Directory UMM :Data Elmu:jurnal:S:Soil Biology And Chemistry:Vol32.Issue10.Sep2000:
Soil Biology & Biochemistry 32 (2000) 1359±1366
www.elsevier.com/locate/soilbio
The decomposition of Lolium perenne in soils exposed to elevated
CO2: comparisons of mass loss of litter with soil respiration and
soil microbial biomass
Alwyn Sowerby a, Herbert Blum b, Tim R.G. Gray a, Andrew S. Ball a,*
a
Department of Biological Sciences, John Tabor Laboratories, Essex University, Wivenhoe Park, Colchester, Essex CO4 3SQ, UK
Department of Plant Sciences, Swiss Federal Institute of Science, Eschikon Research Station, Eschikon, Lindau CH 8315, Switzerland
b
Received 15 June 1999; received in revised form 10 December 1999; accepted 21 February 2000
Abstract
Two key questions regarding the eects of elevated atmospheric CO2 on soil microbial biomass are, (a) will future levels of
elevated CO2 aect the amount of microbial biomass in soil? and (b) how will any observed changes impact on C-¯ux from
soils? These questions were addressed by examining soil microbial biomass, and in situ estimations of soil respiration in
grassland soils exposed to free air carbon dioxide enrichment (60 Pa). Corresponding measurements of plant litter mass loss
were taken using litter bags, ensuring that ambient litter was decomposed in ambient soil, and elevated CO2 grown litter was
decomposed in soils exposed to elevated CO2. Signi®cantly greater levels of microbial biomass ( p < 0.05, paired t-test) were
detected in soils exposed to elevated CO2 (1174.1 compared to 878.9 mg N gÿ1 dry soil for ambient CO2 exposed soils). This
corresponded with a signi®cant increase ( p < 0.005, paired t-test) in in situ soil respiration from the elevated CO2 acclimatised
soils (28.7 compared to 20.4 mmol CO2 m2 hÿ1 from soils exposed to ambient CO2). However, when soil respiration was
calculated per unit of microbial biomass, no dierences in activity per unit biomass were detected (approx. 0.02 mmol CO2 m2
hÿ1 unit biomassÿ1), suggesting that increased soil microbial biomass, rather than increased activity was responsible for the
observed dierences. The mass loss of litter was greater in the elevated CO2 acclimatised soils ( p < 0.05, ANOVA), even though
the initial nutrient ratios of the litter were not signi®cantly dierent. 7 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Decomposition; Elevated CO2; Microbial biomass; Soil; Soil respiration; Lolium perenne
1. Introduction
Resource-balance models predict that plants respond
to a higher availability of above-ground resources by
increasing the relative allocation to nutrient acquisition
(Garnier, 1991). Indeed it has been demonstrated that
elevated CO2 favours investment of biomass in roots
(and their exudates) relative to that in leaves (Stulen
and Den Hertog, 1993), especially when plant growth
is nutrient limited. Therefore, the most pronounced
* Corresponding author. Tel.: +44-1206-873332; fax: +44-1206873416.
E-mail address: andrew@essex.ac.uk (A.S. Ball).
plant responses to elevated atmospheric CO2 concentration are probably going to be below-ground (Rogers
et al., 1994).
It is not thought that elevated atmospheric CO2 will
have any direct impacts on the soil community, as the
concentration of CO2 within the soil atmosphere is suf®ciently high to not be aected by the predicted
increase in atmospheric CO2 (O'Neill, 1994). However,
there are several indirect eects which may impact on
the soil community which could potentially alter rates
of carbon turnover in the terrestrial ecosystem.
Increased root production could result in an increase
in the C allocated below-ground (Norby, 1994). The
consequences of increased C input to the soil community from roots remains unknown. A more conclus-
0038-0717/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 3 8 - 0 7 1 7 ( 0 0 ) 0 0 0 4 5 - 6
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A. Sowerby et al. / Soil Biology & Biochemistry 32 (2000) 1359±1366
ively researched indirect eect of elevated CO2 is that
of altered litter quality entering the decomposition process eecting the rate of decomposition, and therefore,
nutrient turnover. A general dilution of nitrogen
within plant tissues when grown in elevated CO2 has
often been observed. In a meta-analysis of the eects
of elevated CO2 on the concentration of nitrogen in
over 75 species of plants, Cotrufo et al. found an overall mean decrease of 14% when plants were grown in
elevated CO2 (Cotrufo et al., 1998a, 1998b). The relative composition of other compounds, such as lignin
has also been aected by the concentration of atmospheric CO2; however, a change in the composition of
these compounds has been far less consistent (Penuelas
and Estiarte, 1998). However, Norby and Cotrufo
(1998) state that there is sucient evidence now to discount this potential eect. A more complete review of
the hypotheses surrounding the potential direct and
indirect impacts of elevated CO2 on below-ground processes can be found in O'Neill (1994).
Previous work looking at the eects of elevated CO2
on the decomposition of litter has generally focused on
only one aspect of the decomposition process. For
example a number of papers have reported dierences
in the mass loss of litter when originally grown in elevated CO2 (Cotrufo and Ineson, 1996; Cotrufo et al.,
1998a). However, without further investigation into
where nutrients lost from the litter are allocated to in
the soil community, it is not clear how these dierences will impact on nutrient cycling. As yet, no published work has considered the eects of elevated
atmospheric CO2 on the decomposition of litter as well
as the corresponding soil microbial biomass. Measurements of soil microbial biomass alone have also varied,
although a trend is observed towards greater microbial
biomass under elevated CO2 when estimated by fumigation or SIR techniques (Kampichler et al., 1998).
Our objectives with this work were to determine the
eects of elevated CO2 on the decomposition of
Lolium perenne litter along with corresponding
measurements of soil respiration and microbial biomass.
identical crop rotations before the establishment of the
FACE experiment, which began in May 1993. The soil
in the FACE rings is a fertile, eutric cambisol. Further
information regarding the site, including all protocols
involved, can be found in Zanetti et al. (1996).
Litter used in this work was harvested from plots
sown in monoculture with L. perenne in 1996. Since
then the plots have been cut and fertilised eight times
a year (NH4NO3; 560 kg N haÿ1 yearÿ1). Aerial litter
samples were cut during May 1998 and oven dried at
658C.
During July 1998, intact soil cores were taken
directly into plastic cylinders (dimensions: 7 cm 20
cm), at random from within plots in each of the
FACE rings. Mature L. perenne plants were extracted
as well as soil, to produce replica mesocosms for the
decomposition of litter. Fig. 1 shows a diagrammatical
representation of the intact cores, which were kept
within the FACE ring they were extracted from for the
duration of the experiment. Twenty-four soil cores
were taken in each ring. The soil cores were left for 1
week after extraction to equilibrate before any protocols were followed.
2.2. Mass loss of litter in the ®eld
Above-ground litter (0.5±0.8 g, harvested May 1998)
2. Materials and methods
2.1. Site and sampling
The Swiss free air carbon dioxide enrichment
(FACE) facility, at Eschikon, Switzerland, examines
the eects of elevated CO2 on a grassland system. The
site consists of six FACE rings, three of which are at
ambient levels of CO2, three of which have raised
levels of CO2 (60 Pa CO2; 1-min average of 60 Pa 2
10% within 92% of the fumigated time, Zanetti et al.,
1996). Each of the treatments at the site has undergone
Fig. 1. Diagrammatical representation of the intact Lolium perenne
soil cores taken at the Swiss FACE site, July 1998.
A. Sowerby et al. / Soil Biology & Biochemistry 32 (2000) 1359±1366
was weighed, then placed in mesh bags (mesh size 1
mm, dimensions 5 cm 7 cm). The bags were then
placed into the top layers of the soil in the intact soil
cores within the plots in the Swiss FACE rings.
Replica bags were removed sequentially throughout
the duration of the experiment to measure the mass
loss of litter from the litter bags. Litter was decomposed in the soil it was originally grown in, ensuring
that litter grown in elevated CO2 was decomposed in
soil acclimatised to elevated CO2. Similarly, litter originally grown in ambient CO2 was decomposed in
ambient soils.
