth gradients starting from the OL horizon
down to 10–30 cm of mineral soil?
(3) Are stable isotope contents in the depth profiles
related to the stoichiometry (C:N and O:C ratio)
of the bulk soil?
(4) Varies the decomposition of OM and the
stabilization of it in soil significantly between
tree species and if yes, are litterfall and/or root
properties important for these processes?
Materials and methods
Study site
The study was conducted at the afforested spoil heap
¨he’, located in the northwest of the lignite
open-cast mine ‘Hambach’ in the Rhineland, Germany
(N 50°56.110,E6°26.560). There, boundary condi-
tions regarding soil, climate, topography and manage-
ment were highly similar, equivalent to a common
garden experiment. The Regosols at the investigated
sites developed on the same sandy gravelly parent
material (Lorenz and Thiele-Bruhn 2019). The car-
bonate-free parent material that was used for the spoil
heap recultivation had a C content of 0.20 ±0.05%
and a C/N molar ratio of 7.5 ±1.2 (Table S1).
Therefore, a relevant impact of old or fossil carbon
from former land use types and the introduction of coal
from lignite mining was excluded (Lorenz and Thiele-
Bruhn 2019). The investigation was carried out in
monocultural stands of Douglas fir (Pseudotsuga
menziesii), black pine (Pinus nigra), European beech
(Fagus sylvatica) and red oak (Quercus rubra) that
were afforested in 1982 on the western slopes of the
spoil heap. Within 35 years after the start of the
afforestation organic layers had developed that were
classified as Moder (Zanella et al. 2018) with slight
differences between tree stands. Dependent on the
thickness of the OH layer, Dysmoder was the dom-
inant humus form that coexisted in some patchy
sections with Eumoder under Douglas fir, beech and
oak, while under pine solely Dysmoder had developed
(Lorenz and Thiele-Bruhn 2019).
Sampling scheme and sample preparation
Each species stand is subdivided in six to ten plots with
a size of 1780 ±660 m
by skid trails established in
slope line. For each of the four stands, five plots were
Biogeochemistry (2020) 151:203–220 205
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selected for sampling of the forest floor, mineral soil,
roots and litterfall (Fig. 1). Sampling points (light grey
circles) within one plot were located at least 25 m
away from forest roads and in the upper parts of the
middle slopes. In April 2016, forest floor samples were
taken with a steel frame (20 cm 920 cm) and
carefully separated into the organic litter, fragmented,
and humified horizon, OL, OF and OH, respectively,
according to Zanella et al. (2018). Afterwards, bulk
soil samples were taken at three different depths
(0–5 cm, 5–10 cm, 10–30 cm) from excavated
50 cm 950 cm 950 cm pits. To ensure representa-
tiveness, forest floor and soil samples (grey rectangles)
were taken from four positions and samples from
similar depths were subsequently pooled. In total 60
forest floor samples and 60 mineral soil samples were
collected (five per depth in each stand, Table S2),
transported and stored at 4 °C for further preparation.
Forest floor samples were dried at 60 °C and visible
roots were carefully sorted out. Mineral soil samples
were passed through a 2 mm sieve, roots were
removed and the soil samples were dried at 60 °C.
All samples were ground and homogenized using a
ball mill (Retsch MM400, Retsch GmbH, Haan,
We performed root sampling 2 years later, in April
2018 and distinguished roots from different horizons.
To do so, we collected five replicate samples of forest
floor and mineral soil (dark grey circles in Fig. 1) with
a distance of 1 m around a tree within each of the five
plots per species stand. Forest floor roots were
collected using a steel frame (20 cm 920 cm).
Underneath, mineral soil roots were collected using
a root auger with a diameter of 8 cm (Eijkelkamp Soil
& Water, Giesbeck, Netherlands). The cores of
mineral soil were divided into the three subsamples
(0–5 cm, 5–10 cm. 10–30 cm). In total, we collected
400 root samples (25 per depth in each species stand,
Table S2) that were transported and stored at 4 °C. In
the laboratory, the forest floor samples were spread out
in plastic bowls and roots were carefully separated
using a tweezer. The roots were carefully washed to
remove adherent soil particles. The mineral soil
samples were put into plastic bowls and immediately
washed with water to separate roots. Roots with a
diameter B5 mm were dried at 105 °C to determine
dry weights and a subset of 80 samples (five per depth
per species) was homogenized using a ball mill
(Retsch MM400, Retsch GmbH, Haan, Germany) for
further chemical analysis.
In each of the five plots per species stand the
litterfall was collected using litter traps made with
nylon mesh (0.5 mm mesh size) that was fixed on a
wooden frame (1 m 91 m). Litter traps were
Fig. 1 Sampling design at the study site ‘Sophienho
35 year-old afforested monocultural stands of Douglas fir
(Pseudotsuga menziesii), black pine (Pinus nigra), European
beech (Fagus sylvatica) and red oak (Quercus rubra) were
investigated. They are located on the western exposed slopes
(inclination: 22.2°±2.2°) of the spoil heap. Each stand is
subdivided in six to ten plots with a size of 1780 ±660 m
skid trails established in slope line. For each of the four tree
stands, five plots were selected for sampling (light grey circles)
of the forest floor (grey rectangles), mineral soil (grey
rectangles), roots (dark grey circles) and litterfall (litter trap)
206 Biogeochemistry (2020) 151:203–220
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installed 1 m above the soil surface and located in the
plot near the central soil sampling point (Fig. 1). In the
timespan from July 2016 to June 2017 the litter traps
were monthly emptied. In the laboratory, the 240
samples were immediately separated into foliar and
non-foliar fractions and dried at 60 °C to determine
dry weights (Ukonmaanaho et al. 2016). The foliar
fraction of the 12 monthly samples of each litter trap
were pooled into one mixed sample. Consequently,
five litterfall samples per tree species resulted in a total
number of 20 litterfall samples (Table S2) that were
homogenized using a ball mill (Retsch MM400,
Retsch GmbH, Haan, Germany) for further chemical
Laboratory analysis
Total contents of C, N and O were determined using an
Elemental Analyser EA3000 (HEKAtech GmbH,
Wegberg, Germany). Soil samples were acidic and
free of carbonate (Lorenz and Thiele-Bruhn 2019),
thus the measured total C content represents organic
C. The contents of the elements were used to calculate
the molar C:N and O:C ratios. The stable isotopes
N were determined by an IsoPrime 100 isotope
ratio mass-spectrometer (IsoPrime Corporation, Chea-
dle, UK) and vario ISOTOPE cube elemental analyzer
(Elementar Analysensysteme GmbH, Hanau, Ger-
many). Stable isotope compositions are reported in
delta notation (d
C%and d
N%) relative to
Vienna Pee-Dee Belemnite (VPDB) for C, using the
international reference materials IAEA-CH-7
(-32.151%VPDB SD ±0.05%) as a standard,
and relative to atmospheric N
for N, using IAEA-N-1
(?0.4%air N2 SD ±0.2%), IAEA-N-2 (?20.3%
air N2 SD ±0.2%) and USGS32 (?180%air N2
SD ±1%) as standard according to Eq. (1):
dsample ¼Rsample
where Rrepresents the ratio of
respectively. The measurement error of d
N was
approximately 0.2%and \0.1%for d
Data processing and statistics
The relationship between the prevalent vertical
decrease of SOC and increase of d
C in depth profiles
was used as a natural indicator of SOC turnover
(Acton et al. 2013). The slope (b) of the linear
regression (y=a?bx) between the mean d
values and their respective log-transformed C con-
centrations (mg C g
) was calculated and is referred
as b
value. The distribution of
N along soil depth
profiles was compared between tree stands using the
soil enrichment factor (e
N). It is defined as
absolute enrichment between the OL horizon and the
10–30 cm mineral soil layer (Hobbie and Ouimette
2009) and was calculated following Eq. (2):
esoil15 N&ðÞ¼d15N1030cm d15 NOL:ð2Þ
The following statistical analyses were conducted
separately for litter inputs (litterfall and roots) and
each depth starting from the OL horizon down to
10–30 cm with the R statistical package version 3.3.2.
(R Core Team 2016). Boxplots and one-way analysis
variance (ANOVA) as pretests were carried out to
inspect the data structure. The residuals of ANOVA
were tested for normality and homoscedasticity using
the Shapiro–Wilk test respectively Levene’s test.
Accordingly, normal distributed and homoscedastic
data were tested for significant differences between
tree species by one-way ANOVA followed by the
Tukey’s honest significant difference (HSD) post hoc
test. Significant differences between tree species for
normal distributed but heteroscedastic data were
tested using Welch-ANOVA followed by a pairwise
ttest with Bonferroni–Holm correction. In case data
was not normal distributed but homoscedastic the
Kruskal–Wallis test was applied followed by the Dunn
test. Variance analyses and necessary pretests were
performed with a significance level of p\0.05. The
results are presented in arithmetic mean ±standard
deviation (SD) for the different tree stands. To
characterize relationships between isotopic (d
N,) and stoichiometric (C:N, O:C) parameters
regression analyses with linear and logarithmic func-
tions were done. Additionally, multiple linear regres-
sion models were generated to analyze explaining
variables for b
and e
N values. For this
purpose, C, N, C:N, O:C, d
C, d
N, as well as the
biomass of litterfall and roots were used as indepen-
dent variables. To simplify the complexity of the
model, parameters were stepwise eliminated that
decrease the quality of the regression model by
assessing R
and pvalues. Finally, the most
Biogeochemistry (2020) 151:203–220 207
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appropriate models that comprise only parameters
with significant portions of the explainable variance
were used to discuss driving factors for b
N values. The results of the regression analyses
were described as significant in cases where p\0.05.
