International Journal of Geosciences, 2012, 3, 333-338
http://dx.doi.org/10.4236/ijg.2012.32036 Published Online May 2012 (http://www.SciRP.org/journal/ijg)
Dominant Factors of the Nature Regulating CO2 Release
from Boreal Forest Land
Ryunosuke Kikuchi1,2, Tamara T. Gorbacheva3
1Faculty of Science and Technology, Ryukoku University, Otsu, Japan
2CERNAS-ESAC, Polytechnic Institute of Coimbra Bencanta, Coimbra, Portugal
3Institute of the North Industrial Ecology Problems, Kola Science Center,
Murmansk, Russia
Received December 22, 2011; revised February 17, 2012; accepted March 14, 2012
ABSTRACT
Temperature is often considered as a primary factor for microbial decomposition of soil organic carbon. Boreal forests
are the large terrestrial carbon pool: if carbon stored in this region is transferred to the atmosphere as CO2 by a warm-
ing-induced acceleration of its decomposition, there will be positive feedback to global warming. It is reported that real
issue regarding the release of carbon from soils to the atmosphere is how natural factors interact to influence decom-
position of soil organic matter, so we observed mass losses (indicating decomposition rates) from litter and litterfall in a
Northern Fennoscandia forest over 3 years under natural conditions. Our field survey has demonstrated that mass losses
from most kinds of sample had moderate correlation with the temperature. Of the various samples, the canopy-gap litter
alone had a greater rate (~70%) of mass loss. It is at least necessary to make a clear distinction of monitoring sites (un-
der the canopy and in the canopy gap) when discussing the effect of climate on soil CO2 release from high-latitude for-
ests. Though temperature, soil moisture and soil properties are prioritized in the issue of soil CO2 release, our results
suggest that the fungi/bacteria rate and the wind-related mix/fragmentation are also important factors to be considered;
however, this speculation is just tentative, and more detail research is called for.
Keywords: Fragmentation; Global Warming; High-Latitude Forest; Microbial Decomposition; Wind
1. Introduction
The conversion of litter carbon (i.e. young soil carbon
above the mineral soil) to CO2 by microbial respiration is
one of the major processes controlling the terrestrial car-
bon budget [1], and most ecosystem models assume that
the temperature sensitivity of decomposition is identical
for all types of organic matter (review in [2]). Global
warming may trigger an unbalance between carbon sinks
provided by plants and carbon sources from decomposi-
tion, potentially causing an acceleration in CO2 release
from the terrestrial ecosystem. Nearly half of the carbon
stored in forested ecosystems is in boreal forests, and it is
a source of anxiety that a notable characteristic of boreal
forest soils is surface accumulations (~500 giga tons C)
of organic carbon [3]. Since high-latitude regions have
warmed faster than other parts in recent decades [4] and
these regions will be intensively subjected to a warming
climate in the future [5], the effect of warming tempera-
tures on boreal forests may result in a dramatic increase
in terrestrial carbon flux to the atmosphere.
Soil organic carbon (SOM) is mainly divided into min-
eral soil and litter (cf. [6]); as degradation of litter pro-
ceeds, SOM is transformed to organic acids and humins
which accumulate in mineral soil. According to the data
from 82 sites on five continents [7], increased tempera-
ture does not stimulate the decomposition of forest-
derived carbon in mineral soil. The dependence of mi-
crobial decomposition on temperature is only known for
young organic soil [8]. Yet another experiment suggests
that SOM decomposition is affected by soil depth and
experiment method, and the temperature sensitivity for
passive SOM does not differ from that for labile SOM
[9]. Anyway, the carbon input to the young organic soil
from biomass is easily released to the atmosphere ac-
cording to the current model- and observation-based con-
cepts [2,8,9].
Not only temperature and soil properties but also soil
moisture are also prioritized in the issue of soil CO2 re-
lease [10]; e.g. there are drying/wetting cycles in natural
conditions [11]. Nevertheless, the real issue regarding the
release of carbon from soils to the atmosphere is how
temperature, soil water content and other factors interact
to influence decomposition of soil organic matter [12].
The climate effect on microbial decomposition of soil
C
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R. KIKUCHI, T. T. GORBACHEVA 334
carbon is generally represented by applying the variable
Q10 temperature function [13]; Q10 values of ~2 at 30˚C -
35˚C, increasing to 4 - 6 at 5˚C - 10˚C. However, it is re-
ported that, after 5 years, there is no effect of a 5˚C warm-
ing on soil CO2 efflux form boreal forests in Sweden [14].