2.3. Chemical composition of litter
Three individual leaves (taken as the part of the leaf
from the base of the stem) from each sample used in
the litter bag experiment outlined above, were ground
separately by pestle and mortar, then transferred to a
foil capsule of a known weight. Percentage composition of carbon and nitrogen, and the carbon to nitrogen ratio (C:N ratio) of the litter sample was then
measured using an automated CHNS/O analyser (Perkin Elmer).
As well as measurements of mass loss of litter from
the litter bags, percentage carbon, nitrogen and the
C:N ratio were measured from the remaining litter in
the bags, after the extractions of the litter bags from
the soil. This was performed on four of the extraction
dates.
Polymeric composition of the samples was estimated
by sequential extraction and gravimetric analysis. In
this case, six leaves were taken from each sample and
cut into 1 cm segments before analysis. Full experimental protocols are given in Harper and Lynch
(1981).
1361
2.5. Microbial biomass
Immediately after each measurement of soil respiration, soil samples were taken using a soil corer (30
cm 3 cm) and placed into sealed plastic bags.
Samples were temporarily stored at 48C before the estimation of microbial biomass (1 h maximum time
before fumigation). On each occasion, three replicates
were taken from each of the plots (in each of the
FACE rings), giving a total number of nine replicates
for each of the treatments. Microbial biomass-N was
determined in the samples taken above. First the
whole core taken by the soil corer (30 cm 3 cm) was
mixed then two sub-samples (10 g) of the soil were
used in a fumigation-extraction method of measuring
microbial biomass, as outlined in Rowell (1995).
2.6. Statistical analysis
Mass loss of litter from litter bags was analysed
using two way analysis of variance (ANOVA). Dierences between the chemical composition of the elevated CO2 and ambient grown litter were identi®ed
using t-tests.
In the statistical analysis FACE rings were paired
(i.e. C1 with F1, C2 with F2 and C3 with F3), as
analysis of the soil in the FACE rings in a previous
report showed dierences in basic soil chemistry
(Nitschelm, 1996).
2.4. Soil respiration
Soil respiration measurements were taken in both
the morning and afternoon during July and August
1998 using a portable gas monitor (EGM-1, PP Systems) linked to a respiration chamber (diameter 10 cm;
soil respiration chamber, PP Systems) at the Swiss
FACE site. The grass sward (including roots) was
removed from plots (approximate dimensions: 0.5 m
1 m) to avoid the inclusion of root/rhizosphere respiration. The root-free soil was then left to equilibrate for
a few days until the readings of soil respiration stabilised. Measurements were taken at random by placing
the respiration chamber over a section of soil for approximately 5 min. Respiratory activity was calculated
from the CO2 accumulation rate within the chamber,
as described by the manufacturers (Anon, 1990), and
expressed as mmol CO2 m2 hÿ1.
Fig. 2. Mass loss of litter from litter bags containing litter cut from
swards in ambient and elevated CO2 (60 Pa CO2) rings. Ambient
grown litter was decomposed in ambient soil, elevated CO2 grown
litter in elevated CO2-acclimatised soil. Error bars show the standard
error of the mean, n = 9.
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A. Sowerby et al. / Soil Biology & Biochemistry 32 (2000) 1359±1366
Table 1
Chemical analysis of litter grown in ambient and elevated CO2 FACE rings in Switzerlanda
% Carbon
% Nitrogen
C:N
Soluble fraction
Hemicellulose
Cellulose and ash
Lignin
Lignin:N
a
Ambient grown litter
Elevated CO2 grown litter
Signi®cance level
45.28
3.53
13.2:1
35.38
27.97
32.43
4.22
1.2:1
45.72
3.29
14.7:1
39.94
29.77
26.88
3.41
1:1
n.s.
n.s.
n.s.
Signi®cance is reported on t-tests, p < 0.05,
p < 0.01,
n.s.
n.s.
Not applicable
p < 0.005.
3. Results
3.1. Chemical composition of harvested litter
The percentage carbon and nitrogen content, and
therefore, the C:N ratio of L. perenne litter harvested
in May 1996 from plants grown in elevated and ambient CO2 was not statistically dierent (C:N 13:1 and
15:1 for ambient and elevated CO2-derived litters, respectively, Table 1). The percentage composition of the
hemi-cellulose and lignin content was also not signi®cantly dierent between ambient and elevated CO2
grown litters (Table 1). However, dierences were
observed in the soluble and cellulose/ash fractions of
the litter (Table 1).
3.2. Mass loss of litter in the ®eld
mass loss between the two litter types (Fig. 2), where
signi®cant dierences were mainly observed in the ®rst
5 days and then later on after approximately 35 days.
3.4. Microbial biomass
Microbial biomass was measured during 3 days in
July and August 1998. Comparing ambient and elevated CO2 rings for dierences in microbial biomass
showed elevated CO2-acclimatised soils to have a signi®cantly greater biomass than ambient soils (Paired ttest p < 0.05; elevated CO2 exposed soil
mean=1174.07 2 129.0 mg N gÿ1 soil, ambient CO2
exposed soil mean=878.94 2 76.58 mg N gÿ1 soil).
Readings of microbial biomass did dier between the
dates; ranging from 641.9 to 1151.9 mg N gÿ1 soil in
the control soil samples, and from 959.7 to 1785.9 mg
N gÿ1 soil in the elevated CO2 soils. However, the el-
A comparison of the mass loss of L. perenne litter
from litter bags showed statistically signi®cant dierences (Fig. 2; two way ANOVA, F = 17.53, p < 0.01).
The litter originally grown in the elevated CO2 was
found to have consistently greater rates of decomposition than the litter grown and decomposed in the
ambient soils. This pattern was seen throughout the
decomposition period, showing that the length of time
during which the litter was in the soil did not change
the eect of the elevated CO2 on the decomposition of
the litter (Two way ANOVA, F = 0.167, not signi®cant).
3.3. Chemical composition of decomposing litter
Analysis of decomposing litter showed a decline in
the C:N ratios of the litter until ratios levelled out at
approximately 10:1 (Fig. 3). The decline in the C:N
ratio of the elevated CO2 grown litter was slower than
the ambient litter (taking approximately 20 and 30
days, respectively). The dierences observed in the loss
of carbon and nitrogen from the litter were mainly
seen between day 5 and 25 in the soil. Interestingly,
this does not correspond to the dierences observed in
Fig. 3. C:N ratio of litter remaining in the litter bags decomposing in
ambient and elevated CO2 (60 Pa CO2) acclimatised soils at the
Swiss FACE site. Ambient grown litter was decomposed in ambient
soil, elevated CO2 grown litter in elevated CO2-acclimatised soil.
Error bars show the standard error of the mean, n = 9.
A. Sowerby et al. / Soil Biology & Biochemistry 32 (2000) 1359±1366
evated CO2 soils consistently held greater microbial
biomass.
3.5. Soil respiration
Measurements of soil respiration showed that ambient soil on average produced 20.4 mmol CO2 m2 hÿ1,
elevated CO2-acclimatised soil produced on average
28.7 mmol CO2 m2 hÿ1. This shows signi®cantly greater
levels of soil respiration from elevated CO2-acclimatised soils than from ambient soils (paired t-test; p <
0.005), when compared on individual sample dates.
However, when soil respiration was calculated per
unit of microbial biomass, ambient soils produced on
average 0.02 mmol CO2 hÿ1 unit biomass-Nÿ1, (standard error; 3.9 10ÿ3), with elevated CO2-acclimatised
soils producing on average 0.03 mmol CO2 hÿ1 unit
biomass-Nÿ1 (standard error; 4.9 10ÿ3). These values
showed no statistically signi®cant dierences in CO2
production per unit of microbial biomass between
ambient and elevated CO2 acclimatised soil samples
(paired t-test).
4. Discussion
4.1. Chemical composition of litter
The chemical analysis of the litter used in the mass
loss work showed no signi®cant dierences in carbon
and nitrogen content, although a trend towards greater
C:N ratios were observed in elevated CO2 grown litters. Early predictions, based on green leaf chemistry,
estimated that elevated atmospheric CO2 would reduce
nitrogen in senesced plant material (O'Neill and
Norby, 1996). However, this has not been realised, as
these initial predictions did not consider the resorption
of leaf constituents before senescence (Norby and
Cotrufo, 1998), or dierences in plant chemistry when
growth is restricted by pots, etc. (O'Neill and Norby,
1996). It is generally agreed that the C:N ratio of litter
is not greater when plants are grown in elevated CO2
(Walker and Steen, 1997). The results from our work
con®rm these ®ndings.