Litter inputs
The total annual foliar litterfall in pine stands was
significantly higher compared to beech stands,
whereas under Douglas fir and oak intermediate
amounts were reached (Table 1). The
C content of
litterfall were significantly highest in Douglas fir
stands and declined in the sequence Douglas fir [
pine Coak [beech. Also, d
N of litterfall was
significantly higher in Douglas fir stands compared
to the other tree species. The litterfall of coniferous
species was characterized by significantly higher C:N
ratios compared to deciduous forest stands. The O:C
ratio of litterfall decreased in the order
oak [beech [pine [Douglas fir.
In contrast to the litterfall, substantially and in part
significantly higher root biomasses were detected in
the upper 30 cm of soil under deciduous tree species
than under conifers (Table 1). In general, the roots of
all tree species were significantly enriched in
4.56 ±0.99%and
N by 3.01 ±0.61%compared
to the litterfall. Douglas fir roots were characterized by
significantly higher d
C values compared to those of
pine and beech, while the significantly lowest d
values were determined in oak roots. Similar differ-
ences between tree stands were found for d
N of roots
as well as for litterfall d
N. Obviously, the isotopic
signatures of roots in the forest floors did not differ
from roots in deeper soil horizons but the tree species
effect was similarly pronounced in each soil depth. In
contrast to the litterfall, the C:N ratio of beech and oak
roots was significantly higher compared to the conif-
erous species (Table S3). The O:C ratio of roots was
highest in the oak stand similar to the litterfall.
Depth profiles of bulk soil d
C and d
N and their
relationship to stoichiometry patterns
The parent material for soil recultivation was charac-
terized by a d
C value of -29.69 ±0.13%and a
N value of -0.89 ±0.09%(Table S1). In
general, with increasing soil depth an enrichment of
C and
N was observed, while the contents of C and
N decreased (Fig. 2). A small deviation from this
pattern occurred for
C in the forest floor horizons,
where a depletion or no significant variation from OL
to OH horizon was detected. d
C varied in a range
from -29.37%(oak, OF) to -26.19%(Douglas fir,
10–30 cm). The coniferous species Douglas fir and
pine caused significantly higher d
C values in the
forest floor compared to beech and oak. In the mineral
soil lowest d
C values were found in the oak stands
(-28.13%to -27.32%), while in the Douglas fir
stands highest d
C values from -26.91%to
-26.19%were measured (Table S4). Throughout
the soil profile, a significant effect of tree species on
C was detected (Fig. 2b).
Compared to d
C, d
N varied in a wider range
from -6.93%(pine, OL) to 0.54%(beech,
10–30 cm) and depth gradients were more pro-
nounced. The forest floor horizons (OL, OF, OH) of
Douglas fir showed significantly higher d
N values
compared to the other tree species (Fig. 2d). In the first
two mineral soil layers (0–5 cm, 5–10 cm) the d
values of all tree species converged, while at a depth
from 10 to 30 cm under beech and Douglas fir
significantly higher values were determined compared
to oak. Consequently, a tree species effect on d
N was
found in the forest floor as well as the deepest
investigated mineral soil layer from 10 to 30 cm.
Regression analyses revealed that d
C and d
were related to the C:N and O:C ratio (Table 2). The
relationships of these two stoichiometric ratios of the
bulk soil were stronger with d
N than with d
C and
were better described by a logarithmic equation rather
than by a linear equation (Tables 2and S5). Along the
soil profile from OL to 10–30 cm depth the C:N ratio
decreased in a range from 52.8 (oak, OL) to 15.7
(beech, 10–30 cm), while the O:C ratio increased from
0.40 (Douglas fir, pine, OL) to 3.07 (beech, 10–30 cm)
(Table S4). With increasing soil depth the decline of
C:N was exponentially correlated to an enrichment of
N (Fig. 3a). Different slopes of the regression lines
showed that the relationship between C:N and d
was differently pronounced dependent on the tree
species. Similarly close regressions were determined
for the relationship between O:C and d
N (Fig. 3b).
Yet, curves exponentially increased, showing
enrichment with increasing O:C ratio.
208 Biogeochemistry (2020) 151:203–220
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Table 1 Biomass and isotopic composition of litterfall and root inputs of different tree species, 35 years after afforestation on a post-mining site
Roots in
Litterfall Roots (general) Forest floor 0–5 cm 5–10 cm 10–30 cm
Biomass Douglas fir 3.59 ±0.99ab 9.01 ±1.83ab 0.37 ±0.19bc 4.23 ±1.72a 1.38 ±0.54a 3.04 ±0.66
[t ha
] Pine 4.54 ±0.73b 6.73 ±1.89a 0.09 ±0.05a 1.30 ±0.44a 1.68 ±1.02a 3.66 ±1.59
Beech 3.30 ±0.34a 18.41 ±6.88bc 0.61 ±0.10c 4.75 ±2.40a 4.29 ±2.77b 8.76 ±6.51
Oak 3.43 ±0.38ab 22.70 ±7.22c 0.29 ±0.18ab 10.66 ±3.19b 4.59 ±3.98b 7.16 ±3.18
pvalues 0.0366 0.0011 0.0003 0.0011 0.0169 0.0797
n 25 400 100 100 100 100
C Douglas fir -31.25 ±0.11c -26.43 ±0.57c -26.14 ±0.43c -26.57 ±0.55c -26.50 ±0.69c -26.52 ±0.65c
[%] Pine -32.19 ±0.34b -27.49 ±0.52b -27.06 ±0.21b -27.47 ±0.57b -27.61 ±0.43b -27.83 ±0.56ab
Beech -33.17 ±0.25a -27.63 ±0.41b -27.64 ±0.43b -27.89 ±0.29b -27.67 ±0.49b -27.32 ±0.28bc
Oak -32.06 ±0.46b -28.89 ±0.56a -28.88 ±0.58a -29.25 ±0.41a -28.90 ±0.63a -28.51 ±0.50a
pvalues 0.0010 < 0.0001 < 0.0001 < 0.0001 0.0001 0.0001
n 25 100 25 25 25 25
N Douglas fir -5.22 ±0.65c -2.54 ±1.39c -2.19 ±1.76c -2.13 ±1.62c -2.69 ±1.19b -3.18 ±1.02b
[%] Pine -8.25 ±0.58ab -5.91 ±0.82b -6.05 ±0.66ab -5.45 ±0.79a -6.08 ±1.01a -6.07 ±0.89a
Beech -7.79 ±0.22b -4.10 ±1.57b -4.89 ±0.40bc -3.65 ±2.33bc -3.19 ±1.88ab -4.66 ±0.45ab
Oak -8.74 ±0.37a -5.42 ±0.98a -6.31 ±0.58a -4.71 ±0.73ab -5.63 ±1.06a -5.02 ±0.87a
pvalues < 0.0001 < 0.0001 0.0014 0.0126 0.0052 0.0005
n 25 100 25 25 25 25
Root properties are given on horizon level as well as sum (biomass) and on average (d
C, d
N) of roots in the forest floor and mineral soil, irrespective of soil horizons (‘‘Roots
general’). Values are mean ±SD. Differences between tree species are marked by different letters. Significant pvalues (\0.05) are highlighted in bold font style. The total
number of samples (n) per parameter and substrate comprises all four tree species
Biogeochemistry (2020) 151:203–220 209
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
values and e
N and the contribution
of litterfall and root inputs
The b
values for beech (-1.14) were significantly
more negative compared to pine (-0.64) and Douglas
fir (-0.81) by a factor of 1.8, 1.4 respectively
(Fig. 4a). For the oak stands intermediate b
(-0.87) were determined. Values for e
N differed
up to a factor of 1.7 with significantly higher values
under beech (7.02%) and pine (6.52%) compared to
oak (4.76%) and Douglas fir (4.05%) (Fig. 4b).
Apparently, b
values differed systematically
between coniferous and deciduous species, while
N depended more on individual tree species.
Using multiple linear regression analyses, the
impact of litterfall and root properties on both indices,
and e
N, was assessed. In total, 49% and
74% of the variability in b
and e
N was
represented by the explaining variables (Table 3).
