ORIGINAL ARTICLE

Annual variation of temperature sensitivity of soil organic carbondecomposition in North peatlands: implications for thermalresponses of carbon cycling to global warming

Wu Xiang Æ Chris Freeman

Received: 14 April 2008 / Accepted: 18 August 2008 / Published online: 3 September 2008

� Springer-Verlag 2008

Abstract Temperature sensitivities of microbial respira-

tion and dissolved organic carbon (DOC) production were

investigated by using a novel method, thermal gradient

(2–20�C) temperature bar, in two typical peatlands (bog

and fen) in North Wales, UK over 12 months. The study

indicated that temperature sensitivity of soil organic carbon

decomposition in North peatlands was regulated not only

by temperature but soil water content, dry–rewet event and

phenologies. Potential decreases of Q10 (CO2) with

increasing soil temperature were confirmed in both peat-

lands, but Q10 (DOC) increase with increasing soil

temperature in both bog and fen sites. These results imply,

if other factors such as the so-called CO2 fertilization effect

are simultaneously taken into account, that the feedback of

global warming induced CO2 release from peatlands to

climate change may be overestimated in current biogeo-

chemical models. However, global warming might have

been nonlinearly accelerating DOC thermal production,

and therefore it helps explaining the causes of remarkable

increase of DOC in surface water in the Northern Hemi-

sphere during last several decades.

Keywords Temperature sensitivity � Soil organic

carbon � Decomposition � Peatlands

Introduction

There is a general expectation that increases in temperature

can accelerate the decomposition of soil organic carbon,

and, consequently, global warming should increase the

release of soil organic carbon to the atmosphere (Davidson

et al. 2000) and prompt the release of dissolved organic

carbon (DOC) from land to aquatic ecosystems (Freeman

et al. 2001). Over the past several decades, extensive

research has been carried out on the thermal response of

soil systems to global warming (Kirschbaum 1995; Mars-

chner and Bredow 2002; Knorr et al. 2005). However, the

understanding of the mechanism and factors regulating

temperature sensitivity of carbon processes in soil is still

very limited and constrains our confidence in projected

changes in carbon cycling. The interpretations of obser-

vation of laboratory and experiments are highly dependent

on mathematic techniques and inadequate model assump-

tions. Despite much research, a consensus has not yet

emerged on the temperature sensitivity of soil carbon

decomposition (Davidson and Janssens 2006).

Peatlands occupy approximately 15% of boreal and

sub-arctic region, and contain approximately one-third of

the world’s soil carbon pool. The high latitudes occupied

by peatlands are expected to see the greatest amount of

climatic warming in the next several decades (Pastor et al.

2003). Whether the huge amounts of C in peatlands will

response positively or negatively to global warming

depends on not only the temperature sensitivity of net

primary productivity but also the temperature sensitivity

of soil organic carbon decomposition rate. There is

increasing evidence to suggest that the temperature sen-

sitivity of decomposition of soil organic matter can be

significantly influenced by temperature and multiple other

confounding factors such as soil types, substrate quality,

W. Xiang (&) � C. Freeman

School of Biological Sciences, University of Wales,

Bangor LL57 2UW, UK

e-mail: bgegxw@yahoo.cn

Present Address:W. Xiang

Key laboratory of Biogeology and Environmental Geology

of Education Ministry, China University of Geosciences,

Wuhan 430074, People’s Republic of China

123

Environ Geol (2009) 58:499–508

DOI 10.1007/s00254-008-1523-6

hydrological condition and plant activities (Davidson et al.

1998; Yuste et al. 2004). While there is a paucity of

information on seasonal and spatial variation of tempera-

ture sensitivity of soil respiration in some terrestrial

ecosystems (Janssens and Pilegaard 2003; Saiz et al. 2006;

Vincent et al. 2006), few investigations on annual varia-

tion of temperature sensitivity of soil organic carbon

decomposition in northern peatlands were performed.

Particularly, few data are available on the temperature

sensitivity of DOC production in northern peatlands,

which is very important for valuable insight into the cause

of obvious DOC increase in surface water across much of

Europe and north USA (Evans et al. 2006). Microbial

respiration and DOC production are the major factors in

relation to soil organic carbon decomposition. Therefore,

further understanding of thermal response of microbial

respiration and DOC production in northern peatlands is of

potential significance for global carbon cycling and cli-

mate warming.

Field investigations have generally involved relating soil

CO2 fluxes or DOC concentration to diurnal or seasonal

variations in temperature. However, while this approach

has the advantage of investigating thermal response in

undisturbed soils, interpretations of microbial responses to

temperature are usually confounded by some co-varying

factors, such as root respiration. Though laboratory incu-

bations are commonly criticized for being unnatural, it has

been regarded as a successful complementary tool to field

survey and widely used in studies of temperature sensi-

tivity of soil carbon decomposition. However, few

laboratory incubations were performed under varying

temperature conditions. In particular, thermal response

estimation by means of Q10 function based on only two

temperatures, e.g., field temperature and simulated refer-

ence soil temperature in some research, should be

interpreted with caution, because incubation results based

on limited temperature points might be masked by sea-

sonally shifting thermal optimum in soil carbon-cycling

processes. A novel laboratory incubation method based on

a thermal gradient (2–20�C) temperature bar has been

successfully used in recent works related to thermal

response of North peatlands (Freeman et al. 2001). This

novel technique provides us a more reliable and convenient

tool to investigate annual variation of temperature sensi-

tivity of microbial respiration and DOC production in

North peatlands.