4.2. Mass loss of litter in the ®eld
The majority of published results have focused on
the decomposition of litter originally grown in elevated CO2, in ambient soils. Previous research has provided variable results. When litter grown in elevated
CO2 is decomposed in soil not exposed previously to
elevated CO2 either no dierence in mass loss was
observed (Kemp et al., 1994; O'Neill and Norby,
1996), or a decline in the mass loss of litter was seen
1363
(Cotrufo et al., 1994; Cotrufo and Ineson, 1995;
Cotrufo and Ineson, 1996; Cotrufo et al., 1998b).
Although no direct eects of elevated CO2 are
expected on the soil community, indirect eects may
become increasingly important and could potentially
amplify to the extent of altering soil community structure and functions. In contrast to the majority of
papers, our data shows increased rates of mass loss
when litter was grown and decomposed in soils
exposed to elevated CO2. Other work reporting the decomposition of litter originally grown in elevated CO2
in soil exposed to elevated CO2 has not shown many
signi®cant dierences (Hirschel et al., 1997; Gahrooee,
1998). However, looking at the data presented by Gahrooee (1998), for one of the species monitored, Quercus
cerris, a clear elevated CO2 eect was observed with
the soil exposed to elevated levels of CO2. Here soils
exposed to elevated CO2 showed a consistent decline
in the mass loss of litter when compared to soils
exposed to ambient conditions. This eect was
observed for both the ambient grown litter and the elevated CO2 grown litter (Gahrooee, 1998).
4.3. Chemical composition of decomposing litter
Although the litters had similar C:N ratios when
entering the soil, almost as soon as the decomposition process began, dierences arose in the C:N
ratio of the litters until the C:N ratio of the litter
reached approximately 10:1, suggesting that the initial decomposition of the litter did not occur in
the same manner. Whether this was the response of
an altered decomposer community, or some other
factor is not clear. Dierences seen in the mass loss
of the litter continued throughout the decomposition
period. However, after the initial 20±30 days, consistent signi®cant dierences were not seen in the
C:N ratio of the dierent litters as they decomposed. This suggests that the C:N ratio is not a
good indicator of the nitrogen or carbon dynamics
of decaying plant material, which is veri®ed by
Wagener and Schimel (1998), who studied the temporal and spatial decomposition of litter on a Alaskan birch forest ¯oor.
Unlike the C:N ratio, or the total nitrogen content,
signi®cant dierences were observed in the initial soluble fraction and the cellulose and ash content of the
litters (Table 1). This may in part explain the dierences between the mass loss of litter from the litter
bags, as described above, as elevated CO2 grown litter
was found to have a higher soluble fraction and a
lower cellulose and ash content, making it theoretically
more degradable than the litter grown in the ambient
conditions.
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A. Sowerby et al. / Soil Biology & Biochemistry 32 (2000) 1359±1366
4.4. Microbial biomass
In our study elevated CO2 exposed soil samples had
consistently greater microbial biomass than ambient
soil samples. Several other published results have
observed increased microbial biomass in elevated CO2
acclimatised soils (Diaz et al., 1993; Zak et al., 1993;
Schenk et al., 1995). Other published results have not
shown an increase in the soil microbial biomass with
elevated CO2 (Kampichler et al., 1998; Runion et al.,
1994). However, the results presented in Kampichler et
al. (1998), are from containers within a controlled environment exposed to elevated CO2, and in Runion et
al. (1994), microbial biomass was estimated using dilution plate counts, a method notoriously biased when
considering the microbial community in soil.
Diaz et al. (1993) hypothesised that the extra carbon
resulting from elevated levels of atmospheric CO2, will
be allocated to the microbial biomass in soil and will
bring about carbon and nutrient accumulation in soil
organic matter. With this hypothesis, increased carbon
inputs will stay within the soil system and not be
returned to the atmosphere immediately. If this was to
occur, increasing levels of CO2 within the atmosphere
may be ameliorated by increased carbon storage in soil
organic matter. From the measurement of microbial
biomass, our ®ndings, in part, support this theory.
4.5. Soil respiration
Overall, the CO2 eux from the bulk soil at the
Swiss FACE site was greater in the rings exposed to elevated CO2. Other published work examining the CO2
eux from soil shows somewhat variable results. CO2
eux was greater in soils planted with Ponderosa pine
(Vose et al., 1997), temperate zone shrubland (Ball et
al., 2000), Scots pine seedlings (Janssens et al., 1998)
and Douglas ®r seedlings (Lin et al., 1999). However,
CO2 eux from a tidal marsh ecosystem showed no
eects of elevated CO2 (Ball and Drake, 1997). Previous measurements of CO2 eux at the Swiss FACE
site showed lower levels of CO2 eux from soils
exposed to elevated CO2 (Ineson et al., 1998). However, readings of gas exchange were limited only to
one control ring and one fumigated ring.
5. Conclusions
Zak et al. (1993) hypothesised that the result of
increased carbon inputs from the atmosphere to the
soil under elevated concentrations of CO2, may
increase rates of turnover and mineralisation of carbon
within the soil system. Theoretically, carbon entering
the soil will be transformed rapidly and returned to
the atmosphere, giving at best a neutral eect on rates
of CO2 accumulation in the atmosphere, and at worse
potentially result in a positive feedback.
Our results do show an increase in overall rates of
soil respiration from soils exposed to elevated CO2.
However, this was the result of an increase in microbial biomass rather than due to an increase in activity of individuals. Our results from the measurement
of mass loss of litter, and the nutritional quality of
that litter as it decomposes, suggest that there may be
a dierence between the decomposer communities in
the soils in the dierent CO2 treatment rings at the
Swiss FACE site. Whether the observed increase in microbial biomass and soil respiration in the FACE rings
is driven by altered or increased litter input is not completely clear as other factors, such as the water content
of the soil, have been shown to eect microbial biomass and soil respiration. However, various environmental parameters have been measured alongside
microbial biomass and soil respiration at the Swiss site
with very few dierences being observed between elevated CO2 exposed soils and ambient soils (data not
shown). This would seem to suggest that any dierences observed in this work were the result of the
plants response to elevated CO2, be it either through a
change in the input of litter to the soil or through
altered rooting strategies or exudates.
Ineson et al. (1998) showed greatly accelerated rates
of N2O production from the soils at the Swiss FACE
site. They speculated that increased available soil-C
had fuelled the increased rates of denitri®cation. The
rate of mass loss of litter from the litter bags in the
soil at the Swiss FACE site was also rapid, when comparing rates with other published work (e.g. Kemp et
al., 1994; Cotrufo and Ineson, 1996; Hirschel et al.,
1997). Clearly a dierence is being observed in soil
exposed to elevated CO2 at the Swiss FACE site. In
corroboration with some of the results published in
Ineson et al. (1998), our results seem to be suggesting
increased rates of C and N cycling within the soils
exposed to elevated CO2 in the Swiss FACE site.
Future work on the eects of elevated CO2 on the
decomposition of litter and the turnover of carbon
within soil needs more precise measurements of the
rate of C and N cycling within the system. Our results
presented here also show there may be dierences in
the process of decomposition in soils exposed to elevated CO2. The use of stable isotopes such as 15 N and
13
C, will become invaluable in this, and have already
provided interesting results to date (Hungate et al.,
1998). Integrated approaches should also be considered, as measurements of microbial biomass alone,
without an indication of the activity of the population
or the eciency of its operation will be meaningless
when attempting to predict how elevated CO2 will
eect the turnover of carbon within the soil community. Several questions still remain. (1) How is elevated
A. Sowerby et al. / Soil Biology & Biochemistry 32 (2000) 1359±1366
atmospheric CO2 bringing about dierences in the soil
community? (2) Is elevated CO2, or any elevated CO2derived eect, changing the diversity of individuals
within the decomposer community, as well as the
population size of the microbial component of the
community? Clearly long-term integrated research will
be needed to answer some of these questions, so as
precise as possible predictions can be made on how elevated CO2 will impact on the turnover of carbon in
the terrestrial system.