Higher b
values were associated with litterfall that
was characterized by higher d
C values and lower
N values. Furthermore, root C:N played a signif-
icant role for b
. Litterfall with lower C:N ratios
and more negative d
N values were related to higher
N. Additionally, higher root d
C values and
lower O:C ratios of roots were associated with higher
Fig. 2 Depth gradients of C (a), d
C(b), N (c) and d
(d) from the OL horizon to 10–30 cm. Coniferous species
Douglas fir (‘D’) and pine (‘P’) are presented by black
symbols and the deciduous species beech (‘B’) and oak (‘O’’ )
by grey symbols. Significant differences between tree species
are marked with ‘‘*’’ (p\0.05), ‘‘**’’ (p\0.01), ‘‘***’
(p\0.001). Detailed information about statistical differences
can be found in Table S4
Table 2 Results of the regression analyses between isotopic
and stoichiometric ratios
Function R
p values
C vs. C/N
Douglas fir y = -0.181 ln(x) -1.763 0.38 0.0003
Pine y = -0.252 ln(x) -3.745 0.32 0.0011
Beech y = -0.191 ln(x) -2.295 0.56 < 0.0001
Oak y = -0.248 ln(x) -3.740 0.30 0.0018
C vs. O/C
Douglas fir y = 0.538 ln(x) ?14.472 0.55 < 0.0001
Pine y = 0.595 ln(x) ?16.336 0.51 < 0.0001
Beech y = 0.550 ln(x) ?15.463 0.76 < 0.0001
Oak y = 0.402 ln(x) ?11.131 0.45 < 0.0001
N vs. C/N
Douglas fir y = -0.141 ln(x) ?2.889 0.79 < 0.0001
Pine y = -0.124 ln(x) ?2.796 0.85 < 0.0001
Beech y = -0.105 ln(x) ?2.772 0.88 < 0.0001
Oak y = -0.180 ln(x) ?2..662 0.75 < 0.0001
N vs. O/C
Douglas fir y = 0.363 ln(x) ?0.521 0.85 < 0.0001
Pine y = 0.234 ln(x) ?0.711 0.86 < 0.0001
Beech y = 0.263 ln(x) ?0.763 0.90 < 0.0001
Oak y = 0.244 ln(x) ?0.602 0.80 < 0.0001
Significant pvalues (\0.05) are highlighted in bold font style
210 Biogeochemistry (2020) 151:203–220
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Fig. 3 Relationship between d
N and the molar ratios of C:N
and O:C. Douglas fir (‘D’) and pine (‘P’) are presented by blue
and green symbols, beech (‘B’) and oak (‘O’) are represented
by yellow and red symbols. Detailed information about statistics
of the logarithmic and linear relationships can be found in
Table 2and Table S5
Fig. 4 Boxplots of b
(slopes of the linear regression between d
C and log C) (a) and e
N enrichment from OL to
10–30 cm) (b) of investigated forest stands. Black rhombuses represent mean values
Table 3 Final models with
explaining variables of the
multiple linear regression
analysis for the
determination of factors
influencing b
Significant pvalues (\
0.05) are highlighted in
bold font style
Model parameter R
Litterfall d
C?litterfall d
N?root C:N 0.46 0.0046
N Litterfall C:N ?litterfall d
N?root O:C ?root d
C 0.74 < 0.0001
Proxy Explaining variables Coefficients pvalues
Intercept 5.63 0.0245
Litterfall d
C 0.21 0.0131
Litterfall d
N-0.09 0.0337
Root C:N -0.01 0.0114
N Intercept 42.74 0.0002
Litterfall C:N -0.04 0.0361
Litterfall d
N-1.14 < 0.0001
Root O:C -18.61 0.0086
Root d
C 1.19 0.0089
Biogeochemistry (2020) 151:203–220 211
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Isotopic signatures of litter inputs
In all investigated forest stands roots were consistently
enriched in
C and
N compared to the litterfall
(Table 1). Several post-photosynthetic allocation
mechanisms can lead to an enrichment of
heterotrophic plant organs compared to leaves (Cer-
nusak et al. 2009). For example, a greater allocation of
depleted C to lignin and lipid pools and an export of
less depleted carbohydrates to roots result in an
enrichment of
C in belowground organs (Hobbie
and Werner 2004; Badeck et al. 2005). The observed
significant impact of tree species on root and litterfall
C is caused by a complex interplay of physiological
differences between the tree species and their response
to environmental conditions, which has been thor-
oughly reviewed by Dawson et al. (2002). The higher
C values of the here investigated coniferous species
in comparison to deciduous trees (Table 1) are mainly
caused by a higher intrinsic water-use-efficiency,
lower stomatal conductance and lower photosynthetic
rates (Brooks et al. 1997). It must be noted that the
C of different plant parts varies on diurnal, seasonal
and annual to interannual time scales (Bru
et al. 2011). Here we use the d
C of the annual litter
inputs as reference points to evaluate the decomposi-
tion of OM along soil profiles (Bowling et al. 2008).
The generally lower d
N values of litterfall
compared to roots are in line with findings of other
studies (Ho
¨gberg et al. 1996; Templer et al. 2007).
This pattern can be assigned to fractionation during N
transformation and transport within the plant that leads
to an assimilation of
N-depleted N in leaves and
enriched N in roots (Pardo et al. 2013). Moreover, the
formation of mycorrhizal symbioses is one of the most
important factors influencing the d
N signature of
leaves. The here investigated tree species Douglas fir,
pine, beech and oak are well known to form symbioses
with ectomycorrhizal (EM) fungi (Wang and Qiu
N-enriched N compounds are preferentially
retained by the fungal biomass, while
N-depleted N
compounds are transported to their host plant (Craine
et al. 2009). The biggest difference between root and
litterfall d
N was determined for beech
(3.69 ±0.96%) followed by oak (3.33 ±0.52%).
This range is in agreement with differences of *4%
observed by Hobbie and Colpaert (2003). According
to them, the amount of ectomycorrhizal mass included
with the roots also determines the enrichment of roots
N compared to foliar tissues. The threefold higher
root biomass of beech and oak compared to Douglas fir
and pine (Table 1) can therefore be responsible for the
highest differences between the plant organs at these
stands. Analyzing the abundance of mycorrhizal fungi
in symbioses with the investigated tree stands was
beyond the scope of the study. However, it is reported
that the EM fungal biomass does not vary significantly
between beech and conifers in temperate forests but
the mechanisms behind the regulation of EM fungal
biomass are highly complex (Awad et al. 2019).
Additional to mycorrhizal associations, the variability
in plant d
N depends on the form of soil N that plants
predominantly acquire (Vallano and Sparks 2013).
Denitrification and nitrification both discriminate
N because
N-depleted nitrate can be
leached from the soil, resulting in
N-enrichment of
the remaining N that can be taken up by plants (Hobbie
and Ho
¨gberg 2012). High nitrate concentrations of
soils under Douglas fir (Zeller et al. 2019) can account
for the significantly highest litterfall d
N values at the
study site ‘Sophienho
¨he’. Beech, with the second
highest d
N values for litter inputs, is recognized as a
tree species that promotes nitrification in soils (An-
drianarisoa et al. 2010). However, a more profound
investigation of specific N cycling processes in the
plant-soil system that potentially influence the natural
abundance of d
N is beyond the scope of this study.
Depth profiles of d
C and d
N and their
relationship to stoichiometry patterns
Within 35 years after afforestation distinct depth
profiles of d
C developed in all investigated forest
stands confirming findings of Brunn et al. (2017), who
demonstrated that three decades after afforestation are
sufficient to yield such profiles. The gradients from OL
to OH in our investigated forest stands were charac-
terized by a decrease or at least no alteration of d
(Fig. 2b). Within the early stages of OM decomposi-
tion water-soluble substances and non-lignified car-
bohydrates are degraded, while the proportion of
lignin residually increases (Berg 2008; Osono et al.
2008). Lignin is characterized by lower d
C values
compared to bulk foliar d
C, while cellulose and
sugars are characterized by higher values (Bowling
et al. 2008). Therefore, a selective preservation of
212 Biogeochemistry (2020) 151:203–220
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lignin, and lignin building blocks, respectively
´rez-Abelenda et al. 2015), which cannot be
attacked by the vast majority of decomposers, can
lead to the depletion of
C downwards through the
organic horizons under Douglas fir, pine and oak. No
significant depletion of
C was found under beech. It
appears that beech maybe belongs to the group of tree
species, where the carbohydrate-dominated early
stage of litter decomposition is so marginal that it
has no measurable impact on d
C values (Berg and
McClaugherty 2014). The fact that the sampling
campaign was in April and the litterfall predominately
occurred in October and November corroborates the
assumption. The time difference between litterfall and
organic-layer sampling can also be responsible for the
high extent of
C enrichment from litterfall to OL
material. In the mineral soil horizons, however, d
of bulk SOM increased with increasing depth. A
relevant contribution of atmospheric
(Francey et al. 1999), to OM at the soil surface
can be excluded because the 35 years old afforested
sites are rather young. Instead,
C-depleted litter from
aboveground plant materials accumulates at the soil
surface, while the contribution of OM that derives
C-enriched roots to SOM formation increases
with soil depth (Bird et al. 2003). Correspondingly, in
our study roots were on average higher with
4.56 ±0.99%compared to the litterfall. Furthermore,
the kinetic fractionation of C isotopes during the
maturation of SOM leads to an enrichment of
C with
increasing depth (Wynn et al. 2006). Within the
microbial metabolism of C sources preferentially
depleted CO
is respired by microorganisms, while the
remaining SOM including the soil microbial biomass
becomes enriched in
C (Werth and Kuzyakov 2010).
Thus, microorganisms fractionate during the C assim-
ilation and/or preferentially use
C-enriched sub-
strates (Schwartz et al. 2007). Especially in mineral
soils of forests
C-enriched microbial-derived OM
has a larger share of bulk SOM d
C values than lignin
or aliphatic biopolymers (Du
¨mig et al. 2013).
Throughout the soil profile, d
C of SOM was affected
by tree identity with consistently highest values in
Douglas fir stands and lowest values in oak stands. In
contrast, Marty et al. (2015) found a negative impact
of the percentage of conifers in Canadian forests on
C values in mineral horizons. They assume that this
was caused by lower microbial activity and/or lower
SOM degradation at sites dominated by conifers. Yet,
in our study the differences in the SOM d
C values
among tree species reflect the isotopic signatures of
the OL horizon that in turn strongly correlated with
litterfall d
C. This explains why OM under Douglas
fir with highest d
C values in litterfall and roots
exhibited the highest d
C values throughout the soil
profile, while they were lowest under oak.
In coincidence with the
C enrichment with
increasing soil depth, gradients of d
N from the OL
down to the 10–30 cm layer of mineral soil had
developed in all forest stands during 35 years of
afforestation (Fig. 2d). The depth distribution of SOM
N mainly results from an interplay of input
signatures and losses that occur during decomposition
processes (Craine et al. 2015). The accumulation of
N-depleted plant litter on the soil surface determines
the gradient from the significantly lower d
N values
of forest floor horizons to the
N-enriched mineral
soil. Thus, the highest d
N values in the forest floor
horizons under Douglas fir reflect the highest d
values of the litterfall and roots of all tree species that
in turn were determined by the nitrate concentrations
of Douglas fir soils (Zeller et al. 2019). The lowest
forest floor d
N values that were observed in the pine
stand are in accordance with other studies revealing
that conifer-dominated sites were
N-depleted com-
pared to deciduous species (Pardo et al. 2007). In
contrast to the d
C depth gradients, d
N increased
consistently throughout the soil profile following a
curve that is typical for N-limited forest ecosystems
dominated by EM fungi (Hobbie and Ouimette 2009).