The major objectives of this study were: (1) to inves-

tigate the annual and spatial variations of temperature

sensitivity of soil organic carbon decomposition in North

peatlands, and (2) to identify the possible factors regu-

lating the thermal response of North peatlands to climate

change.

Materials and methods

Sample collection, preparation and incubation

Samples were collected monthly from an ombrotrophic bog

and a minerotrophic fen from February 2006 to January

2007. The bog peat was collected from Marchlyn Mawr

located in North Wales dominated by Sphagnum species

and other familiar bog plants such as hare’s-tail cotton-

grass. The fen peat was collected from Cors Erddreiniog

located in Anglesey dominated by Sphagnum, Juncus and

great fen-sedge (Fig. 1). Both of the peatlands are typical

in North Wales. The surface layer of vegetation was

removed and peat was collected to a depth of approxi-

mately 10–15 cm. For each field site, water table and

temperature of air and soil were measured, while three

subsamples were collected and mixed. The mixed peat

samples were homogenized by hand after removing most

of larger visible roots. A 2-cm thick layer of each peat was

placed in temperature gradient bars within two layers of

cling-film to prevent water loss without inhibiting gas

exchange. Samples were incubated in a temperature gra-

dient bar for 2 weeks, along which the soil temperature was

respectively controlled as 2, 4, 6, 8, 10,…,18 and 20�C.

The details of the method by using gradient bar can be

found in Fenner et al. (2005).

Gas and hydrochemical analysis

Fluxes of carbon dioxide were determined following 2

weeks of incubation by removing the cling-film from the

Fig. 1 Location of studied area

500 Environ Geol (2009) 58:499–508

123

sample surface, placing nine or ten 90 mm 9 13 mm Petri

dish bases upside down on the peat at equal distances along

the temperature gradient bar. The dishes were pressed

approximately 2 mm into the soil. The increase in trace gas

concentration above the initial background concentration

after 45 min was used to estimate gaseous fluxes from the

soil. Four replicate gas samples from each position were

analyzed by gas chromatography.

Dissolved organic carbon was tested in peat leachable

water obtained from each sample. Briefly, a peat cube

(1 cm3) was gently homogenized for 60 s with 9 ml

deionized water in a stomacher (Seward Colworth model

400). Following centrifugation (14,2809g, 25 min),

leachable water was obtained after through 0.45 lm filter.

The water samples were stored at 4�C before analysis.

DOC was measured by using Total Organic Carbon ana-

lyzer (Shimadzu 5000, Japan).

The apparent molecular weight distribution of DOC in

leachable water was determined by using high-pressure

size exclusion chromatography method of Zhou et al.

(2000).

Calculation of temperature sensitivity of microbial

respiration and DOC production

Temperature sensitivity of microbial respiration or DOC

production was estimated by means of Q10 function:

Q10 ¼ ðR2=R1Þ½10=ðT2�T1Þ� ð1Þ

where R2 and R1 are the rates of CO2 emission or DOC

concentration at two temperatures, T2 and T1. In this study,

T2 = 20�C and T1 = 2�C. R2 and R1 were estimated by

calculating linear regression slopes of CO2 flux or DOC

concentration against temperature.

Results and discussion

Annual variation of temperature sensitivity of microbial

respiration

Figure 2a showed a significant positive correlation between

Q10 (CO2), the temperature sensitivity of microbial respi-

ration and soil temperature from February to May in the

bog site. However, a remarkable depression of Q10 (CO2)

was found during warm summer (June to July) and caused

a negative correlation between Q10 (CO2) and soil tem-

perature from May to August. Q10 (CO2) increased during

August, but did not reach the values close to values of

spring. Then, Q10 (CO2) gradually fell with decrease in soil

temperature. In particular, the lowest value of Q10 (CO2)

was observed in November. However, a negative correla-

tion between Q10 (CO2) and soil temperature was found

from November to January. All the values of Q10 (CO2) in

the bog site are [1.0 and the highest value (3.0) appeared

in May.

For the fen site, on the contrary, a significant negative

correlation between Q10 (CO2) and soil temperature was

found from February to May (Fig. 2b). Especially, the

highest value of Q10 (CO2) during the year appeared in

coldest winter (March). During the summer, a groove of

Q10 (CO2) in July was also found. But, unlike the bog site,

no significant increase of Q10 (CO2) in June. After August,

Q10 (CO2) decreases with decreasing soil temperature.

However, from October to November, unexpected lower

Q10 (CO2) appeared. Then, apparent increases of Q10 (CO2)

in December of 2006 and January of 2007.