Considering this report, we observed the decomposition
rates over 3 years in a common spruce forest of the North-
ern Fennoscandia (Figure 1(a)) in order to evaluate the
climate effect under natural conditions. The obtained re-
sults are presented in this paper.
2. Materials and Methods
Sixteen sample plots are located in a subarctic region of
the Kola Peninsula (67˚51'N 32˚47'E to 66˚50'N 30˚12'E)
(see Figure 1(a)). The monitoring site is situated in
spruce forest with green mosses and dwarf shrubs (cf.
picture in Figure 1(a)), and the O horizon in this site
contains a rich amount of carbon (430 to 650 g·kg–1) [15].
According to a previous study on the biogeochemical cy-
cle in this field [15], the annual input of biomass to the
monitored litter layer is mainly composed of Picea ob-
ovata Ldb (0.47 t·ha–1 needles, 0.47 t·ha–1 wood/branches
and below 0.01 t·ha–1 bark) and dwarf shrubs (0.34 t·ha–1
Vaccinium myrtillus L. leaves, 0.22 t·ha–1 Empetrum
hermaphroditum leaves and 0.03 t·ha–1 Vaccinium vitis-
idaea leaves).
2.1. Sampling and Observation
In our survey, litter is defined as plant materials (residues)
which have already been in contact with the soil. The
surface accumulations (O horizon) of SOM in boreal
forests are basically identified as fresh litter (L layer),
partially decomposed but recognizable Formultningss-
kiktet (F layer) and relatively homogenous humus (H
layer) [16]. We took litter samples mainly from the L la-
yer (fresh litter) under the canopy and in the canopy gap
(Figure 1(b)) in October of the 1st year; a small portion
of F layer material was inadvertently mixed in, but we
avoided the mixing of H layer and E horizon materials
with the samples.
The sampled litter contained reproductive and dead
parts of the aforementioned various plants such as Picea
obovata Ldb, Vaccinium myrtillus L., Empetrum hermap-
hroditum and Vaccinium vitis-idaea.
In our survey, litterfall (browned needles, leaves, etc.)
is defined as plant materials which are still attached to
the living plants. Avoiding yellow coloring and leachate
loss, fresh litterfalls were also picked off from tree
branches and dwarf shrubs present in the survey field in
October of the 1st year.
After each collected sample of 10 g was put in a syn-
thetic mesh bag (10 × 10 cm) with breathability, these
bags (15 replicates of a sample in each plot) were sealed
(a)
(b)
Figure 1. Study area and arrangement of field survey. (a)
Study area in boreal biogeogr aphy of European region: the
border of the boreal region is shown with a bold line, and
the distribution of spruce (Picea abies ssp. obovata) species
is shown with bold dots (redrawn from [17]); (b) Sample
collection and observation sites in the study area: in our
field survey, litter is defined as dead plant materials which
are present on the soil, and litterfall is defined as plant
materials which are still attached to the living plants. Each
sample was put in a mesh bag, and these bags were placed
under the canopy and in the canopy gaps; i.e. all the sam-
ples underwent the natural decomposition pr ocess.
to prevent additional litterfalls from entering the bags,
some of the sealed bags (n = 4 in each plot) were analy-
zed in the same month as the collection (i.e. 0 year), and
the rest were placed under the canopy and in the canopy
gaps (Figure 1(b)); that is, all the samples underwent the
natural decomposition process in the mesh bags during
the survey period.
2.2. Measurement and Interval
Mass loss (percentage of the original mass) can be used
to predict biodegradation-related carbon loss (indicating
the rate of microbial CO2 respiration), and its measure-
ment can be easily conducted by using a weight-mea-
suring scale (review in ref. [18]). The sample bags were
dried in a desiccator and then measured with respect to
the following items: weight change of the sample bag (i.e.
gross mass loss) and hydroscopic coefficient for conver-
sion of sample weight to dry-base weight. Using quite a
simple method—mass loss of the sample, we attempted
to assess the climate sensitivity of a carbon pool so as not
to isolate the decomposition of soil carbon from the local
climate (i.e. natural conditions).

R. KIKUCHI, T. T. GORBACHEVA 335
If data of total mass loss from soil incubations longer
than one year are used to assess the SOM dependence on
temperature, there is a possibility that the obtained value
may be underestimated because respiration rates at all
temperatures are close to zero at the later stage of in-
cubation [9]. Viewed in this light, we set an interval of
one year for data collection in the survey field. We ca-
rried out measurement using the samples each October
over 3 years.