The clearly higher d
N values under beech and
Douglas fir compared to oak and pine are in accor-
dance with observations made in a common garden
experiment in Poland (Angst et al. 2019). With
increasing depth and ongoing decomposition, SOM
becomes preferentially
N-enriched due to microbial
activity coupled with an increasing proportion of
enriched microbial derived compounds (Lerch et al.
2011). The individual SOM d
N depth gradients of
tree species converged in the upper two mineral soil
horizons (0–5, 5–10 cm) and diverged again with
increasing depth implying that SOM turnover differed
under the influence of tree species. Additionally, tree
species and their mycorrhizal symbionts, respectively,
also contributes to the d
N depth profiles by their N
uptake from soil (Handley and Raven 1992; Callesen
et al. 2013). The type of mycorrhizal association
mainly drives the form of N acquisition of temperate
Biogeochemistry (2020) 151:203–220 213
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tree species (Liese et al. 2017) and leads to differences
N enrichments in soil profiles between EM and
arbuscular mycorrhizal fungi dominated systems
(Hobbie and Ho
¨gberg 2012). The here investigated
tree species are all dominated by EM fungi with
similar fungal biomasses (Wang and Qiu 2006; Awad
et al. 2019) and thus, the N transfer from the soil to the
host plant is presumably not significantly different
between tree species. Anyhow, the turnover of SOM
and the N uptake by plants are highly interrelated and
therefore both mechanisms will have contributed to
the d
N depth profiles.
The negative relationship between d
N and soil
C:N ratio may result from increasing loss of
depleted N in the form of nitrate leaching or denitri-
fication as consequence of decreasing N retention
(Marty et al. 2019). However, this option is likely
subordinate because N-limited temperate forests are
characterized by a largely closed internal N cycle,
where N-losses are generally low due to a high
competition for this growth-limiting resource (Ren-
nenberg et al. 2009). Rather, the relationship of d
to the soil C:N is best explained by the increase in OM
decomposition with increasing soil depth. The rela-
tionship between d
C and soil C:N ratio, that is also
negative, supports this assumption (Baisden et al.
2002). The well-known decline of the C:N ratio with
increasing depth (Marı
´n-Spiotta et al. 2014) is mostly
attributed to OM decay because substrate that accu-
mulates at the soil surface has significantly higher C:N
ratios compared to decomposers and their products
(Manzoni et al. 2010; Paul 2016). The slopes of the
specific regression lines (Fig. 3a) were more driven by
the enrichment of
N with increasing depth than by
the C:N ratio. Steeper slopes in beech and pine stands
were associated with the significantly higher e
values of beech and pine compared to oak and Douglas
fir. Nonetheless, the significant relationship between
N and the C:N ratio emphasizes the potential of
depth gradients as proxy for OM decay. Kramer et al.
(2017) found that changes in organo-mineral associ-
ations can drive depth trends of C:N and d
N more
than the microbial decay. However, this effect was
largely reduced, since forest stands with uniform
mineral phase were investigated in this study. Fur-
thermore, the O:C ratio that represents the state of
chemical oxidation (Fan et al. 2018) of SOM was also
significantly related to d
N and d
C throughout the
soil profile. Oxidation is accompanied with microbial
breakdown and depolymerization of plant residues
followed by assimilation of C in microbial biomass as
well as mineralization at the same time (Lehmann and
Kleber 2015). Thus, with increasing depth the rise of
the O:C ratio indicated the progressive oxidative
degradation of OM and correlated with the enrichment
N and
C in SOM. All this led to the assumption
that the depth trends of SOM d
N and d
C resulted
mainly from the decomposition of OM.
values and e
N and the contribution
of litterfall and root inputs
In well-drained forest soils, like our study site
¨he’, the linear regression function of d
and the logarithm of SOC with soil depth, termed as
value, is a suitable indicator of isotopic
fractionation during decomposition (Brunn et al.
2014). Physical soil mixing processes that could also
have a contribution to the isotopic fractionation with
soil depth (Acton et al. 2013) can be excluded because
during field surveys no earthworms or signs of
significant bioturbation processes were found (Lorenz
and Thiele-Bruhn 2019). However, steeper regression
slopes, and more negative b
values respectively,
indicate higher rates of
C enrichment through the
soil depth profile and enhanced organic matter turn-
over (Garten 2006; Wang et al. 2018). Tree species
had a significant effect on b
values. The most
negative b
values and therefore the highest rates of
SOM turnover were determined in beech forest stands,
while reduced SOM turnover at coniferous sites was
indicated by less negative b
values. This is in
accordance with the view that turnover rates, espe-
cially in the early-stage of decomposition, of decid-
uous species litter are generally higher compared to
conifers (Augusto et al. 2015 and references in there).
Long-term studies ([10 years) suggest that there are
also significant differences in the remaining masses
after decomposition between tree species (Harmon
et al. 2009; Prescott 2010). This can be addressed to
significantly higher N contents and lower C:N ratios in
the litterfall of beech and oak (Table S3), because it is
well documented that these parameters correlate well
with decomposition (Fernandez et al. 2003; Laganie
et al. 2010; Vesterdal et al. 2012). Our findings are
confirmed by other studies revealing that more neg-
ative b
values were related to higher N contents
and lower C:N ratios of litterfall (Garten et al. 2000;
214 Biogeochemistry (2020) 151:203–220
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
Garten 2006; Wang et al. 2015). The multiple linear
regression analysis, which included different quantity
and quality properties of the litter inputs, pointed out
that the isotopic signatures of litterfall and root C:N
account for nearly half of the variation (46%) in b
values. This implies on the one hand that the initial
isotopic composition of the aboveground litter plays a
crucial role, for evaluating the
C enrichment in soil
depth profiles in the context of SOM turnover.
Camino-Serrano et al. (2019) figured out that litter
C is the key to predict and model d
C depth
profiles. On the other hand, the C and N stoichiometry
of root biomass seems to be of high importance for
values in forest soils. Belowground inputs are
still less researched but knowledge is growing that
these inputs have a significantly contribution to OM
formation (Angst et al. 2018; Poirier et al. 2018). For
example, Kramer et al. (2010) demonstrated that root-
derived C is the major ([60%) source of C for
microbes in temperate deciduous forest soils, while
also in boreal forest soils 50 to 70% of the stored C
derives from roots and root-associated microorgan-
isms (Clemmensen et al. 2013). However, 54% of the
variation in b
values cannot be explained with the
here investigated properties of litterfall inputs and root
material suggesting that SOM decomposition depends
additionally on other factors.
The use of bulk SOM d
N as proxy for turnover
and stabilization of SOM bears an uncertainty due to
factors like the variability of the initial abundance of
N in litter inputs (Ho
¨gberg 1997). Thus, we evalu-
ated soil enrichment factors (e
N) to level out the
different isotopic signatures of litter inputs. Like the
values, e
N values were significantly
affected by tree species but the differences between
tree species are not consistent for e
N and b
values. If mainly microbial turnover of SOM would
drive both parameters in similar trajectories, this
would result in a negative relationship between both
indices. This applies for Douglas fir, oak and in
particular for beech with oppositely high or low b
and e
N values, respectively (Fig. 4). However,
soils under pine were characterized by highest b
well as high e
N values. This indicates that in pine
C and
N enrichments with soil depth are
decoupled from each other and not systematically
interlinked. Nel et al. (2018) reported on a general link
between d
C and d
N on global scale and with soil
depth but they also mentioned that this link varied with
local influences from biota, disturbance and environ-
mental conditions. Multiple linear regression analysis
revealed that litterfall C:N and d
N as well as root
O:C and d
C account for 74% of the variability in
N. This highlights the importance of litterfall and
root inputs for the
N enrichment with soil depth. The
general view that the higher decomposition rates of
litter with high N content and low C:N ratio (Garten
et al. 2000) are associated with both, high
N and
enrichment does not match with the finding of high
N enrichment in soils under pine. This must be
caused by other processes. The high C:N ratio of the
N-poor litterfall (Table S3) suggests that the N supply
in the upper 30 cm of soil in the pine stands is limited.
The significantly lowest root biomasses of the typical
deep rooting tap root system (Burylo et al. 2011)
combined with its low turnover rates (Yuan and Chen
2010) will also provide not much N. Under such
N-limited conditions EM fungi are able to oxidize OM
primarily as N source rather than as a source of
metabolic C (Lindahl and Tunlid 2015). Thus, EM
fungi in the pine stands can compete directly with
decomposers for soil N resources and exacerbate the
N-limitation of free-living decomposers (Averill
2016). Therefore, the soil microbial community could
adapt their N-utilization strategy to an efficient re-use
of organic N derived from their own bio- and
necromass. The multiple recycling of these N sources
will lead to an ongoing enrichment of
N without
higher rates of decomposition and could end up in the
discrepancy between b
and e
N values. Fur-
thermore, it has been reported that under conditions
with low C availability, N-containing organic com-
pounds can be primarily used as source of C and
energy. Consequently, dissimilated
N-depleted N
will be exported and the microbial cell gets enriched
relative to its source (Dijkstra et al. 2008). Conse-
quently, both mechanisms, the effective recycling of
microbial derived N under N-limited conditions as
well as the dissimilation of
N-enriched N, when the
relative availability of C is low, could lead to high
N and low b
values in pine stands.