Compared with the fen site, the bog site has obvious

higher average values of Q10 (CO2). This difference

may be partly due to different substrate qualities. The

Arrhenius function [k = aexp(-Ea/RT)] reveals that the

temperature sensitivity of decomposition increases with

increasing molecular complexity of the substrate, which is

characterized by low decomposition rates and high acti-

vation energies (Davidson and Janssens 2006). Rain-fed

bog is acidic, nutrient-poor peatland and fen is relatively

neutral, nutrient-rich peatland. In general, the high-quality

substrate of soil organic-C in nutrient-rich fen is expected

to be more abundant than in bog. Analytical results of the

Fig. 2 Annual variation of temperature sensitivity of soil CO2 efflux.

a Bog, b fen (note: no April CO2 data)

Environ Geol (2009) 58:499–508 501

123

apparent molecular weight distribution of DOC indi-

cated higher proportion of larger apparent molecular

(100,000*10,000 Da) in bog DOC compared with the

fen data, which implied less readily decomposable mate-

rial in bog site for microbial respiration (Fig. 3). The

more complex molecular attributes of soil organic-C in

bog might be the major reason for the relatively higher

temperature sensitivity of microbial respiration in bog

site, though other confounding factors such as vegetation

types and decomposer community might play an impor-

tant role in modifying the temperature response of

microbial respiration.

The relationships between Q10 (CO2) and soil temper-

ature for both peatland sites are shown in Fig. 4. Overall,

potentially decreasing trends of Q10 (CO2) were observed

at higher temperature. This is in agreement with the

Arrhenius theory which predicts that the Q10 of chemical

reactions decreases with increasing temperature. The sim-

ilar seasonal changes in Q10 have also been found in other

terrestrial ecosystems (Kirschbaum 1995; Janssens and

Pilegaard 2003; Luo et al. 2001). However, the Q10 (CO2)

in this study is actually an apparent temperature sensitivity

which is affected not only by the inherent kinetic properties

based on molecular structure but also by the environmental

constraints such as hydrologic condition. A few studies

have recently shown that seasonal values of Q10 are neg-

atively correlated with temperature, but positively related

to a limited range of soil water content (Xu and Qi 2001;

Janssens and Pilegaard 2003). Research by Li et al. (2000)

indicates that influence of temperature on soil respiration is

much more significant than that of soil water contend when

temperature is \15�C, but the confounded effect of tem-

perature and soil water content was enhanced when

temperature is[15�C. In fact, soil water content is, in most

cases, negatively correlated with soil temperature, which

precludes distinguishing between the effects of the two

confounded factors on CO2 flux, particularly in summer

(Table 1). Therefore, potentially decreasing trends of

Q10 (CO2) at higher temperature observed in this study are

no longer reflection of temperature sensitivity only, but

combined responses to changes in temperature, moisture

conditions and other variables.

Despite the relatively lower summertime Q10, the

absolute responses of soil respiration, the release amount of

CO2, to temperature in many terrestrial systems, including

peatlands, have been found to be higher in summer than in

Fig. 3 Comparison of the apparent molecular weight distribution of

DOC between bog and fen

Fig. 4 The relationship between Q10 (CO2) and soil temperature. aBog, b fen (Data fitting excluded October and November data)

Table 1 Annual variation of water table elevation (cm) in bog and

fen

Month Water table elevation (cm)

Bog Fen

February (2006) -6 -2

March -1.5 0

April -3 -1

May -5.2 -2

June -8.3 -9.5

July -9 -13.8

August -2 -10

September 0 -8

October 1.5 1

November 1.5 3

December -2.5 0

January (2007) -4.5 -2

502 Environ Geol (2009) 58:499–508

123

winter (Chapman and Thurlow 1996), which implies that

importance of temperature decreased and soil moisture

assumed increased significance in controlling microbial

respiration. Nevertheless, it has also been found that over

drought-stress may cause rapid declines in soil respiration

rates (Davidson et al. 1998; Bowden et al. 1998). Drought

may reduces the thickness of soil water films, thus inhib-

iting diffusion of extracellular enzymes and soluble

organic-C substrates and lowering substrate availability at

reaction microsites (Davidson and Janssens 2006). There-

fore, drought mechanism might be useful to explain the

lower values of Q10 (CO2) in summer.

However, drought mechanism is unlikely to account for

the significantly lower Q10 (CO2) in rainy October and

November occurred in the bog and the fen, both of which

are significantly deviated from the optimal data fitting and

much lower than expected (square points in Fig. 4). The

reason for so lower Q10 values (or insensitivity) is pos-

sibly related to sharp transformation from oxidizing to

reducing conditions caused by rising water table. During

early autumn, rain caused soil water content to increase

but it is still under moderate moisture condition. After

September, in addition to decreased soil temperature,

precipitation remarkably increased and led to water table

rapidly covered the soil surface (Table 1). Higher water

table slows oxygen diffusion and sharply changed soil

environment from relatively aerobic to anaerobic condi-

tion. Bowden et al. (1998) found reduced soil respiration

rates under very dry and very wet condition. Harper et al.