Acknowledgements
The award of a NERC studentship to Alwyn
Sowerby is gratefully acknowledged. Technical assistance provided by Tania Cresswell-Maynard in UK
and Werner Wild in Switzerland is also appreciated.
References
Anon, 1990. Operators Manual Version 1.8. PP Systems. Hitchens,
UK.
Ball, A.S., Milne, E., Drake, B.G., 2000. Elevated atmospheric-carbon dioxide increases soil respiration in a midsuccessional lowland forest. Soil Biology and Biochemistry, in press.
Ball, A.S., Drake, B.G., 1997. Short-term decomposition of litter
produced by plants grown in ambient and elevated atmospheric
CO2 concentrations. Global Change Biology 3, 29±35.
Cotrufo, M.F., Ineson, P., Scott, A., 1998a. Elevated CO2 reduces
the nitrogen concentration of plant tissues. Global Change
Biology 4, 43±54.
Cotrufo, M.F., Briones, M.J.I., Ineson, P., 1998b. Elevated CO2
eects ®eld decomposition rate and palatability of tree leaf litter:
importance of changes in substrate quality. Soil Biology and
Biochemistry 30, 1565±1571.
Cotrufo, M.F., Ineson, P., 1996. Elevated CO2 reduces ®eld decomposition rates of Betula pendula (Roth.) leaf litter. Oecologia
106, 525±530.
Cotrufo, M.F., Ineson, P., 1995. Eects of enhanced atmospheric
CO2 and nutrient supply on the quality and subsequent decomposition of ®ne roots of Betula pendula Roth. and Picea sitchensis
(Bong.) Carr. Plant and Soil 170, 267±277.
Cotrufo, M.F., Ineson, P., Rowland, A.P., 1994. Decomposition of
tree leaf litters grown under elevated CO2: eect of litter quality.
Plant and Soil 163, 121±130.
Diaz, S., Grime, J.P., Harris, J., McPherson, E., 1993. Evidence of a
feedback mechanism limiting plant response to elevated carbon
dioxide. Nature 364, 616±617.
Gahrooee, F.R., 1998. Impacts of elevated atmospheric CO2 on litter
quality, litter decomposability and nitrogen turnover rate of two
oak species in a Mediterranean forest ecosystem. Global Change
Biology 4, 667±677.
Garnier, E., 1991. Resource capture, biomass allocation and growth
in herbaceous plants. Trends in Ecology and Evolution 6, 126±
131.
Harper, S.H.T., Lynch, J.M., 1981. The chemical components and
decomposition of Wheat straw leaves, internodes and nodes.
Journal of Science, Food and Agriculture 32, 1057±1062.
Hirschel, G., Korner, Ch., Arnone III, J.A., 1997. Will rising atmos-
1365
pheric CO2 aect leaf litter quality and in situ decomposition
rates in native plant communities? Oecologia 110, 387±392.
Hungate, B.A., Holland, E.A., Jackson, R.B., Chapin III, F.S.,
Mooney, H.A., Field, C.B., 1998. The fate of carbon in grasslands under carbon dioxide enrichment. Nature 388, 576±579.
Ineson, P., Coward, P.A., Hartwig, U.A., 1998. Soil gas ¯uxes of
N2O, CH4 and CO2 beneath Lolium perenne under elevated CO2:
the Swiss free air carbon dioxide enrichment experiment. Plant
and Soil 198, 89±95.
Janssens, F., Peeters, A., Tallowin, J.R.B., Bakker, J.P., Bekker,
R.M., Fillat, F., Oomes, M.J.M., 1998. Relationship between soil
chemical factors and grassland diversity. Plant and Soil 202, 69±
78.
Kampichler, C., Kandeler, E., Bardgett, R.D., Jones, T.H.,
Thompson, L.J., 1998. Impact of elevated atmospheric CO2 concentration on soil microbial biomass and activity in a complex,
weedy ®eld model. Global Change Biology 4, 335±346.
Kemp, P.R., Waldecker, D.G., Owensby, C.E., Reynolds, J.F.,
Virginia, R.A., 1994. Eects of elevated CO2 and nitrogen fertilisation pretreatments on decomposition on tall grass prairie leaf
litter. Plant and Soil 165, 115±127.
Lin, G., Ehleringer, J.R., Rygiewicz, P.T., Johnson, M., Tingey,
D.T., 1999. Elevated CO2 and temperature impacts on dierent
components of soil CO2 eux in Douglas-®r terracosms. Global
Change Biology 5, 157±168.
Nitschelm, J.J., 1996. Using stable isotopes to trace soil C input in a
pasture subject to free air CO2 enrichment. Masters Thesis,
University of Saskatchewan, Canada.
Norby, R.J., Cotrufo, M.F., 1998. A question of litter quality.
Nature 396, 17±18.
Norby, R.J., 1994. Issues and perspectives for investigating root responses to elevated atmospheric carbon dioxide. Plant and Soil
165, 9±20.
O'Neill, E.G., 1994. Responses of soil biota to elevated atmospheric
carbon-dioxide. Plant and Soil 165, 55±65.
O'Neill, E.G., Norby, R.J., 1996. Litter quality and decomposition
rates of foliar litter produced under CO2 enrichment. In: Koch,
G.W., Mooney, H.A. (Eds.), Carbon Dioxide and Terrestrial
Ecosystems. Academic Press, London, UK, pp. 87±103.
Rogers, H.H., Runion, G.B., Krupa, S.V., 1994. Plant-responses to
atmospheric CO2 enrichment with emphasis on roots and the rhizosphere. Environmental Pollution 83, 155±189.
Penuelas, J., Estiarte, M., 1998. Can elevated CO2 aect secondary
metabolism and ecosystem function? Trends in Ecology and
Evolution 13, 20±23.
Rowell, D.L., 1995. Soil Science: Methods and Applications.
Longman, UK.
Runion, G.B., Curl, E.A., Rogers, H.H., Backman, P.A.,
Rodriguezcabana, R., Helms, B.E., 1994. Eects of free-air CO2
enrichment on microbial-populations in the rhizosphere and phyllosphere of cotton. Agricultural and Forest Meteorology 70, 117±
130.
Schenk, U., Manderscheid, R., Hugen, J., Weigel, H.-J., 1995.
Eects of CO2 enrichment and intraspeci®c competition on biomass partitioning, nitrogen content and microbial biomass carbon
in soil of perennial ryegrass an white clover. Journal of
Experimental Botany 46, 987±993.
Stulen, I., Den Hertog, J., 1993. Root-growth and functioning under
atmospheric CO2 enrichment. Vegetatio 104, 99±115.
Vose, J.M., Elliot, K.J., Johnson, D.W., Tingey, D.T., Johnson,
M.G., 1997. Soil respiration response to three years of elevated
CO2 and N fertilisation in Ponderosa pine (Pinus ponderosa
Doug, ex Laws.). Plant and Soil 190, 19±28.
Wagener, S.M., Schimel, J.P., 1998. Strati®cation of soil ecological
processes: a study of the birch forest ¯oor in the Alaskan taiga.
Oikos 81, 63±74.
Walker, B., Steen, W., 1997. The terrestrial biosphere and global
1366
A. Sowerby et al. / Soil Biology & Biochemistry 32 (2000) 1359±1366
change: implications for natural and managed ecosystems. A synthesis of GCTE and related research. IGBP Science No.1. ISSN
0284-8015.
Zak, D.R., Pregitzer, K.S., Curtis, P.S., Teeri, J.A., Fogel, R.,
Randlett, D.L., 1993. Elevated atmospheric CO2 and feedback
between carbon and nitrogen cycles. Plant and Soil 151, 105±117.
Zanetti, S., Hartwig, U.A., Luscher, A., Hebeisen, T., Frehner, M.,
Fischer, B.U., Hendrey, G.R., Blum, H., Nosberger, J., 1996.