The post-mining site ‘‘Sophienho
¨he’’ represented a
suitable site to characterize the influence of tree
species on the natural abundance of
C and
N in soil
Biogeochemistry (2020) 151:203–220 215
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
depth gradients. As an advantage to many common
garden experiments, an interference from old C
sources originating from former land use was negli-
gible. Additionally, 35 years after the afforestation
were sufficient to generate tree species-specific depth
gradients. Hence, evidence was provided that differ-
ences in isotopic signatures of SOM originated from
the input of plant litter and its decomposition products.
The significantly different d
C and d
N values in
the OM of forest floor and mineral soil reflected the
signatures of the litter inputs (litterfall and roots) that
were tree species specific. Along the soil profile, both
isotopes were significantly related to the C:N and O:C
ratio indicating that the enrichment of
C and
with increasing soil depth is driven by processes that
presumably can be assigned to microbial decomposi-
tion of OM. Consequently, when
C and
N of bulk
SOM are used to evaluate decomposition and stabi-
lization of OM, the isotopic signatures of litter inputs
should be considered as well. Differences in b
values indicated different turnover of SOM between
tree species with higher decomposition rates in
deciduous forest stands compared to conifers. The
quality of litterfall and root inputs (N content, C:N,
O:C ratio) as well as the initial isotopic signatures of
litterfall contributed to the regulation of OM decom-
position. Yet, 54% of the variance in b
, and 26% in
N respectively, cannot be explained with the here
investigated litterfall and root properties showing that
SOM decomposition depends additionally on other—
presumably microbial driven—factors. The corre-
spondence of e
N values with b
values in
three of the four investigated forest stands (Douglas
fir, beech, oak) suggests that the
C and
enrichment with increasing depth followed similar
principles. However, the conditions under pine did not
follow the systematic link between
C and
enrichment. This is presumably due to specific N
cycling mechanisms mediated by microorganisms that
were adapted to conditions of limited N availability
and the relatively low availability of C.
It is concluded that typical pattern of
C and
enrichment with increasing soil depth are due to
maturation and ongoing turnover of SOM. However,
under the influence of tree species the enrichment of
both isotopes did not follow similar trajectories in
general because of microorganisms that can create
specific utilization strategies depending on the litter
quality. It was possible to obtain this finding by
combining stable isotope analysis with the classical
determination of stoichiometry ratios (C:N, O:C).
Acknowledgements The authors thank the colleagues of the
Soil Science Department of Trier University, P. Ziegler and M.
Ortner, and the students, K. Becker, L. von Drathen, S. Stein and
L. Schneider, for assistance during laboratory and field work.
We also thank the certified facility in Functional Ecology (PTEF
OC 081) from UMR 1137 EEF and UR 1138 BEF in the research
centre INRAE Grand-Est–Nancy, and in particular S. Moutama,
J. Ph. Gallais for sample preparation and weighting and C.
Hossann, for performing/supervising isotopic analyses. Many
thanks to Oliver Brendel (INRA, Champenoux, France) for
suggestions in the data interpretation. This research did not
receive any specific grant from funding agencies in the public,
commercial, or not-for-profit sectors. The UR BEF is supported
by the French National Research Agency through the Cluster of
Excellence ARBRE (ANR-11-LABX-0002-01). Additional
support was provided by the mobile lab (M-POETE) of
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... Bulk soil organic C and total N concentrations are among parameters that are regularly assessed in forest ecosystem research to analyze soil organic matter (SOM) pools and ecosystem development. Furthermore, stoichiometric ratios such as the carbon:nitrogen (C:N) ratio and stable isotope composition (δ 13 C and δ 15 N) of SOM also provide a powerful tool for investigating spatial and temporal SOM dynamics and particularly, SOM turnover and stability, including fire disturbances [67][68][69][70]. ...
... Forest soils are characterized by the continuous inputs of fresh plant litter and roots that are steadily mixed and undergo microbial decomposition downward the soil profile [71][72][73]. Our data clearly demonstrate that Albic Podzols under pristine and fireaffected pine forests are supplied by a litter fall containing wide C:N and 13 C and 15 N depleted organic matter, as it was shown earlier for boreal and temperate forests [68,70]. With increasing soil depths and the aging of SOM [74], the content of C and N and C:N ratios in forest soils tend to decrease, and, in opposite, δ 13 C and δ 15 N show a trend toward enrichment by heavier isotopes [67][68][69][70]. ...
... Our data clearly demonstrate that Albic Podzols under pristine and fireaffected pine forests are supplied by a litter fall containing wide C:N and 13 C and 15 N depleted organic matter, as it was shown earlier for boreal and temperate forests [68,70]. With increasing soil depths and the aging of SOM [74], the content of C and N and C:N ratios in forest soils tend to decrease, and, in opposite, δ 13 C and δ 15 N show a trend toward enrichment by heavier isotopes [67][68][69][70]. These depth patterns are specific in Albic Podzols as the eluvial E horizon lying below an organic layer is strongly depleted by C and N in comparison to the deeper illuvial Bs horizon [15]. ...
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Fires are one of the most widespread factors of changes in the ecosystems of boreal forests. The paper presents the results of a study of the morphological and physicochemical properties and soil organic matter (SOM) of Albic Podzols under pine forests (Pinus sylvestris L.) of the middle taiga zone of Siberia (Krasnoyrsky kray) with various time passed after a surface fire (from 1 to 121 years ago). The influence of forest fires in the early years on the chemical properties of Albic Podzols includes a decrease in acidity, a decrease in the content of water-soluble compounds of carbon and nitrogen and an increase in the content of light polycyclic aromatic hydrocarbons (PAHs) in organic and upper mineral horizons. Podzols of pine forests that were affected by fires more than forty-five years ago are close to manure forest soils according to most physical and chemical properties. Significant correlations were found between the thickness (r = 0.75, p < 0.05), the moisture content (r = 0.90, p < 0.05) of organic horizons and the content of ∑PAHs in the organic horizon (r = −0.71, p < 0.05) with the time elapsed after the fire (i.e., from 1 to 121 years). The index of the age of pyrogenic activity (IPA) calculated as the ratio of ∑ PAHs content in the organic horizon to ∑ PAHs at the upper mineral horizon is significantly higher in forests affected by fires from 1 to 23 years than for plots with «older» fires (45-121 years). Thus, the article presents the conserved and most changing factors under the impact of fires in the boreal forests of Russia.
... However, Wang et al. (2014) found that biochar increased labile organic C only in the first month after application, and Anderson et al. (2011) found that microbial abundances in soils significantly increased 12 weeks after biochar application, which suggested that, in the short term, biochar could also accelerate the degradation of SOC through the release of labile organic C from biochar and providing a favour environment for microbes. In addition, the prevalence of SOC decreases and δ 13 C increases with soil depth has promoted the use of the δ 13 C value as an indicator for the formation and alteration of SOC pools (Nguyen et al. 2018;Lyu et al. 2019;Lorenz et al. 2020); accordingly, soil δ 13 C might vary in response to the impacts of biochar application on soil C pools. ...
... This result also indicated that the short-term "positive priming" of biochar on soil C and N pools was significant only at the 0-5 cm depth and after 3 months of biochar application. Accordingly, in contrast to soil total C, soil δ 13 C, which is usually combined with SOC as a proxy for evaluating SOM turnover (Lorenz et al. 2020), increased with biochar application rates. Moreover, our study site was located in an urban forest and close to a motorway; therefore, fuel combustion from vehicles was one of the main sources of NOx (Bai et al. 2012), and negative δ 15 N values were detected in the topsoil after 3 months of biochar application in our study. ...
... Biochar effectively regulated the processes of N cycling and SOM turnover, which was characterized by the linear relationship between δ 15 N and δ 13 C, although generally, biochar does not significantly affect soil C and N pools (Lyu et al. 2019;Lorenz et al. 2020). Increased soil labile pools that were derived from the stimulation of microorganisms by biochar might have resulted in a dominance of labile pools during the processes of SOM turnover and N cycling in the sixth month after biochar application, and this result also suggested the key role of the duration after biochar application in affecting SOM turnover and N cycling processes. ...
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Purposes Biochar has received widespread attention as a means for improving carbon (C) sequestration and soil fertility over the long term. However, information on its short-term effects on the soil C and nitrogen (N) pools is scarce, particularly in poor soils that are subjected to prescribed burning. Materials and methods In an effort to better understand the short-term effects of biochar on the soil C and N pools, a half-year field study was conducted in a suburban forest that is subjected to prescribed burning in subtropical Australia. In this experiment, biochar was applied to the soil once at rates of 0, 5 and 10 t ha⁻¹, and soil samples were collected for the top 20 cm soil profile under the canopies of leguminous Acacia leiocalyx (A. leiocalyx) and Acacia disparimma (A. disparimma) in the third month (August 2019) and the sixth month after biochar application (November 2019). At this site, we measured soil total C, total N, δ¹³C and δ¹⁵N. Results We observed that biochar generally impacted the soil C and N pools in the third month after biochar application: soil total C, particularly at the 0–5 cm depth, significantly decreased with increased biochar application rates. Soil C and N pools, particularly at the 10–20 cm depth, varied with sampling times, and soil total C and N under the A. leiocalyx canopy were significantly higher, while soil δ¹³C and δ¹⁵N were lower in the sixth month relative to the third month. Soil δ¹³C and δ¹⁵N were primarily linearly related to soil total C and N in the third month, while the linear relationship was closer than that between soil δ¹³C and δ¹⁵N and the labile C and N pools in the sixth month, which were regulated by biochar application rates. Conclusions Biochar application significantly decreased the soil C and N pools at the 0–5 cm depth in the third month after biochar application. The soil C and N pools and soil labile C and N pools were responsible for the changes in the processes of soil organic matter (SOM) turnover and N cycling that were revealed by soil δ¹³C and δ¹⁵N, and the changes were governed by the biochar application rates and the time elapsed after biochar application. The influence of understorey legume Acacia species, particularly A. leiocalyx, on N inputs and C sequestration in the poor forest soils was significantly enhanced in the sixth month after biochar application.