(2005) used Mielnick–Dugas model to reveal the com-

bined responses of soil CO2 flux to soil temperature and

moisture, and indicated that the highest soil CO2 fluxes

occurred at high soil temperatures and intermediate soil

water contents. Furthermore, their results also confirmed

that both over drying and very high soil water content

strongly suppressed the temperature response of soil CO2

flux, while cool soil temperatures suppressed the soil

moisture response. In addition, extreme wet condition

might have changed soil microbial communities, and

consequently affected soil carbon decomposition pro-

cesses. Research by Waldrop and Firestone (2006)

supported the hypothesis that microbial community com-

position and function is altered when microbes are

exposed to new extremes in environmental conditions,

which caused not only generally slower degradative

enzymatic activities but also shit of soil microbial com-

munities. Unfortunately, our knowledge about the effect

of environmental constraints on microbial community

change is still poorly understood. Even such, it’s reason-

able to attribute the insensitive response of Q10 for

microbial respiration to changes in environmental tem-

perature in October and November mainly to the over wet

condition at lower temperature.

In addition, bog site showed a significant positive cor-

relation between Q10 (CO2) and soil temperature within

approximately 2*10�C which unlike monotonic decreas-

ing changes in Q10 (CO2) occurred in the fen (Fig. 4). The

similar results, increasing tendency in Q10 with increasing

temperature, have been reported in other soil systems

(Sjogersten and Wookey 2002). However, comparison

between the reported data should be very careful because

some of the tendency results were analyzed based on lab-

oratory incubation temperature rather than field

temperature. For example, Sjogersten and Wookey (2002)

calculated Q10 based on tundra soil incubation experiments

at three different temperature pairs. They found Q10 (CO2)

obtained from the 7–12�C range were significantly

(P \ 0.001) higher than the 2–7 and 12–17�C temperature

ranges. As Q10 calculation based on only two temperature

points could be easily masked by seasonally shifting ther-

mal optimum in soil carbon-cycling processes (Fenner

et al. 2005), their Q10 changes might be the reflection of

soil process (e.g., CO2 efflux) thermal optimum rather than

the response to seasonal temperature change.

The significant positive correlation between Q10 (CO2)

and soil temperature in bog site before May might result

from phenologies. It has been found that higher seasonal

Q10 may be affected by seasonal variation in carbon allo-

cation. Davidson et al. (2006) reported springtime Q10 of

soil respiration was always higher than springtime Q10 of

total ecosystem respiration, but the reverse was observed in

the autumn at the Howland forest of Maine, USA. They

suggested that these seasonal differences in apparent tem-

perature sensitivities of soil respiration and total ecosystem

respiration could partly result from different phenologies of

aboveground and belowground process, and different

temperature sensitivities of these processes. Boone et al.

(1998) reported Q10’s of 2.5 and 3.5, respectively, in

trenched plots without roots and control plots with roots in

temperate forests. Curiel et al. (2004) compared the sea-

sonal changes of Q10 for soil respiration between a

deciduous hardwood forest compared with an adjacent

evergreen conifer forest. They argued that the seasonal Q10

reflects the greater seasonality of photosynthesis and sub-

sequent supply of substrate belowground in the deciduous

hardwood site compared with the evergreen conifer site,

and the higher seasonal Q10 observed in the hardwood

forest can be explained partly by phenological responses to

seasons. In this study, for bog stand, the highest Q10 (CO2)

appeared in spring which is plant growing period. Though

most of visible larger roots, including living and dead

roots, were removed before incubation in this study, root-

derived factors might still influence the springtime appar-

ent Q10 (CO2) through following several pathways. First,

it’s very difficult or even impossible to remove all the roots

from peat materials during preparation for incubation.

Environ Geol (2009) 58:499–508 503

123

Many fine roots, including fresh root hairs growing during

spring periods, still make more or less contribution to the

apparent Q10 (CO2) though CO2 originating from root

respiration might be very small after removing most of the

roots. Second, even if all roots had been removed before

incubation, significant increase in root exudates exist in

peat samples might exert a strong influences on Q10 (CO2).

Strictly speaking, CO2 efflux measured in this study mainly

including not only the CO2 derived by microbial decom-

position of soil organic matter, but also CO2 evolved by

microbial utilization of exudates or other rhizodeposits.

Kuzyakov (2002) argued that root respiration contributes

only about 40–50% to the root-derived CO2 efflux, the

remaining 50–60% comprise the microbial decomposition

of root exudates and other rhizodeposits. Cheng et al.