Stimulation of symbiotic N2 ®xation in Trifolium repens L. under
elevated atmospheric pCO2 in a grassland ecosystem. Plant
Physiology 112, 575±583.
www.elsevier.com/locate/soilbio
The decomposition of Lolium perenne in soils exposed to elevated
CO2: comparisons of mass loss of litter with soil respiration and
soil microbial biomass
Alwyn Sowerby a, Herbert Blum b, Tim R.G. Gray a, Andrew S. Ball a,*
a
Department of Biological Sciences, John Tabor Laboratories, Essex University, Wivenhoe Park, Colchester, Essex CO4 3SQ, UK
Department of Plant Sciences, Swiss Federal Institute of Science, Eschikon Research Station, Eschikon, Lindau CH 8315, Switzerland
b
Received 15 June 1999; received in revised form 10 December 1999; accepted 21 February 2000
Abstract
Two key questions regarding the eects of elevated atmospheric CO2 on soil microbial biomass are, (a) will future levels of
elevated CO2 aect the amount of microbial biomass in soil? and (b) how will any observed changes impact on C-¯ux from
soils? These questions were addressed by examining soil microbial biomass, and in situ estimations of soil respiration in
grassland soils exposed to free air carbon dioxide enrichment (60 Pa). Corresponding measurements of plant litter mass loss
were taken using litter bags, ensuring that ambient litter was decomposed in ambient soil, and elevated CO2 grown litter was
decomposed in soils exposed to elevated CO2. Signi®cantly greater levels of microbial biomass ( p < 0.05, paired t-test) were
detected in soils exposed to elevated CO2 (1174.1 compared to 878.9 mg N gÿ1 dry soil for ambient CO2 exposed soils). This
corresponded with a signi®cant increase ( p < 0.005, paired t-test) in in situ soil respiration from the elevated CO2 acclimatised
soils (28.7 compared to 20.4 mmol CO2 m2 hÿ1 from soils exposed to ambient CO2). However, when soil respiration was
calculated per unit of microbial biomass, no dierences in activity per unit biomass were detected (approx. 0.02 mmol CO2 m2
hÿ1 unit biomassÿ1), suggesting that increased soil microbial biomass, rather than increased activity was responsible for the
observed dierences. The mass loss of litter was greater in the elevated CO2 acclimatised soils ( p < 0.05, ANOVA), even though
the initial nutrient ratios of the litter were not signi®cantly dierent. 7 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Decomposition; Elevated CO2; Microbial biomass; Soil; Soil respiration; Lolium perenne
1. Introduction
Resource-balance models predict that plants respond
to a higher availability of above-ground resources by
increasing the relative allocation to nutrient acquisition
(Garnier, 1991). Indeed it has been demonstrated that
elevated CO2 favours investment of biomass in roots
(and their exudates) relative to that in leaves (Stulen
and Den Hertog, 1993), especially when plant growth
is nutrient limited. Therefore, the most pronounced
* Corresponding author. Tel.: +44-1206-873332; fax: +44-1206873416.
E-mail address: andrew@essex.ac.uk (A.S. Ball).
plant responses to elevated atmospheric CO2 concentration are probably going to be below-ground (Rogers
et al., 1994).
It is not thought that elevated atmospheric CO2 will
have any direct impacts on the soil community, as the
concentration of CO2 within the soil atmosphere is suf®ciently high to not be aected by the predicted
increase in atmospheric CO2 (O'Neill, 1994). However,
there are several indirect eects which may impact on
the soil community which could potentially alter rates
of carbon turnover in the terrestrial ecosystem.
Increased root production could result in an increase
in the C allocated below-ground (Norby, 1994). The
consequences of increased C input to the soil community from roots remains unknown. A more conclus-
0038-0717/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 3 8 - 0 7 1 7 ( 0 0 ) 0 0 0 4 5 - 6
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A. Sowerby et al. / Soil Biology & Biochemistry 32 (2000) 1359±1366
ively researched indirect eect of elevated CO2 is that
of altered litter quality entering the decomposition process eecting the rate of decomposition, and therefore,
nutrient turnover. A general dilution of nitrogen
within plant tissues when grown in elevated CO2 has
often been observed. In a meta-analysis of the eects
of elevated CO2 on the concentration of nitrogen in
over 75 species of plants, Cotrufo et al. found an overall mean decrease of 14% when plants were grown in
elevated CO2 (Cotrufo et al., 1998a, 1998b). The relative composition of other compounds, such as lignin
has also been aected by the concentration of atmospheric CO2; however, a change in the composition of
these compounds has been far less consistent (Penuelas
and Estiarte, 1998). However, Norby and Cotrufo
(1998) state that there is sucient evidence now to discount this potential eect. A more complete review of
the hypotheses surrounding the potential direct and
indirect impacts of elevated CO2 on below-ground processes can be found in O'Neill (1994).
Previous work looking at the eects of elevated CO2
on the decomposition of litter has generally focused on
only one aspect of the decomposition process. For
example a number of papers have reported dierences
in the mass loss of litter when originally grown in elevated CO2 (Cotrufo and Ineson, 1996; Cotrufo et al.,
1998a). However, without further investigation into
where nutrients lost from the litter are allocated to in
the soil community, it is not clear how these dierences will impact on nutrient cycling. As yet, no published work has considered the eects of elevated
atmospheric CO2 on the decomposition of litter as well
as the corresponding soil microbial biomass. Measurements of soil microbial biomass alone have also varied,
although a trend is observed towards greater microbial
biomass under elevated CO2 when estimated by fumigation or SIR techniques (Kampichler et al., 1998).
Our objectives with this work were to determine the
eects of elevated CO2 on the decomposition of
Lolium perenne litter along with corresponding
measurements of soil respiration and microbial biomass.
identical crop rotations before the establishment of the
FACE experiment, which began in May 1993. The soil
in the FACE rings is a fertile, eutric cambisol. Further
information regarding the site, including all protocols
involved, can be found in Zanetti et al. (1996).
Litter used in this work was harvested from plots
sown in monoculture with L. perenne in 1996. Since
then the plots have been cut and fertilised eight times
a year (NH4NO3; 560 kg N haÿ1 yearÿ1). Aerial litter
samples were cut during May 1998 and oven dried at
658C.
During July 1998, intact soil cores were taken
directly into plastic cylinders (dimensions: 7 cm 20
cm), at random from within plots in each of the
FACE rings. Mature L. perenne plants were extracted
as well as soil, to produce replica mesocosms for the
decomposition of litter. Fig. 1 shows a diagrammatical
representation of the intact cores, which were kept
within the FACE ring they were extracted from for the
duration of the experiment. Twenty-four soil cores
were taken in each ring. The soil cores were left for 1
week after extraction to equilibrate before any protocols were followed.
2.2. Mass loss of litter in the ®eld
Above-ground litter (0.5±0.8 g, harvested May 1998)
2. Materials and methods
2.1. Site and sampling
The Swiss free air carbon dioxide enrichment
(FACE) facility, at Eschikon, Switzerland, examines
the eects of elevated CO2 on a grassland system. The
site consists of six FACE rings, three of which are at
ambient levels of CO2, three of which have raised
levels of CO2 (60 Pa CO2; 1-min average of 60 Pa 2
10% within 92% of the fumigated time, Zanetti et al.,
1996). Each of the treatments at the site has undergone
Fig. 1. Diagrammatical representation of the intact Lolium perenne
soil cores taken at the Swiss FACE site, July 1998.
A. Sowerby et al. / Soil Biology & Biochemistry 32 (2000) 1359±1366
was weighed, then placed in mesh bags (mesh size 1
mm, dimensions 5 cm 7 cm). The bags were then
placed into the top layers of the soil in the intact soil
cores within the plots in the Swiss FACE rings.
Replica bags were removed sequentially throughout
the duration of the experiment to measure the mass
loss of litter from the litter bags. Litter was decomposed in the soil it was originally grown in, ensuring
that litter grown in elevated CO2 was decomposed in
soil acclimatised to elevated CO2. Similarly, litter originally grown in ambient CO2 was decomposed in
ambient soils.