(a) (b)
2.3. Data Processing
The warmth index is defined as the yearly sum of the
mean monthly temperature minus 5˚C for the months
with the mean temperature above 5˚C (cf. [19]). There is
the limitation in applying the Q10 value to the rate of
SOM decomposition—the Q10 value is not adequate
when simulating the effect of temperature on decom-
position below 5˚C [20,21]. Considering the filed con-
ditions and the Q10 temperature function (i.e. the tem-
perature-decomposition relationship), we modified the
interval of warmth index in order to adapt the index to
the cold region of our survey; that is, the yearly sum of
the daily mean temperature minus 5˚C for the days with a
mean temperature above 5˚C.
Mass loss of sample was determined as a percentage of
the original mass, and their rates (%) were compared
using statistic analysis of variance (ANOVA) with a
probability value of 0.05.
3. Results and Interpretation
CO2 production by microorganisms is measurable at
–39˚C [22], and microbial decomposition in the arctic
ecosystem is relatively independent of temperature when
moisture content is less than 20% [23]. The climate con-
ditions recorded in our field were not so severe to micro-
organisms: the annual mean temperature varied between
+0.2˚C and –1.8˚C over our field survey. During the
plant-growing season (mid-June to mid-September) at
the daily mean temperature of 14.7˚C ± 1.4˚C standard
deviation (SD), the site under the canopy was 5.2˚C ±
1.8˚C (SD) cooler than the canopy gap, the throughfall
(precipitation passing through the canopy) averaged
62.8% ± 11.9% (SD) of gross rainfall, and the soil
moisture in the monitored litter layer was 52.9% ± 2.9%
(SD) under the canopy and 60.4% ± 1.6% (SD) in the
canopy gap, respectively.
As stated in the section entitled materials ands meth-
ods, we shortened the interval of warmth index order to
adapt the index to the cold region of our survey. This
warmth index and the mass-loss rates of each sample are
plotted in Figure 2.
Mass losses from all the samples occurred (p < 0.05)
during the 1st year (warmth index of 1530˚C) at the rate
Figure 2. Variations of mass loss (n = 4 in each plot) in
sample types as a function of warmth index with monitoring
site (a) canopy gap and (b) under canopy as parameter.
Warmth index in this figure means the sum of daily mean
temperature above 5˚C: 1530˚C from October the 1st year
to September the 2nd year, 2710˚C from October the 1st
year to September the 3rd year, and 4161˚C from October
the 1st year to September the 4th year.
of about 20% except for woody litterfalls and the canopy-
gap litter (Figure 2). The early-stage (over the 1st year)
decomposition rates range from ~10% near the Arctic
Circle to ~40% in northern Germany [24]. It is consi-
dered that the measured mass losses except those for the
above-mentioned two samples are approximately inter-
mediate between the Arctic value and the northern Ger-
many value.
The climate effect on decomposition of soil carbon is
represented by a water-stress function and the variable
Q10 temperature function [25,26]. The canopy gap was
~5˚C warmer and ~7% wetter than the under-canopy
floor, so this warmer and wetter microclimate seemed to
cause a greater rate of mass loss; however, as shown in
Figure 2, no clear difference between mass loss in the
canopy gap and mass loss under the canopy was ob-
served (p ≥ 0.05) (excluding the litter samples).
4. Discussion
Of the various samples, special attention should be paid
to the woody litterfalls and the canopy-gap litter because
these samples showed their own patterns of mass loss.
4.1. Woody Litterfalls
The mass-loss rates from woody litterfalls both in the
canopy gap and under the canopy were quite small
(~10% over 3 years). This low rate can be interpreted as
follows: before any weight loss, woody debris usually
has a long lag time which is related to the substrate size;
following the lag phase, the debris begins to weather and
fragment, and mass leaching and microbial activity occur
[27]. Therefore, the decomposition of woody litterfalls
was slow, that is, its mass loss was small.

R. KIKUCHI, T. T. GORBACHEVA 336
4.2. Hypothesis for Explaining the Mass Loss of
Canopy-Gap Litter
As compared with the above-mentioned data in northern
Germany (~40%) and near the Arctic Circle (~10%) (cf.
[24]), the mass-loss rate of ~70% from the canopy-gap
litter is significantly great; by contrast, the rate of ~20%
from the litter under the canopy is rational. Can we
logically interpret the obtained results to find the reason
why only the canopy-gap litter had a very high rate of
mass loss? It must be the most important rate-regulating
factor under natural conditions.