(1993) found, in an experiment with 3-week-old wheat

plants, that root respiration and rhizo-microbial respiration

contributed, on average, 40.6 and 59.4% of total rhizo-

sphere respiration, respectively. In the spring and early

summer, the exudation intensity of growing plants has been

found to be very high, and it leads to increased microbial

growth and activity in the rhizosphere (Kuzyakov and

Cheng 2001). From winter to spring, root exudates increase

with growing root biomass and enhanced photosynthesis in

peatlands. Therefore microbial decomposition of root ex-

udates might exert a strong influence on spring apparent

Q10 (CO2) for nutrient-poor bog stand. In addition, stimu-

lation of microbial growth and activity around roots could

increase the mineralization of native soil organic matter or

corresponding rhizosphere priming effect (Kuzyakov

2002), which might consequently affect springtime Q10

(CO2) for the bog site in this study. Interestingly, no sig-

nificant ‘‘spring-stimulation’’ effect on Q10 (CO2) was

found for fen site. These might be partly explained by

different substrate availability. For nutrient-poor bog,

increasing supply of energy-rich root exudates could boost

up not only the labile substrate availability but also

microbial growth, which in turn influenced the temperature

sensitivity of microbial respiration. In contrast, nutrient-

rich fen may supply relatively abundant higher quality

substrates for microbial utilization, which leads to less

importance of root exudates effect, compared with other

environmental constraints, on apparent temperature sensi-

tivity of microbial respiration.

Annual variation of temperature sensitivity

of soil DOC production

Annual variation of temperature sensitivity of soil DOC

production is shown in Fig. 5. It can be seen from Fig. 5a

that most of Q10 (DOC) values in bog site are[1 and peak

values appeared in August and September. For fen

site, similarly, the highest value of Q10 (DOC)

appeared in August rather than the warmest July (Fig. 5b).

Figures 2 and 5 showed the synchronous appearance of the

peaks of Q10 (CO2) and Q10 (DOC) in August after sum-

mer. These phenomena might be partly attributed to dry–

rewet event caused by rain after drought summer (Table 1).

Many studies showed that there was a burst of respiration

when air-dry soil was re-wetted (Birch 1958). Borken et al.

(2006) have found that prolonged summer droughts

decrease primarily heterotrophic respiration in the O hori-

zon, which could cause increases in the storage of soil

organic carbon in soil. In addition, lower water flow in

drought summer may efficiently decrease the export of

DOC from peat to aqueous systems and consequently

stored more soil organic carbon in peat by adsorbing DOC.

Precipitation in August led to moderate increase in soil

moisture, which is expected to release at least part of the

soil organic carbon stored during drought summer and

increased substrate availability to microbes. Studies have

shown water stable aggregates to decline by 20–60% in

soils exposed to wet–dry cycles when compared to soils

maintained moist (Soulides and Allison 1961; Degens and

Sparling 1995). Disruption of soil aggregates could expose

insoluble soil organic matter which would otherwise be

protected from microbial attack. In addition to release of

liable soil carbon caused by rewetting, moderate soil

moisture also improved the drought-stress on soil micro-

organism and thereby increasing their activity. Combining

Fig. 5 Annual variation of temperature sensitivity of soil DOC

production. a Bog, b fen

504 Environ Geol (2009) 58:499–508

123

both of the processes increased the temperature sensitivi-

ties of both of Q10 (CO2) and Q10 (DOC) in whatever

rewetting bog or fen site.

Though the highest values of Q10 (DOC) appeared in

August for both peat sites, the corresponding values of Q10

(CO2) are not the highest and even lower than expected

values on the basis of soil temperature alone. One of the

explanations is that some of the DOC, such as aromatic

carbon, released from peat during rewetting periods is

relatively stable for microbe utilization. Lundquist et al.

(1999) found that DOC content is not a reliable indicator of

C availability to microorganisms. Their finding was sup-

ported by Zsolnay and Steindl (1991) who found different

carbon mineralization rates in samples with comparable

DOC concentrations, which suggests that DOC is not the

only C substrate for mineralization. Another possible rea-

son is related to the converting reaction from SOC to DOC

by microbes utilization. The release of sequestered carbon

from soil aggregates in dry–rewet events including both

DOC adsorbed by peat and some SOC protected from

microbes attack during dry periods. The previously pro-

tected SOC is generally microbially labile organic carbon

sources, and some of them are converted to DOC while

some to CO2. However, the reaction converting SOC to

DOC is independent of CO2 production and has an inde-

pendent reaction rate constant (Chow et al. 2006).

Temperature effect on DOC production and CO2 release in

rewetting peat would be quite different from that before

drying. Unfortunately, the complex relationship between

carbon availability and DOC production, and the capacity

of the microbial community to utilize the previously pro-

tected SOC, remain poorly understood.

The relationship between Q10 (DOC) and soil tempera-

ture was shown in Fig. 6. It indicates potentially positive

correlations between Q10 (DOC) and soil temperature for

both peat sites, which implies that warming may accelerate

the production of DOC in peat soil. Comparing with bog

site, the effect of warming on temperature sensitivity of

dissolved organic compounds production in fen soil seems

more remarkable with linear fitting slope value of 0.0166,

approximately 1.25 times of the value for bog site. How-

ever, it should be accentuated that the effect of higher

temperature is frequently accompanied by drying pro-

cesses, which is one of the main factors controlling soil

microbial respiration.

If we assume that the respiration rate is an indicator of

carbon availability to microorganisms, the ratio of respi-

ration (CO2) to DOC could, to some extend, be used to

indicate changes in available soil carbon relative to DOC.