... Garten et al. (2000) reported strong negative correlations between δ 13 C and the logarithm of SOC concentrations at different depths, and they proposed that the slope of this relationship (β) represents the rate of microbial processing of SOC down the soil profile. While this β-index has proven to be robust in several studies (e.g., Garten, 2006;Lorenz et al., 2020;Marty et al., 2015b), it is based on the unverified assumption that the rate of SOM migration down the soil profile is related to the rate of microbial processing. For this reason, the β-index may be imprecise, as several pedogenic processes, such as bioturbation and podzolization, result in SOC migration independently from microbial processing. ...
... Анализ состава стабильных изотопов углерода (δ 13 С) представляет собой один из важных методологических подходов к исследованию пространственно-временной вариабельности почвенного органического вещества (ПОВ) [18,20]. δ 13 С рассматривается в качестве интегрального показателя процессов трансформации органического вещества [16] и, таким образом, имеет значительный потенциал для оценки динамики почвенного углерода [16,21]. ...
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Изучены пространственные и внутрипрофильные вариации состава стабильных изотопов углерода органического вещества почв центральной части западного побережья озера Байкал. Территория характеризуется контрастной ландшафтной структурой и вариативностью климатических условий, отражающихся на составе стабильных изотопов углерода почв. Значения δ13С органического вещества и опада находятся в диапазоне от –27,93 до –18,19‰ и свидетельствуют о преобладании растительности с С3-типом фото- синтеза. Тем не менее в степных ландшафтах отмечены единичные представители растений с С4- и CAM-фотосинтезом. Значения δ13С возрастают в направлении от лесных почв предгорий Приморского хребта к степным почвам Приольхонского плато. Такая тенденция отражает снижение влагообеспеченности, являющейся основным лимитирующим фактором развития почв Приольхонья и определяющей через дискриминацию 13С в растительных тканях состав стабильных изотопов углерода органического вещества почв. Вне зависимости от условий педогенеза для почв исследуемой территории характерно снижение значений δ13С с глубиной. Однако выраженность такого градиента определяется локальными сочетаниями факторов почвообразования. Исходя из различий исследуемых почв по коэффициентам наклона линейных регрессий (β), предполагается более интенсивный оборот углерода в почвах склонов северо-западных экспозиций, а также прибрежных ландшафтов и отрицательных форм рельефа, где влияние воздушных масс с озера Байкал обусловливает меньшее иссушение профиля в летний период и обеспечивает более благоприятный гидротермический режим почв для микробиологической активности. При этом вариации β не сопровождаются существенными колебаниями в значениях C:N и pH, что может свидетельствовать о несущественной роли внутрипочвенных факторов и перекрытии их влияния эффектом влагодефицита на интенсивность оборота углерода в почвах Приольхонья.
... As soil moisture in our experiment was higher throughout the year under F. sylvatica than P. abies -a trend that became more pronounced under drought (Grams et al., 2021) -we would expect higher C content in HF in the topsoil under F. sylvatica compared to P. abies. An additional valuable tool to identify SOM stability makes use of the relation between the vertical enrichment of 13 C and the simultaneous decrease of SOC with depth (Acton et al., 2013;Brunn et al., 2014;Lorenz et al., 2020). The extent of SOC decomposition can be approximated from the slopes of linear relationships between δ 13 C values and log 10 C content and these so-called beta values can be interpreted both in terms of C processing, as well as the potential for C to form organomineral associations (Brunn et al., 2017). ...
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Reduced carbon assimilation by trees is often considered to lower the overall carbon sink function of drought-stressed forests. However, soil organic carbon (SOC) stocks may respond differently to drought than ecosystem carbon flux dynamics, leading to imprecise predictions of soil carbon sequestration when one value is inferred from the other. As a major component of soil organic matter, SOC is the largest actively cycling terrestrial carbon reservoir, and thus fulfills various important ecosystem services. Yet, there is uncertainty about how SOC quantity and quality respond to drought in temperate forests. This study addressed the depth distribution of SOC stocks and soil organic matter stability in a forest exposed to artificial drought for five consecutive growing seasons below clusters of temperate mature deciduous beech (Fagus sylvatica L.) and coniferous spruce (Picea abies (L.) Karst.). In addition to SOC stock determination, we measured concentrations of water-extractable organic carbon (WEOC), performed density fractionation, and determined beta values of SOC (slopes of linear regressions between δ 13 C of soil and log-transformed SOC content throughout soil depth profiles). Following drought, SOC stocks down to 30 cm depth increased by a factor of 1.5 under P. abies while they did not change with drought under F. sylvatica. Under both species, SOC stocks in the mineral topsoil (0-5 cm soil depth) increased by >80 % with drought, increasing the relative contribution of this thin depth section to total SOC from 5 % to >30 %. At 5-15 cm soil depth, SOC stocks decreased with drought under F. sylvatica but not under P. abies. With drought, carbon in the free light fraction (fLF) increased under F. sylvatica but declined marginally under P. abies. Results from density fractionation and beta values suggest decreased soil organic matter stability under F. sylvatica and increased stability under P. abies. Greater SOC accumulation suggests that the belowground carbon sink strength of drought-stressed forests increases, which contrasts with reduced ecosystem carbon uptake under drought.
... Wang et al. (2019) discovered that the alleviation of soil N restriction and exacerbation of P restriction could promote the fractionation of plant N isotope; Zhao et al. (2019) found that high C/N ratio could increase the rate of microbial decomposition, accelerate the loss of 12 C and thus enrich the 13 C in soil. Lorenz et al. (2020) proposed that microbes could optimize their resource utilization strategy according to litter quality, nutrients utilization e ciency and restriction status. In conclusion, soil stoichiometry, microorganism and soil fertility are closely related. ...
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Background and aims Understanding the relationship between carbon, nitrogen and their stable isotope ¹³C, ¹⁵N and soil stoichiometry may assist to reveal the distribution pattern and stability mechanism of nutrient elements in karst ecosystem. Methods Four plantations of Zanthoxylum planispinum var. dintanensis (5–7, 10–12, 20–22 and 30–32 years) in the karst plateau gorge area of Guizhou Province, China, were selected as the research objects to clarify the variation characteristics and interaction effects of leaf, litter, soil C, N and their isotopes with plantation age, and to explore the relationship between soil stoichiometry and the ¹³C, ¹⁵N of Zanthoxylum planispinum var. dintanensis plantation. Results (1) the ¹³C in leaf, litter and soil were − 28.04‰±0.59‰, -26.85‰±0.67‰ and − 19.39‰±1.37‰, respectively, correspondingly, the contents of ¹⁵N were 2.01‰±0.99‰, 2.91‰±1.32‰ and 3.29‰±0.69‰, respectively. The contents of the ¹³C and ¹⁵N can be rank ordered as soil > litter > leaf; (2) with the increase of plantation age, the soil ¹³C decreased; the leaf and litter ¹⁵N increased first then decreased; the litter ¹³C and soil ¹⁵N did not vary significantly; (3) the litter layer positively correlated to soil ¹³C, and negatively correlated to ¹⁵N; (4) redundancy analysis showed that soil microbial biomass carbon (MBC) and bacteria/fungi (BAC/FUN) were the dominant factors affecting C and N isotope natural abundances. Conclusions This study indicated that the species and acidity of soil microbial can affect the C and N isotope natural abundance.
... It was unexpected to observe the highest C:N ratio of soils under BEF (Table 2). Indeed, the relatively low C:N ratio of beech residues is commonly recognized [119,120]. The high C:N ratio due to the lowest TN content found in the soils under BEF might be attributable to the fast degradation rate of such residues [121,122]. ...
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Pedodiversity is considered the cornerstone of biodiversity. This work aimed to (1) assess pedodiversity according to vegetation, topographic factors, and lithology and to (2) identify the major soil-forming factors on soil organic matter (SOM) stock at a 0–30 cm depth. These goals were reached using data from 147 georeferenced soil profiles distributed along 400–1000 m (≤1000) and 1000–2134 m (>1000) altitudinal gradients in the northern part of the Apennine chain in Italy. Soils showed mainly weak or incipient development (i.e., Entisols and Inceptisols), which could be attributed to sand-based lithology, high slope gradients, and low SOM accumulation rates, which promote soil erosion processes. However, higher pedodiversity was observed at >1000 m than at ≤1000 m, likely due to the higher vegetation cover diversity and climate variability; Spodosols and Mollisols were also found. A greater SOM stock was found at >1000 than ≤1000 m, and vegetation seemed to not affect SOM amounts, suggesting a greater influence of climate on SOM content compared to vegetation. Considering ecosystem conservation, the observed spatial pedodiversity could be considered a critical basis for the protection of soil resources and pedodiversity itself in mountain regions.
... The composition of stable carbon isotopes (δ 13 С) is one of the important characteristics of soil organic matter (SOM) and is considered an integral indicator of the transformation processes of organic matter [16]. Thus, its analysis is crucial for the study of SOM spatiotemporal variability [18,20] and has a significant potential for assessing the dynamics of soil carbon [16,21]. ...