There is a clear difference between litter and litterfalls;
litter (picked from the O horizon) has already been in
contact with the soil, but litterfalls (picked from the
plants) have not yet had such contact (refer to the section
titled materials ands methods). Therefore, the following
hypothesis is built up to interpret this specific phenol-
menon.
1) Fungi/bacteria ratio: in our survey field, the F
layer under the canopy contains great amounts of fungi―
i.e. 1.73 - 8.65 mg·g–1 fungi mycelium and 0.10 - 0.25
mg· g –1 sporidium [28]. However, these amounts clearly
decrease in the canopy-gap F layer―i.e. 0.52 mg·g–1
fungi mycelium and 0.03 mg·g–1 sporidium [28]. The
bacteria community shows a different trend: 0.01 mg·g–1
under the canopy and 0.03 mg·g–1 in the canopy gap [28].
It is suggested that the changes in specie composition of
fungi potentially influence the accumulation of recalci-
trant soil organic matter derived from lignin and lignin-
like substances [29].
It is also reported that the mean percent fungal-to-
bacterial respiratory is 84-to-16 at a 6.0 pH in North
German spruce, and the metabolic quotient qCO2 (i.e.
respiration per unit biomass) declines with increasing
fungal presence [30]. Furthermore, the effect of soil war-
ming on the fungal population is still uncertain [31,32].
Based on the microbial difference, it is possible to inter-
pret the reason why the canopy-gap litter alone had a
greater rate of mass loss as follows: mite-like small ani-
mals break down dead plant materials on the soil into
fine pieces in a process called fragmentation; litter frag-
mentation (reduction of litter size and increase of surface
area) results in the establishment of a soil bacteria popu-
lation [33]―bacteria growth is especially affected by
fragmentation size because fungi can penetrate sub-
stances more easily than bacteria. However, a high rate
of litter fragmentation by mite-like small animals is not
expected under a harsh and cold climate in high-latitude
regions.
2) Wind-related mix and fragmentation: air tempe-
ratures within the forest in the afternoon are cooler than
the temperatures in nearby cleared areas [34]. Openings
(canopy gaps) in a moderate to dense tree stand become
warm air pockets during the day, and these openings
often act as natural chimneys, leading to accelerate the
rate of local updraft [34]. This updraft gives vibration,
rotation, inversion and so on to the litter present on the
canopy-gap floor, contributing to litter fragmentation
(Figure 3).
During the updraft-related process, litter is mixed with
soil bacteria as well as fungi. The main benefit of frag-
mentation is the fast leaching of toxic phenolics asso-
ciated with fragmented litter [35]. The different contents
of phenolics support this leaching theory: the content (6.0
mM·kg–1) of phenolics in the canopy-gap litter is much
lower than that (14 mM·kg–1) in the litter under the
canopy [36]. Consequently, litter becomes more bioavai-
lable not only to fungi but also to bacteria in the canopy
gap.
5. Conclusions
Of the various samples, the canopy-gap litter alone had a
greater rate of mass loss in the monitored boreal forest.
Although temperature, soil moisture and soil properties
are currently prioritized in the issue of soil CO2 release,
the obtained results suggest that it is necessary to make a
clear distinction of monitoring sites (under the canopy
and in the canopy gap) as well as make a clear distinction
between litter and litterfall when discussing the effect of
climate on soil CO2 release from high-latitude forests.
Furthermore, it is also suggested that the type of mi-
crobial community and the wind effect are essential fac-
tors to be considered: 1) fungal mycelia are abundant in
the boreal tree area (under the canopy in particular); 2)
the bacteria community is abundant in the canopy gap
(i.e. open space); and 3) there is a possibility that the rate
of local updraft may make litter materials more bio-
available to the bacteria community (i.e. an increase in
the soil-CO2 release). However, this interpretation is only
tentative, and more detail research is called for.
Figure 3. Conceptual schematics of wind effect on litter de-
composition in a canopy gap for explaining the high mass-
loss rate from the canopy-gap litter. Openings (canopy gaps)
often act as natural chimneys and accelerate the rate of
local updraft (a current of rising air). The swaying motion
of litter in the wind facilitates the mixing of litter with soil
bacteria and fungi, the leaching of phenolics from litter
pieces, and litter fragmentation. Consequently, microbial
accessibility to litter rises.

R. KIKUCHI, T. T. GORBACHEVA 337
6. Acknowledgements
Thanks are due to Ms E. Belova (Kola Science Center)
for field assistance, to Ms S. Sverchkova (Kola Science
Center) for laboratorial assistance, to Dr J. Maxwell
(University of Bristol) for helpful advice on text structure,
and to Ms C. Lentfer for English review.
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