We calculated the mole ratio of CO2 to DOC and it’s

temperature sensitivity—Q10 (CO2/DOC). The relationship

between Q10 (CO2/DOC) and soil temperature was shown

in Fig. 7. The Q10 (CO2/DOC) of bog site, with most

values approximately ranged from 1.5 to 2.5, are obviously

higher than the fen data, most of which are \1.6. This

suggests that increasing temperature significantly raised

microbial utilization ratio of DOC for bog, in which DOC

is the most important carbon source for microbial com-

munity. However, the importance of DOC as a carbon

source in fen might decrease when compared with bog. In

despite of these differences, for both bog and fen, Q10

(CO2/DOC) potentially decreased with increasing soil

temperature, which implies relatively more CO2 trans-

formed from DOC in winter compared with in summer.

Warmer climate might relatively be more propitious to

thermal production of DOC rather than CO2 release from

peatlands.

Implications for the thermal response of Northern

peatland to global warming

With the continuing increase in Greenhouse gases in the

atmosphere, global temperature increases are expected to

continue and become even more pronounced. The high

latitudes occupied by peatlands are expected to see the

greatest amount of climatic warming in the next several

decades (Pastor et al. 2003). If carbon stored in North

peatlands is transferred to the atmosphere by a warming-

induced acceleration of its decomposition, a positive

Fig. 6 The relationship between Q10 (DOC) and soil temperature. aBog, b fen

Environ Geol (2009) 58:499–508 505

123

feedback to climate change would occur. However, the real

issue about 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

(Davidson et al. 2000). Although soil temperature may

often account for a large fraction of seasonal variation in

soil CO2 fluxes, other factors such as soil water content

may influence soil respiration. Our observational data

suggested that summertime Q10 (CO2) was obviously

depressed by warming-induced lower soil water content,

which implies increasing importance of hydrologic condi-

tion controlled by precipitation and evapotranspiration.

Both precipitation and evapotranspiration are likely to

change in many regions of the world owing to global

warming, tipping the hydrologic balance toward summer-

time drying of many mid-continental peatlands and

wetlands (Davidson and Janssens 2006). Though aerobic

condition may expose large stocks of carbon substrates to

aerobic decomposition, soil CO2 fluxes are often affected

by decreasing soil moisture. The results in this study that

potentially decreasing tendency in Q10 (CO2) with

increasing temperature in both peat sites may be at least

partly attributed to lower soil water contend. In addition to

the environmental factors, the constraints to decomposition

still include soil carbon substrates of varying chemical

complexity, which are usually called as ‘‘labile pool’’ and

‘‘recalcitrant pool’’, each with its own inherent kinetic

properties. Many studies have found a similar experimental

result that an initial increase in soil CO2 efflux appeared in

response to experimental warming and this CO2 efflux

pulse often disappears within a few years (Melillo et al.

2002; Eliasson et al. 2005). One interpretation has been

proposed that only decomposition of the most labile soil

carbon pool was sensitive to the warming, and the

decomposition of more recalcitrant SOM was temperature

insensitive. Another interpretation is so-called ‘‘acclima-

tion’’ mechanism which proposed that soil microbial

communities may acclimate to warming and return to

previously respiration levels within a relatively short time.

Though the explanations regarding the cause are still

controversial, the experimental results implied that accel-

eration of CO2 release caused by warming seems to be less

than we expected. Moreover, Davidson et al. (2000) con-

sider that the key to climatic sensitivity of soil carbon is not

total ecosystem respiration but decomposition rates. If

other factors such as the so-called CO2 fertilization effect

are simultaneously taken into account, we might, at least,

get a conclusion that the feedback of global warming

induced CO2 release from peatlands to climate change may

be overestimated in current biogeochemical models. The

net effect of soil carbon decomposition caused by global

warming on atmospheric CO2 loading over the next dec-

ades to centuries is, therefore, likely to be small.