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Spatial and depth-profile variations in stable carbon isotopic composition of soil organic matter have been studied in the central part of the western coast of Lake Baikal. The contrasting landscape structure and significant climatic variability of the area strongly affect the soil stable carbon isotopic composition. The δ13С values of organic matter and litter vary in the range from –27.93 to –18.19‰ and indicate the predominance of C3-pathway vegetation. However, in the steppe landscapes, single representatives of plants with C4 and CAM photosynthesis types have been noted. The δ13С values increase in the direction from the forest soils of the foothills of the Primorsky Range to the steppe soils of the Olkhon Plateau. This trend reflects a decrease in humidity, the main limiting factor of the soil development in the Olkhon region. Insufficient moisture determines the stable carbon isotopic composition of soil organic matter through 13C discrimination in plant tissues. Regardless of the soil-forming conditions, δ13С values increase with the depth in the soils of the studied area. However, the gradient rates are determined by local combinations of environmental factors. According to the differences in the slope coefficients of linear regressions (β) in the studied soils, more intensive carbon turnover is assumed in the soils of coastal landscapes, negative landforms, and the slopes of northwestern exposures. Under such conditions, air masses from Lake Baikal cause a significantly lower drying of the soil profile in summer and provide a more favorable water and temperature regime for microbiological activity. At the same time, β variations are not accompanied by significant fluctuations in C/N ratio and pH values, which may indicate an insignificant role of edaphic factors and overlapping of their influence by the effect of moisture deficiency on the carbon turnover intensity in the soils of the Olkhon region.
... Instead, vertical changes in either δ 13 C or δ 15 N of forest soils have been extensively studied (Lorenz et al. 2020 and references therein), and some studies further investigated co-variations in the δ 13 C and δ 15 N of soils across depth (Bohlen et al. 2004;Billings and Richter 2006;Boström et al. 2007;Bekele et al. 2013). Vertical distributions of δ 13 C and δ 15 N are unique features of natural forest soils that have rarely been disturbed, unlike croplands and grasslands (Han et al. 2020). ...
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This review analyzes the data on co-variations in δ¹³C and δ¹⁵N of soils with land-use types, management, and disturbance obtained from literature to explore potential implications of the dual isotopes in the study of soil organic matter (SOM) sources and C and N processes. Overall, croplands (δ¹³C and δ¹⁵N were − 20.3 ± 4.4‰ and + 7.6 ± 4.4‰, respectively) had greater isotopic values than grasslands (‒26.3 ± 3.0‰ and + 5.4 ± 1.1‰, respectively) and forests (− 26.0 ± 1.1‰ and + 4.3 ± 2.2‰, respectively). For intensively managed lands such as croplands and grasslands, application of organic inputs such as manure and compost of which isotopic signatures differed from the indigenous SOM was the main driver of co-variations in the δ¹³C and δ¹⁵N of SOM. For natural forests, both δ¹³C and δ¹⁵N of SOM co-increased with soil depth, reflecting heavy isotope enrichment during microbial stabilization of SOM and the potential influence of ¹³C-depleted atmospheric CO2 and ¹⁵N-depleted N deposition on the upper soils. Such vertical co-enrichments of ¹³C and ¹⁵N were disturbed by a land-use conversion to other lands including croplands. Though there were indications that land management practices such as tillage in croplands and grazing in grasslands, land-use changes, and land disturbance including forest fire might also affect both δ¹³C and δ¹⁵N, more data need to be accumulated to find a general trend of the isotopic variations of SOM. Analysis of both δ¹³C and δ¹⁵N may enlarge understanding of changes in SOM sources and soil C and N cycling by land-use types, management, change, and disturbance.
... The soil C/N ratio can represent the decomposition rate of organic N and microorganisms; the higher soil C/N ratio, the lower the decomposition rate of organic N; and the opposite is true for microorganisms [15]. In addition, it was found that microbes can optimize their resource utilization strategy according to litter quality, nutrient utilization efficiency, and restriction status, thus affecting the δ 13 C and the δ 15 N composition [16]. In conclusion, soil stoichiometry and microorganisms are tightly connected to δ 13 C and δ 15 N. Yet, the variation of forest δ 13 C and δ 15 N with plantation age and the mechanism of soil stoichiometry driving C and N isotope fractionation are still uncertain and need to be further studied. ...
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Understanding the relationships between carbon; nitrogen, their stable isotopes δ13C and δ15N, and soil stoichiometry may further our understanding of the regulatory mechanisms of the soil quality index on the equilibrium on isotopic fractionation. Four plantations of Zanthoxylum planispinum var. dintanensis (5–7, 10–12, 20–22 and 30–32 years) in the karst plateau gorge area, Guizhou Province, China, were selected to determine the variation characteristics and interactions between leaves, leaf litter, soil carbon (C), soil nitrogen (N) and their isotopes with plantation age, and to explore the relationship between soil stoichiometry and the isotopes δ13C and δ15N. The results were as follows: (1) the δ13C in leaves, litter, and soil were −28.04‰ ± 0.59‰, −26.85‰ ± 0.67‰, and −19.39‰ ± 1.37‰, respectively. The contents of δ15N were 2.01‰ ± 0.99‰, 2.91‰ ± 1.32‰, and 3.29‰ ± 0.69‰, respectively. The contents of δ13C and δ15N were ranked in the order, soil > litter > leaf. (2) With increasing plantation age, the soil 13C decreased; the leaf and the litter δ15N increased first then decreased, and the litter δ13C and the soil δ15N did not vary significantly. (3) The litter layer was positively correlated with soil δ13C and negatively correlated to δ15N. (4) Redundancy analysis showed that the soil microbial biomass carbon (MBC) and the bacteria/fungi (BAC/FUN) were the dominant factors affecting the natural abundance of C and N isotopes
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Abstract Soil organic carbon (SOC) is a crucial component of the terrestrial carbon cycle and its turnover time in models is a key source of uncertainty. Studies have highlighted the utility of δ13C measurements for benchmarking SOC turnover in global models. We used 13C as a tracer within a vertically discretized soil module of a land‐surface model, Organising Carbon and Hydrology In Dynamic Ecosystems‐ Soil Organic Matter (ORCHIDEE‐SOM). Our new module represents some of the processes that have been hypothesized to lead to a 13C enrichment with soil depth as follows: 1) the Suess effect and CO2 fertilization, 2) the relative 13C enrichment of roots compared to leaves, and 3) 13C discrimination associated with microbial activity. We tested if the upgraded soil module was able to reproduce the vertical profile of δ13C within the soil column at two temperate sites and the short‐term change in the isotopic signal of soil after a shift in C3/C4 vegetation. We ran the model over Europe to test its performance at larger scale. The model was able to simulate a shift in the isotopic signal due to short‐term changes in vegetation cover from C3 to C4; however, it was not able to reproduce the overall vertical profile in soil δ13C, which arises as a combination of short and long‐term processes. At the European scale, the model ably reproduced soil CO2 fluxes and total SOC stock. These findings stress the importance of the long‐term history of land cover for simulating vertical profiles of δ13C. This new soil module is an emerging tool for the diagnosis and improvement of global SOC models.
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Douglas fir trees presumable stimulate nitrification in the soil. We studied in 21 French Douglas fir forests if and how nitrification is modulated by soil properties, past land use and current forest management. Soil (0–10 cm depth) was collected and initial concentrations of N-NH4+ and N-NO3−, potential net nitrogen mineralization (PNM) and net nitrification (PNN) rates and microbial biomass were measured. At 11 of the 21 sites, annual nitrate fluxes in the soil were measured using anion exchange resin bags. Soils contained between 2.3 to 29.4 mg N-NO3− kg soil−1. About 86% (±14%) of mineral N was nitrate. The proportion of nitrate increased to almost 100% during incubation. PNN varied from 0.10 mg N kg soil−1 day−1 to 1.05 mg N kg soil−1 day−1 (21 sites). Neither the initial nitrate concentration nor PNN was related to soil chemistry (pH, % C, %N, P, CEC), microbial biomass, texture, past land use or thinning. In situ net nitrate accumulation (NNA) estimated with resins beds varied from 4 to 100 kg N-NO3− ha−1 yr−1 (11 sites). It was positively correlated with base saturation, clay content, ELLENBERG N, temperature and negatively with soil organic N, C/N ratio and precipitation.
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The exchange of carbon between soil organic carbon (SOC) and the atmosphere affects the climate1,2 and-because of the importance of organic matter to soil fertility-agricultural productivity3. The dynamics of topsoil carbon has been relatively well quantified4, but half of the soil carbon is located in deeper soil layers (below 30 centimetres)5-7, and many questions remain regarding the exchange of this deep carbon with the atmosphere8. This knowledge gap restricts soil carbon management policies and limits global carbon models1,9,10. Here we quantify the recent incorporation of atmosphere-derived carbon atoms into whole-soil profiles, through a meta-analysis of changes in stable carbon isotope signatures at 112 grassland, forest and cropland sites, across different climatic zones, from 1965 to 2015. We find, in agreement with previous work5,6, that soil at a depth of 30-100 centimetres beneath the surface (the subsoil) contains on average 47 per cent of the topmost metre's SOC stocks. However, we show that this subsoil accounts for just 19 per cent of the SOC that has been recently incorporated (within the past 50 years) into the topmost metre. Globally, the median depth of recent carbon incorporation into mineral soil is 10 centimetres. Variations in the relative allocation of carbon to deep soil layers are better explained by the aridity index than by mean annual temperature. Land use for crops reduces the incorporation of carbon into the soil surface layer, but not into deeper layers. Our results suggest that SOC dynamics and its responses to climatic control or land use are strongly dependent on soil depth. We propose that using multilayer soil modules in global carbon models, tested with our data, could help to improve our understanding of soil-atmosphere carbon exchange.