2.3. Chemical composition of litter
Three individual leaves (taken as the part of the leaf
from the base of the stem) from each sample used in
the litter bag experiment outlined above, were ground
separately by pestle and mortar, then transferred to a
foil capsule of a known weight. Percentage composition of carbon and nitrogen, and the carbon to nitrogen ratio (C:N ratio) of the litter sample was then
measured using an automated CHNS/O analyser (Perkin Elmer).
As well as measurements of mass loss of litter from
the litter bags, percentage carbon, nitrogen and the
C:N ratio were measured from the remaining litter in
the bags, after the extractions of the litter bags from
the soil. This was performed on four of the extraction
dates.
Polymeric composition of the samples was estimated
by sequential extraction and gravimetric analysis. In
this case, six leaves were taken from each sample and
cut into 1 cm segments before analysis. Full experimental protocols are given in Harper and Lynch
(1981).
1361
2.5. Microbial biomass
Immediately after each measurement of soil respiration, soil samples were taken using a soil corer (30
cm 3 cm) and placed into sealed plastic bags.
Samples were temporarily stored at 48C before the estimation of microbial biomass (1 h maximum time
before fumigation). On each occasion, three replicates
were taken from each of the plots (in each of the
FACE rings), giving a total number of nine replicates
for each of the treatments. Microbial biomass-N was
determined in the samples taken above. First the
whole core taken by the soil corer (30 cm 3 cm) was
mixed then two sub-samples (10 g) of the soil were
used in a fumigation-extraction method of measuring
microbial biomass, as outlined in Rowell (1995).
2.6. Statistical analysis
Mass loss of litter from litter bags was analysed
using two way analysis of variance (ANOVA). Dierences between the chemical composition of the elevated CO2 and ambient grown litter were identi®ed
using t-tests.
In the statistical analysis FACE rings were paired
(i.e. C1 with F1, C2 with F2 and C3 with F3), as
analysis of the soil in the FACE rings in a previous
report showed dierences in basic soil chemistry
(Nitschelm, 1996).
2.4. Soil respiration
Soil respiration measurements were taken in both
the morning and afternoon during July and August
1998 using a portable gas monitor (EGM-1, PP Systems) linked to a respiration chamber (diameter 10 cm;
soil respiration chamber, PP Systems) at the Swiss
FACE site. The grass sward (including roots) was
removed from plots (approximate dimensions: 0.5 m
1 m) to avoid the inclusion of root/rhizosphere respiration. The root-free soil was then left to equilibrate for
a few days until the readings of soil respiration stabilised. Measurements were taken at random by placing
the respiration chamber over a section of soil for approximately 5 min. Respiratory activity was calculated
from the CO2 accumulation rate within the chamber,
as described by the manufacturers (Anon, 1990), and
expressed as mmol CO2 m2 hÿ1.
Fig. 2. Mass loss of litter from litter bags containing litter cut from
swards in ambient and elevated CO2 (60 Pa CO2) rings. Ambient
grown litter was decomposed in ambient soil, elevated CO2 grown
litter in elevated CO2-acclimatised soil. Error bars show the standard
error of the mean, n = 9.
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A. Sowerby et al. / Soil Biology & Biochemistry 32 (2000) 1359±1366
Table 1
Chemical analysis of litter grown in ambient and elevated CO2 FACE rings in Switzerlanda
% Carbon
% Nitrogen
C:N
Soluble fraction
Hemicellulose
Cellulose and ash
Lignin
Lignin:N
a
Ambient grown litter
Elevated CO2 grown litter
Signi®cance level
45.28
3.53
13.2:1
35.38
27.97
32.43
4.22
1.2:1
45.72
3.29
14.7:1
39.94
29.77
26.88
3.41
1:1
n.s.
n.s.
n.s.
Signi®cance is reported on t-tests, p < 0.05,
p < 0.01,
n.s.
n.s.
Not applicable
p < 0.005.
3. Results
3.1. Chemical composition of harvested litter
The percentage carbon and nitrogen content, and
therefore, the C:N ratio of L. perenne litter harvested
in May 1996 from plants grown in elevated and ambient CO2 was not statistically dierent (C:N 13:1 and
15:1 for ambient and elevated CO2-derived litters, respectively, Table 1). The percentage composition of the
hemi-cellulose and lignin content was also not signi®cantly dierent between ambient and elevated CO2
grown litters (Table 1). However, dierences were
observed in the soluble and cellulose/ash fractions of
the litter (Table 1).
3.2. Mass loss of litter in the ®eld
mass loss between the two litter types (Fig. 2), where
signi®cant dierences were mainly observed in the ®rst
5 days and then later on after approximately 35 days.
3.4. Microbial biomass
Microbial biomass was measured during 3 days in
July and August 1998. Comparing ambient and elevated CO2 rings for dierences in microbial biomass
showed elevated CO2-acclimatised soils to have a signi®cantly greater biomass than ambient soils (Paired ttest p < 0.05; elevated CO2 exposed soil
mean=1174.07 2 129.0 mg N gÿ1 soil, ambient CO2
exposed soil mean=878.94 2 76.58 mg N gÿ1 soil).
Readings of microbial biomass did dier between the
dates; ranging from 641.9 to 1151.9 mg N gÿ1 soil in
the control soil samples, and from 959.7 to 1785.9 mg
N gÿ1 soil in the elevated CO2 soils. However, the el-
A comparison of the mass loss of L. perenne litter
from litter bags showed statistically signi®cant dierences (Fig. 2; two way ANOVA, F = 17.53, p < 0.01).
The litter originally grown in the elevated CO2 was
found to have consistently greater rates of decomposition than the litter grown and decomposed in the
ambient soils. This pattern was seen throughout the
decomposition period, showing that the length of time
during which the litter was in the soil did not change
the eect of the elevated CO2 on the decomposition of
the litter (Two way ANOVA, F = 0.167, not signi®cant).
3.3. Chemical composition of decomposing litter
Analysis of decomposing litter showed a decline in
the C:N ratios of the litter until ratios levelled out at
approximately 10:1 (Fig. 3). The decline in the C:N
ratio of the elevated CO2 grown litter was slower than
the ambient litter (taking approximately 20 and 30
days, respectively). The dierences observed in the loss
of carbon and nitrogen from the litter were mainly
seen between day 5 and 25 in the soil. Interestingly,
this does not correspond to the dierences observed in
Fig. 3. C:N ratio of litter remaining in the litter bags decomposing in
ambient and elevated CO2 (60 Pa CO2) acclimatised soils at the
Swiss FACE site. Ambient grown litter was decomposed in ambient
soil, elevated CO2 grown litter in elevated CO2-acclimatised soil.
Error bars show the standard error of the mean, n = 9.
A. Sowerby et al. / Soil Biology & Biochemistry 32 (2000) 1359±1366
evated CO2 soils consistently held greater microbial
biomass.
3.5. Soil respiration
Measurements of soil respiration showed that ambient soil on average produced 20.4 mmol CO2 m2 hÿ1,
elevated CO2-acclimatised soil produced on average
28.7 mmol CO2 m2 hÿ1. This shows signi®cantly greater
levels of soil respiration from elevated CO2-acclimatised soils than from ambient soils (paired t-test; p <
0.005), when compared on individual sample dates.
However, when soil respiration was calculated per
unit of microbial biomass, ambient soils produced on
average 0.02 mmol CO2 hÿ1 unit biomass-Nÿ1, (standard error; 3.9 10ÿ3), with elevated CO2-acclimatised
soils producing on average 0.03 mmol CO2 hÿ1 unit
biomass-Nÿ1 (standard error; 4.9 10ÿ3). These values
showed no statistically signi®cant dierences in CO2
production per unit of microbial biomass between
ambient and elevated CO2 acclimatised soil samples
(paired t-test).
4. Discussion
4.1. Chemical composition of litter
The chemical analysis of the litter used in the mass
loss work showed no signi®cant dierences in carbon
and nitrogen content, although a trend towards greater
C:N ratios were observed in elevated CO2 grown litters. Early predictions, based on green leaf chemistry,
estimated that elevated atmospheric CO2 would reduce
nitrogen in senesced plant material (O'Neill and
Norby, 1996). However, this has not been realised, as
these initial predictions did not consider the resorption
of leaf constituents before senescence (Norby and
Cotrufo, 1998), or dierences in plant chemistry when
growth is restricted by pots, etc. (O'Neill and Norby,
1996). It is generally agreed that the C:N ratio of litter
is not greater when plants are grown in elevated CO2
(Walker and Steen, 1997). The results from our work
con®rm these ®ndings.