However, DOC, another important product of soil car-

bon decomposition, might be a more interesting but

controversial issue in coming years. Peatlands has been

considered to supply most of the DOC entering boreal

lakes and rivers and the Arctic Ocean (Pastor et al. 2003),

which is of great significance not only to global carbon

cycling but to safety of aquatic ecological system and our

drinking water. Since 1988, there has been, on average, a

91% increase in DOC concentrations of UK lakes

and streams in the Acid Waters Monitoring Network

(AWMN). Similar DOC increases have been observed in

surface waters across much of Europe and North America

(Evans et al. 2006). Several explanations have been pro-

posed. First, increase in DOC might reflect warming-

induced increased SOM decomposition rates (Freeman

et al. 2001); Second, there is the possible stimulation of

plant primary production by elevated atmospheric carbon

dioxide (Freeman et al. 2004); Both of the two explana-

tions about the causes of rising DOC focused on the

carbon cycle related to climate change. However, Evans

et al. (2006) proposed a new explanation that the increase

in DOC concentration has been caused by the decrease in

sulfur deposition that has occurred in northern Europe and

North America over the past 20 years, as clean-air legis-

lation has taken effect. Roulet and Moore (2006) made a

excellent review about above hypothesis and they

Fig. 7 The relationship between temperature sensitivity of mole ratio

of CO2 to DOC [Q10 (CO2/DOC)] and soil temperature. a Bog, b fen

506 Environ Geol (2009) 58:499–508

123

concluded that no proposed explanation is particularly

convincing. Our annual observational data in this study

indicated that temperature sensitivity of DOC production

[Q10 (DOC)] in both types of peatlands increased with

increasing temperature. This finding may help, at least in

part, in explaining the cause of obviously rising DOC,

because warming-induced soil DOC production rate in

peatlands is not evenly increasing but with nonlinear

acceleration. What is certain, unfortunately, is that our

finding still can not perfectly explain the cause of rising

DOC, further researches are still wanting. However, our

observational data underscore the importance of driving

mechanism related to climate change. In fact, more and

more evidences are converging to demonstrate that factors

related to climate change appear likely to be the most

important driving force to rising DOC.

Summary

This study confirmed that the annual variation of tem-

perature sensitivity of soil carbon decomposition in north

peatlands was influenced not only by the type of peat but

also by temperature and hydraulic condition, which are the

main factors controlling the thermal response of soil car-

bon decomposition. For bog, none linear thermal response

of Q10 (CO2) was found, while apparent monotone

decrease of Q10 (CO2) with increasing soil temperature

was observed in fen. Drought or extreme wet may

remarkably decrease the temperature sensitivity of

microbial respiration, but dry–rewet event may increase

Q10 (CO2), which suggests change of frequency and tim-

ing of drought and precipitation in the future will

remarkably influence the extent of thermal response of

peatlands to global warming. However, potentially

decreasing tendency in Q10 (CO2) with increasing tem-

perature in both peat sites in this study, combining with

other authors’ research results, implies that the feedback

of CO2 release from peatlands to global warming may be

overestimated based on current biogeochemical models.

Furthermore, the apparent increases of Q10 (DOC) with

increasing temperature in both bog and fen sites suggests

that climate change may accelerate DOC thermal pro-

duction, offering a potential explanation of the increase of

DOC in surface water in the Northern Hemisphere during

last several decades.

Acknowledgments This work was financial supported by grants

from Leverhulme Trust (F00174h) and Dr. W. Xiang want to thank

CSC and NSFC (40772205) for their support. We particularly want to

thank J. Williamson and C. Yvonne for their help with sample

collecting.

References

Birch HF (1958) The effect of soil drying on humus decomposition

and nitrogen availability. Plant Soil 10:9–31

Boone RD, Nadelhoffer KJ, Canary JD et al (1998) Roots exert a

strong influence on the temperature sensitivity of soil respiration.

Nature 396:570–572

Borken W, Savage K, Davidson EA, Trumbore SE (2006) Effects of

experimental drought on soil respiration and radiocarbon efflux

from a temperate forest soil. Glob Change Biol 12:177–193

Bowden RD, Newkirk KM, Rullo GM (1998) Carbon dioxide and

methane fluxes by a forest soil under laboratory-controlled

moisture and temperature conditions. Soil Biol Biochem

30:1591–1597

Chapman SJ, Thurlow M (1996) The influence of climate on CO2 and

CH4 emissions from organic soils. Agric For Meteorol 79:205–

217

Cheng WX, Coleman DC, Carroll CR, Hoffman CA (1993) In situ

measurement of root respiration and soluble C concentrations in

the rhizosphere. Soil Biol Biochem 25:1189–1196

Chow AT, Tanji KK, Gao S, Dahlgren RA (2006) Temperature, water

content and wet–dry cycle effects on DOC production and

carbon mineralization in agricultural peat soils. Soil Biol

Biochem 38:477–488

Curiel YJ, Janssens IA, Carrara A et al (2004) Annual Q10 of soil

respiration reflects plant phenological patterns as well as

temperature sensitivity. Glob Change Biol 10:161–169

Davidson EA, Janssens IA (2006) Temperature sensitivity of soil

carbon decomposition and feedbacks to climate change. Nature

440:165–173

Davidson EA, Belk E, Boone RD (1998) Soil water content and

temperature as independent or confounded factors controlling

soil respiration in a temperate mixed hardwood forest. Glob

Change Biol 4:217–227

Davidson EA, Trumbore SE, Amundson R (2000) Soil warming and

organic carbon content. Nature 408:789–790

Davidson EA, Janssens IA, Luo YQ (2006) On the variability of

respiration in terrestrial ecosystems: moving beyond Q10. Glob

Change Biol 12:154–164

Degens BP, Sparling GP (1995) Repeated wet–dry cycles do not

accelerate the mineralisation of organic C involved in the macro-

aggregation of a sandy soil. Plant Soil 175:197–203

Eliasson PE, McMurtrie RE, Pepper DA, Stromgren M, Linder S,

Agren GI (2005) The response of heterotrophic CO2 flux to soil

warming. Glob Change Biol 11:167–181

Evans CD, Chapman PJ, Clark JM, Monteith DT, Cresser MS (2006)