The purpose of this research was to evaluate tree species effects on quantitative and qualitative soil organic matter (SOM) properties of forest floors and mineral soil layers. Additionally, the contribution of soil microbial biomass to SOM was studied in five forest stands with different dominant tree species. The study was conducted at the afforested spoil heap ‘Sophienhöhe’ located at the lignite open-cast mine Hambach near Jülich, Germany. The 35 year-old afforested sites consisted of monocultural stands of Douglas fir (Pseudotsuga mienziesii), pine (Pinus nigra), beech (Fagus sylvatica) and red oak (Quercus rubra) as well as a mixed deciduous stand site planted mainly with hornbeam (Carpinus betulus), lime (Tilia cordata) and common oak (Quercus robur). There, boundary conditions regarding soil, climate, topography and management were highly similar, equivalent to a common garden experiment but on landscape level. Because the parent material used for site recultivation was free from organic matter or coal material, the SOM accumulation is a result of in situ soil development. Tree species had a significant effect on soil organic carbon (SOC) stocks, stoichiometric patterns of C, hydrogen (H), nitrogen (N), oxygen (O) and sulfur (S) and the microbial biomass carbon (MBC) content in the forest floor and the top mineral soil layers (0–5 cm, 5–10 cm, 10–30 cm). In general, forest floor SOC stocks were significantly higher in coniferous forest stands compared to deciduous tree species. Differences in SOM quantity became less pronounced with increasing depth, while stoichiometric molar ratios of SOM as indices of litter turnover and SOM composition differed also in deeper layers. Differences in H:C and O:C ratios among tree species clearly increased along the depth gradient in mineral soils, indicating that SOM turnover by oxidative processes depends on tree species. Differences in depth gradients of the microbial quotient (MBC to SOC ratio) among tree species emphasized differences in the microbial C turnover. Furthermore, the relationship between the microbial quotient and SOM stoichiometry (C:N and C:S ratio) became stronger with increasing soil depth. This suggests that N and especially S limitation determined the microbial turnover of SOM in deeper mineral soil layers.
Functionally, ectomycorrhizal (ECM) and saprotrophic (SAP) fungi belong to different guilds, and they play contrasting roles in forest ecosystem C-cycling. SAP fungi acquire C by degrading the soil organic material, which precipitates massive CO2 release, whereas, as plant symbionts, ECM fungi receive C from plants representing a channel of recently assimilated C to the soil. In this study, we aim to measure the amounts and identify the drivers of ECM and SAP fungal biomass in temperate forest topsoil. To this end, we measured ECM and SAP fungal biomass in mineral topsoils (0–12 cm depth) of different forest types (pure European beech, pure conifers, and mixed European beech with other broadleaf trees or conifers) in a range of about 800 km across Germany; moreover, we conducted multi-model inference analyses using variables for forest and vegetation, nutritive resources from soil and roots, and soil conditions as potential drivers of fungal biomass. Total fungal biomass ranged from 2.4 ± 0.3 mg g−1 (soil dry weight) in pure European beech to 5.2 ± 0.8 mg g−1 in pure conifer forests. Forest type, particularly the conifer presence, had a strong effect on SAP biomass, which ranged from a mean value of 1.5 ± 0.1 mg g−1 in broadleaf to 3.3 ± 0.6 mg g−1 in conifer forests. The European beech forests had the lowest ECM fungal biomass (1.1 ± 0.3 mg g−1), but in mixtures with other broadleaf species, ECM biomass had the highest value (2.3 ± 0.2 mg g−1) among other forest types. Resources from soil and roots such as N and C concentrations or C: N ratios were the most influential variables for both SAP and ECM biomass. Furthermore, SAP biomass were driven by factors related to forest structure and vegetation, whereas ECM biomass was mainly influenced by factors related to soil conditions, such as soil temperature, moisture, and pH. Our results show that we need to consider a complex of factors differentially affecting biomass of soil fungal functional groups and highlight the potential of forest management to control forest C-storage and the consequences of changes in soil fungal biomass.
Rising atmospheric CO2 concentrations have increased interest in the potential for forest ecosystems and soils to act as carbon (C) sinks. While soil organic C contents often vary with tree species identity, little is known about if, and how, tree species influence the stability of C in soil. Using a 40‐year‐old common garden experiment with replicated plots of eleven temperate tree species, we investigated relationships between soil organic matter (SOM) stability in mineral soils and 17 ecological factors (including tree tissue chemistry, magnitude of organic matter inputs and their turnover, microbial community descriptors, and soil physico‐chemical properties). We measured five SOM stability indices, including heterotrophic respiration, C in aggregate‐occluded particulate organic matter (POM) and mineral‐associated SOM, and bulk SOM δ¹⁵N and ∆¹⁴C. The stability of SOM varied substantially among tree species and this variability was independent of the amount of organic C in soils. Thus, when considering forest soils as C sinks, the stability of C stocks must be considered in addition to their size. Further, our results suggest tree species regulate soil C stability via the composition of their tissues, especially roots. Stability of SOM appeared to be greater (as indicated by higher δ¹⁵N and reduced respiration) beneath species with higher concentrations of nitrogen and lower amounts of acid‐insoluble compounds in their roots, while SOM stability appeared to be lower (as indicated by higher respiration and lower proportions of C in aggregate‐occluded POM) beneath species with higher tissue calcium contents. The proportion of C in mineral‐associated SOM and bulk soil ∆¹⁴C, though, were negligibly dependent on tree species traits, likely reflecting an insensitivity of some SOM pools to decadal‐scale shifts in ecological factors. Strategies aiming to increase soil C stocks may thus focus on particulate C pools, which can more easily be manipulated and are most sensitive to climate change. This article is protected by copyright. All rights reserved.
Plant roots contribute substantially to the formation of stable soil organic matter (SOM), and there is evidence that species differ in their contribution to SOM stabilization. However, it remains unclear what specific root traits contribute to the three SOM stabilization mechanisms: recalcitrance against decomposition, occlusion in soil aggregates and interaction with soil minerals and metals. This is likely because research is highly fragmented and hampered by disciplinary barriers. By reviewing both plant functional ecology and soil science literature, we identified 18 different traits: architectural, morphological, physiological, symbiotic and chemical root characteristics, influencing the three SOM stabilization mechanisms. We found that traits increasing root recalcitrance promote short term stabilization by slowing decomposition, but that traits reducing recalcitrance contribute to long term stabilization by reaction of microbial products with mineral surfaces. Root length density, mycorrhizal association and rhizodeposition contribute to microaggregation. These and other traits, such as hemicellulose, soluble compounds, and high root branching index, favor macroaggregation. For stabilization by minerals and metals, those root traits promoting higher microbial activity: root nitrogen, hemicellulose and soluble compound concentrations are fundamental, while polyphenols, and litter Al and Mn also contribute to complexification and stabilization. Root depth distribution is the most important trait to control root C storage and stabilization in the subsoil; once roots have reached deeper soil layers, other traits, such as rhizodeposition and root chemistry, influence interaction with minerals and metals. Both mycorrhizal presence and root suberin promote SOC stabilization by means of all three mechanisms, indicating that these are important targets for continued work. Surprisingly, morphological traits commonly measured, namely specific root length and root diameter, poorly relate to stabilization mechanisms. Alternative traits such as chemical composition of the different root orders, root apex characteristics, quantity and quality of rhizodeposits as well as mycorrhizal fungal traits, should be further investigated. For future research, this review highlights the need to evaluate root decomposition and root-C stabilization concomitantly over the long-term, considering simultaneously root litter quality, estimated by root traits, the microbial products and properties of the soil matrix. The information accrued in this review can be used to evaluate the potential of plant species and cultivars to promote SOM stabilization, based on their root traits.
Significant relationships have been observed between soil N isotopic natural abundance (δ15N) and both climate and soil characteristics across a large range of ecosystems over the globe, suggesting strong and consistent effects of these variables on N cycling. However, whether the strength and the nature of these relationships vary at regional scales and with soil depth is less documented, especially in northern cold and N-limited forest eco- systems. In this study, we analyzed δ15N in soil horizons at 21 forest sites in eastern Quebec along a gradient of concomitant decreasing N deposition and temperature (MAAT) and increasing precipitation (MAP). We hy- pothesized that both soil δ15N and the magnitude of increase in soil δ15N would decrease along this gradient, in accordance with relationships reported at a global scale. The data show an increase in δ15N with soil depth, although it remained constant or sharply decreased between the B- and the C-horizon at most sites. The natural abundance of 15N in the forest floor (FF), in the B-horizon and in the C-horizon averaged 2.2 ± 0.9‰, 6.2 ± 1.2‰and 4.9 ± 2.0‰, respectively while total soil profile δ15N ranged from 3.8‰to 7.4‰. Contrary to our hypothesis, soil δ15N was poorly correlated with climate, vegetation and most soil metrics. As a consequence, there was no spatial gradient in soil δ15N values and in the magnitude of increase in δ15N with soil depth across the study area. Soil C:N ratio was the only variable significantly correlated with soil δ15N. Multivariate models including the C:N ratio explained 47%, 60% and 36% of the inter-sites δ15N variation in B-horizon, C-horizon and total soil, respectively. In contrast with global scale studies, which have reported higher soil δ15N at sites with low soil C:N ratio, the relationship between these two variables was positive across the study area. The possible influence of ecto-myccorhizal association on this pattern is discussed. Overall, our data show that soil δ15N is controlled by complex mechanisms influenced by several variables with potential antagonist effects. Climate and most soil metrics appear to have no direct influence in the cold and N-limited forest ecosystems studied here and soil C:N ratio can affect soil δ15N in an opposite manner to what has been commonly observed.