4.2. Mass loss of litter in the ®eld
The majority of published results have focused on
the decomposition of litter originally grown in elevated CO2, in ambient soils. Previous research has provided variable results. When litter grown in elevated
CO2 is decomposed in soil not exposed previously to
elevated CO2 either no dierence in mass loss was
observed (Kemp et al., 1994; O'Neill and Norby,
1996), or a decline in the mass loss of litter was seen
1363
(Cotrufo et al., 1994; Cotrufo and Ineson, 1995;
Cotrufo and Ineson, 1996; Cotrufo et al., 1998b).
Although no direct eects of elevated CO2 are
expected on the soil community, indirect eects may
become increasingly important and could potentially
amplify to the extent of altering soil community structure and functions. In contrast to the majority of
papers, our data shows increased rates of mass loss
when litter was grown and decomposed in soils
exposed to elevated CO2. Other work reporting the decomposition of litter originally grown in elevated CO2
in soil exposed to elevated CO2 has not shown many
signi®cant dierences (Hirschel et al., 1997; Gahrooee,
1998). However, looking at the data presented by Gahrooee (1998), for one of the species monitored, Quercus
cerris, a clear elevated CO2 eect was observed with
the soil exposed to elevated levels of CO2. Here soils
exposed to elevated CO2 showed a consistent decline
in the mass loss of litter when compared to soils
exposed to ambient conditions. This eect was
observed for both the ambient grown litter and the elevated CO2 grown litter (Gahrooee, 1998).
4.3. Chemical composition of decomposing litter
Although the litters had similar C:N ratios when
entering the soil, almost as soon as the decomposition process began, dierences arose in the C:N
ratio of the litters until the C:N ratio of the litter
reached approximately 10:1, suggesting that the initial decomposition of the litter did not occur in
the same manner. Whether this was the response of
an altered decomposer community, or some other
factor is not clear. Dierences seen in the mass loss
of the litter continued throughout the decomposition
period. However, after the initial 20±30 days, consistent signi®cant dierences were not seen in the
C:N ratio of the dierent litters as they decomposed. This suggests that the C:N ratio is not a
good indicator of the nitrogen or carbon dynamics
of decaying plant material, which is veri®ed by
Wagener and Schimel (1998), who studied the temporal and spatial decomposition of litter on a Alaskan birch forest ¯oor.
Unlike the C:N ratio, or the total nitrogen content,
signi®cant dierences were observed in the initial soluble fraction and the cellulose and ash content of the
litters (Table 1). This may in part explain the dierences between the mass loss of litter from the litter
bags, as described above, as elevated CO2 grown litter
was found to have a higher soluble fraction and a
lower cellulose and ash content, making it theoretically
more degradable than the litter grown in the ambient
conditions.
1364
A. Sowerby et al. / Soil Biology & Biochemistry 32 (2000) 1359±1366
4.4. Microbial biomass
In our study elevated CO2 exposed soil samples had
consistently greater microbial biomass than ambient
soil samples. Several other published results have
observed increased microbial biomass in elevated CO2
acclimatised soils (Diaz et al., 1993; Zak et al., 1993;
Schenk et al., 1995). Other published results have not
shown an increase in the soil microbial biomass with
elevated CO2 (Kampichler et al., 1998; Runion et al.,
1994). However, the results presented in Kampichler et
al. (1998), are from containers within a controlled environment exposed to elevated CO2, and in Runion et
al. (1994), microbial biomass was estimated using dilution plate counts, a method notoriously biased when
considering the microbial community in soil.
Diaz et al. (1993) hypothesised that the extra carbon
resulting from elevated levels of atmospheric CO2, will
be allocated to the microbial biomass in soil and will
bring about carbon and nutrient accumulation in soil
organic matter. With this hypothesis, increased carbon
inputs will stay within the soil system and not be
returned to the atmosphere immediately. If this was to
occur, increasing levels of CO2 within the atmosphere
may be ameliorated by increased carbon storage in soil
organic matter. From the measurement of microbial
biomass, our ®ndings, in part, support this theory.
4.5. Soil respiration
Overall, the CO2 eux from the bulk soil at the
Swiss FACE site was greater in the rings exposed to elevated CO2. Other published work examining the CO2
eux from soil shows somewhat variable results. CO2
eux was greater in soils planted with Ponderosa pine
(Vose et al., 1997), temperate zone shrubland (Ball et
al., 2000), Scots pine seedlings (Janssens et al., 1998)
and Douglas ®r seedlings (Lin et al., 1999). However,
CO2 eux from a tidal marsh ecosystem showed no
eects of elevated CO2 (Ball and Drake, 1997). Previous measurements of CO2 eux at the Swiss FACE
site showed lower levels of CO2 eux from soils
exposed to elevated CO2 (Ineson et al., 1998). However, readings of gas exchange were limited only to
one control ring and one fumigated ring.
5. Conclusions
Zak et al. (1993) hypothesised that the result of
increased carbon inputs from the atmosphere to the
soil under elevated concentrations of CO2, may
increase rates of turnover and mineralisation of carbon
within the soil system. Theoretically, carbon entering
the soil will be transformed rapidly and returned to
the atmosphere, giving at best a neutral eect on rates
of CO2 accumulation in the atmosphere, and at worse
potentially result in a positive feedback.
Our results do show an increase in overall rates of
soil respiration from soils exposed to elevated CO2.
However, this was the result of an increase in microbial biomass rather than due to an increase in activity of individuals. Our results from the measurement
of mass loss of litter, and the nutritional quality of
that litter as it decomposes, suggest that there may be
a dierence between the decomposer communities in
the soils in the dierent CO2 treatment rings at the
Swiss FACE site. Whether the observed increase in microbial biomass and soil respiration in the FACE rings
is driven by altered or increased litter input is not completely clear as other factors, such as the water content
of the soil, have been shown to eect microbial biomass and soil respiration. However, various environmental parameters have been measured alongside
microbial biomass and soil respiration at the Swiss site
with very few dierences being observed between elevated CO2 exposed soils and ambient soils (data not
shown). This would seem to suggest that any dierences observed in this work were the result of the
plants response to elevated CO2, be it either through a
change in the input of litter to the soil or through
altered rooting strategies or exudates.
Ineson et al. (1998) showed greatly accelerated rates
of N2O production from the soils at the Swiss FACE
site. They speculated that increased available soil-C
had fuelled the increased rates of denitri®cation. The
rate of mass loss of litter from the litter bags in the
soil at the Swiss FACE site was also rapid, when comparing rates with other published work (e.g. Kemp et
al., 1994; Cotrufo and Ineson, 1996; Hirschel et al.,
1997). Clearly a dierence is being observed in soil
exposed to elevated CO2 at the Swiss FACE site. In
corroboration with some of the results published in
Ineson et al. (1998), our results seem to be suggesting
increased rates of C and N cycling within the soils
exposed to elevated CO2 in the Swiss FACE site.
Future work on the eects of elevated CO2 on the
decomposition of litter and the turnover of carbon
within soil needs more precise measurements of the
rate of C and N cycling within the system. Our results
presented here also show there may be dierences in
the process of decomposition in soils exposed to elevated CO2. The use of stable isotopes such as 15 N and
13
C, will become invaluable in this, and have already
provided interesting results to date (Hungate et al.,
1998). Integrated approaches should also be considered, as measurements of microbial biomass alone,
without an indication of the activity of the population
or the eciency of its operation will be meaningless
when attempting to predict how elevated CO2 will
eect the turnover of carbon within the soil community. Several questions still remain. (1) How is elevated
A. Sowerby et al. / Soil Biology & Biochemistry 32 (2000) 1359±1366
atmospheric CO2 bringing about dierences in the soil
community? (2) Is elevated CO2, or any elevated CO2derived eect, changing the diversity of individuals
within the decomposer community, as well as the
population size of the microbial component of the
community? Clearly long-term integrated research will
be needed to answer some of these questions, so as
precise as possible predictions can be made on how elevated CO2 will impact on the turnover of carbon in
the terrestrial system.