Alternative explanations for rising dissolved organic carbon

export from organic soils. Glob Change Biol 12:2044–2053

Fenner N, Freeman C, Reynolds B (2005) Observations of a

seasonally shifting thermal optimum in peatland carbon-cycling

processes: implications for the global carbon cycle and soil

enzyme methodologies. Soil Biol Biochem 37:1814–1821

Freeman C, Evans CD, Monteith DT, Reynolds B, Fenner N (2001)

Export of organic carbon from peat soils. Nature 412:785

Freeman C, Fenner N, Ostle NJ et al (2004) Export of dissolved

organic carbon from peatlands under elevated carbon dioxide

levels. Nature 430:195–198

Harper CW, Blair JM, Fay PA, Knapp AK, Carlisle JD (2005)

Increased rainfall variability and reduced rainfall amount

decreases soil CO2 flux in a grassland ecosystem. Glob Change

Biol 11:322–334

Janssens IA, Pilegaard K (2003) Large seasonal changes in Q10 of soil

respiration in a beech forest. Glob Change Biol 9:911–918

Environ Geol (2009) 58:499–508 507

123

Kirschbaum MUF (1995) The temperature dependence of soil organic

matter decomposition, and the effect of global warming on soil

organic c storage. Soil Biol Biochem 27:753–760

Knorr W, Prentice IC, House JI, Holland EA (2005) Long-term

sensitivity of soil carbon turnover to warming. Nature 443:298–

300

Kuzyakov Y (2002) Separating microbial respiration of exudates from

root respiration in non-sterile soils: a comparison of four

methods. Soil Biol Biochem 34:1621–1631

Kuzyakov Y, Cheng W (2001) Photosynthesis controls of rhizosphere

respiration and organic matter decomposition. Soil Biol Biochem

33:1915–1925

Li LH, Wang QB, Bai YF (2000) Soil respiration of a Leymuschinensis grassland in the Xilin River Basin as affected by over-

grazing and climate. Acta Phytoecol Sin 24:680–686 (in

Chinese)

Lundquist EJ, Jackson LE, Scow KM (1999) Wet–dry cycles affect

dissolved organic carbon in two California agricultural soils. Soil

Biol Biochem 31(7):1031–1038

Luo Y, Wan S, Hui D et al (2001) Acclimatization of soil respiration

to warming in a tall grass prairie. Nature 413:622–625

Marschner B, Bredow A (2002) Temperature effects on release and

ecologically relevant properties of dissolved organic carbon in

sterilized and biologically active soil samples. Soil Biol

Biochem 34:459–466

Melillo JM, Steudler PA, Aber JD, Newkirk K, Lux H, Bowles FP,

Catricala C, Magill A, Ahrens T, Morrisseau S (2002) Soil

warming and carbon-cycle feedbacks to the climate system.

Science 298:2173–2175

Pastor J, Solin J, Bridgham SD, Updegraff K, Harth C, Weishampel P,

Dewey B (2003) Global warming and the export of dissolved

organic carbon from boreal peatlands. Oikos 100:380–386

Roulet N, Moore TR (2006) Browning the waters. Nature 444:283–

284

Saiz G, Green C, Butterbach-Bahl K, Kiese R, Avitabile V, Farrell E

(2006) Seasonal and spatial variability of soil respiration in four

Sitka spruce stands. Plant Soil 287:161–176

Sjogersten S, Wookey PA (2002) Climatic and resource quality

controls on soil respiration across a forest–tundra ecotone in

Swedish Lapland. Soil Biol Biochem 34:1633–1646

Soulides D, Allison F (1961) Effect of drying and freezing soils on

carbon dioxide productions, available mineral nutrients, aggre-

gation and bacterial population. Soil Sci 91:291–298

Vincent G, Shahriari AR, Lucot E, Badot PM, Epron D (2006) Spatial

and seasonal variations in soil respiration in a temperate

deciduous forest with fluctuating water table. Soil Biol Biochem

38:2527–2535

Waldrop MP, Firestone MK (2006) Response of microbial commu-

nity composition and function to soil climate change. Microb

Ecol 52:716–724

Xu M, Qi Y (2001) Spatial and seasonal variations of Q10 determined

by soil respiration measurement at a Sierra Nevadan forest. Glob

Biogeochem Cycles 15:687–696

Yuste JC, Janssens IA, Carrara A, Ceulemans R (2004) Annual Q10 of

soil respiration reflects plant phonological patterns as well as

temperature sensitivity. Glob Change Biol 10:161–169

Zhou Q, Cabaniss SE, Maurice PA (2000) Consideration in the use of

high-pressure size exclusion chromatography (HPSEC) for

determining molecular weights of aquatic humic substances.

Water Res 34:3505–3514

Zsolnay A, Steindl H (1991) Geovariability and biodegradability of

the water-extractable organic material in an agricultural soil. Soil

Biol Biochem 23:1077–1082

508 Environ Geol (2009) 58:499–508

123