Acknowledgements
The award of a NERC studentship to Alwyn
Sowerby is gratefully acknowledged. Technical assistance provided by Tania Cresswell-Maynard in UK
and Werner Wild in Switzerland is also appreciated.
References
Anon, 1990. Operators Manual Version 1.8. PP Systems. Hitchens,
UK.
Ball, A.S., Milne, E., Drake, B.G., 2000. Elevated atmospheric-carbon dioxide increases soil respiration in a midsuccessional lowland forest. Soil Biology and Biochemistry, in press.
Ball, A.S., Drake, B.G., 1997. Short-term decomposition of litter
produced by plants grown in ambient and elevated atmospheric
CO2 concentrations. Global Change Biology 3, 29±35.
Cotrufo, M.F., Ineson, P., Scott, A., 1998a. Elevated CO2 reduces
the nitrogen concentration of plant tissues. Global Change
Biology 4, 43±54.
Cotrufo, M.F., Briones, M.J.I., Ineson, P., 1998b. Elevated CO2
eects ®eld decomposition rate and palatability of tree leaf litter:
importance of changes in substrate quality. Soil Biology and
Biochemistry 30, 1565±1571.
Cotrufo, M.F., Ineson, P., 1996. Elevated CO2 reduces ®eld decomposition rates of Betula pendula (Roth.) leaf litter. Oecologia
106, 525±530.
Cotrufo, M.F., Ineson, P., 1995. Eects of enhanced atmospheric
CO2 and nutrient supply on the quality and subsequent decomposition of ®ne roots of Betula pendula Roth. and Picea sitchensis
(Bong.) Carr. Plant and Soil 170, 267±277.
Cotrufo, M.F., Ineson, P., Rowland, A.P., 1994. Decomposition of
tree leaf litters grown under elevated CO2: eect of litter quality.
Plant and Soil 163, 121±130.
Diaz, S., Grime, J.P., Harris, J., McPherson, E., 1993. Evidence of a
feedback mechanism limiting plant response to elevated carbon
dioxide. Nature 364, 616±617.
Gahrooee, F.R., 1998. Impacts of elevated atmospheric CO2 on litter
quality, litter decomposability and nitrogen turnover rate of two
oak species in a Mediterranean forest ecosystem. Global Change
Biology 4, 667±677.
Garnier, E., 1991. Resource capture, biomass allocation and growth
in herbaceous plants. Trends in Ecology and Evolution 6, 126±
131.
Harper, S.H.T., Lynch, J.M., 1981. The chemical components and
decomposition of Wheat straw leaves, internodes and nodes.
Journal of Science, Food and Agriculture 32, 1057±1062.
Hirschel, G., Korner, Ch., Arnone III, J.A., 1997. Will rising atmos-
1365
pheric CO2 aect leaf litter quality and in situ decomposition
rates in native plant communities? Oecologia 110, 387±392.
Hungate, B.A., Holland, E.A., Jackson, R.B., Chapin III, F.S.,
Mooney, H.A., Field, C.B., 1998. The fate of carbon in grasslands under carbon dioxide enrichment. Nature 388, 576±579.
Ineson, P., Coward, P.A., Hartwig, U.A., 1998. Soil gas ¯uxes of
N2O, CH4 and CO2 beneath Lolium perenne under elevated CO2:
the Swiss free air carbon dioxide enrichment experiment. Plant
and Soil 198, 89±95.
Janssens, F., Peeters, A., Tallowin, J.R.B., Bakker, J.P., Bekker,
R.M., Fillat, F., Oomes, M.J.M., 1998. Relationship between soil
chemical factors and grassland diversity. Plant and Soil 202, 69±
78.
Kampichler, C., Kandeler, E., Bardgett, R.D., Jones, T.H.,
Thompson, L.J., 1998. Impact of elevated atmospheric CO2 concentration on soil microbial biomass and activity in a complex,
weedy ®eld model. Global Change Biology 4, 335±346.
Kemp, P.R., Waldecker, D.G., Owensby, C.E., Reynolds, J.F.,
Virginia, R.A., 1994. Eects of elevated CO2 and nitrogen fertilisation pretreatments on decomposition on tall grass prairie leaf
litter. Plant and Soil 165, 115±127.
Lin, G., Ehleringer, J.R., Rygiewicz, P.T., Johnson, M., Tingey,
D.T., 1999. Elevated CO2 and temperature impacts on dierent
components of soil CO2 eux in Douglas-®r terracosms. Global
Change Biology 5, 157±168.
Nitschelm, J.J., 1996. Using stable isotopes to trace soil C input in a
pasture subject to free air CO2 enrichment. Masters Thesis,
University of Saskatchewan, Canada.
Norby, R.J., Cotrufo, M.F., 1998. A question of litter quality.
Nature 396, 17±18.
Norby, R.J., 1994. Issues and perspectives for investigating root responses to elevated atmospheric carbon dioxide. Plant and Soil
165, 9±20.
O'Neill, E.G., 1994. Responses of soil biota to elevated atmospheric
carbon-dioxide. Plant and Soil 165, 55±65.
O'Neill, E.G., Norby, R.J., 1996. Litter quality and decomposition
rates of foliar litter produced under CO2 enrichment. In: Koch,
G.W., Mooney, H.A. (Eds.), Carbon Dioxide and Terrestrial
Ecosystems. Academic Press, London, UK, pp. 87±103.
Rogers, H.H., Runion, G.B., Krupa, S.V., 1994. Plant-responses to
atmospheric CO2 enrichment with emphasis on roots and the rhizosphere. Environmental Pollution 83, 155±189.
Penuelas, J., Estiarte, M., 1998. Can elevated CO2 aect secondary
metabolism and ecosystem function? Trends in Ecology and
Evolution 13, 20±23.
Rowell, D.L., 1995. Soil Science: Methods and Applications.
Longman, UK.
Runion, G.B., Curl, E.A., Rogers, H.H., Backman, P.A.,
Rodriguezcabana, R., Helms, B.E., 1994. Eects of free-air CO2
enrichment on microbial-populations in the rhizosphere and phyllosphere of cotton. Agricultural and Forest Meteorology 70, 117±
130.
Schenk, U., Manderscheid, R., Hugen, J., Weigel, H.-J., 1995.
Eects of CO2 enrichment and intraspeci®c competition on biomass partitioning, nitrogen content and microbial biomass carbon
in soil of perennial ryegrass an white clover. Journal of
Experimental Botany 46, 987±993.
Stulen, I., Den Hertog, J., 1993. Root-growth and functioning under
atmospheric CO2 enrichment. Vegetatio 104, 99±115.
Vose, J.M., Elliot, K.J., Johnson, D.W., Tingey, D.T., Johnson,
M.G., 1997. Soil respiration response to three years of elevated
CO2 and N fertilisation in Ponderosa pine (Pinus ponderosa
Doug, ex Laws.). Plant and Soil 190, 19±28.
Wagener, S.M., Schimel, J.P., 1998. Strati®cation of soil ecological
processes: a study of the birch forest ¯oor in the Alaskan taiga.
Oikos 81, 63±74.
Walker, B., Steen, W., 1997. The terrestrial biosphere and global
1366
A. Sowerby et al. / Soil Biology & Biochemistry 32 (2000) 1359±1366
change: implications for natural and managed ecosystems. A synthesis of GCTE and related research. IGBP Science No.1. ISSN
0284-8015.
Zak, D.R., Pregitzer, K.S., Curtis, P.S., Teeri, J.A., Fogel, R.,
Randlett, D.L., 1993. Elevated atmospheric CO2 and feedback
between carbon and nitrogen cycles. Plant and Soil 151, 105±117.
Zanetti, S., Hartwig, U.A., Luscher, A., Hebeisen, T., Frehner, M.,
Fischer, B.U., Hendrey, G.R., Blum, H., Nosberger, J., 1996.
Stimulation of symbiotic N2 ®xation in Trifolium repens L. under
elevated atmospheric pCO2 in a grassland ecosystem. Plant
Physiology 112, 575±583.