Water-mediated responses of ecosystem carbon fluxes to climatic change in a temperate steppe.

Abstract

Global warming and a changing precipitation regime could have a profound impact on ecosystem carbon fluxes, especially in arid and semiarid grasslands where water is limited. A field experiment manipulating temperature and precipitation has been conducted in a temperate steppe in northern China since 2005. A paired, nested experimental design was used, with increased precipitation as the primary factor and warming simulated by infrared radiators as the secondary factor. The results for the first 2 yr showed that gross ecosystem productivity (GEP) was higher than ecosystem respiration, leading to net C sink (measured by net ecosystem CO(2) exchange, NEE) over the growing season in the study site. The interannual variation of NEE resulted from the difference in mean annual precipitation. Experimental warming reduced GEP and NEE, whereas increased precipitation stimulated ecosystem C and water fluxes in both years. Increased precipitation also alleviated the negative effect of experimental warming on NEE. The results demonstrate that water availability plays a dominant role in regulating ecosystem C and water fluxes and their responses to climatic change in the temperate steppe of northern China.

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Research
Blackwell Publishing Ltd
Water-mediated responses of ecosystem carbon fluxes to
climatic change in a temperate steppe
Shuli Niu
1
, Mingyu Wu
1,2
, Yi Han
1,2
, Jianyang Xia
1,2
, Linghao Li
1
and Shiqiang Wan
1
1
Key Laboratory of Vegetation and Environmental Change, Institute of Botany, the Chinese Academy of Sciences, Beijing 100093, China;
2
Graduate University
of the Chinese Academy of Sciences, Yuquanlu, Beijing 100049, China
Summary
• Global warming and a changing precipitation regime could have a profound
impact on ecosystem carbon fluxes, especially in arid and semiarid grasslands where
water is limited. A field experiment manipulating temperature and precipitation has
been conducted in a temperate steppe in northern China since 2005.
• A paired, nested experimental design was used, with increased precipitation as
the primary factor and warming simulated by infrared radiators as the secondary
factor.
• The results for the first 2 yr showed that gross ecosystem productivity (GEP) was
higher than ecosystem respiration, leading to net C sink (measured by net ecosystem
CO
2
exchange, NEE) over the growing season in the study site. The interannual
variation of NEE resulted from the difference in mean annual precipitation. Experi-
mental warming reduced GEP and NEE, whereas increased precipitation stimulated
ecosystem C and water fluxes in both years. Increased precipitation also alleviated
the negative effect of experimental warming on NEE.
• The results demonstrate that water availability plays a dominant role in regulating
ecosystem C and water fluxes and their responses to climatic change in the temperate
steppe of northern China.
Key words:
carbon, evapotranspiration, global warming, grassland, precipitation.
New Phytologist
(2008)
177
: 209–219
© The Authors (2007). Journal compilation ©
doi
: 10.1111/j.1469-8137.2007.02237.x
New Phytologist
(2007)
Author for correspondence:
Shiqiang Wan
Tel:
+
86 10 6283 6512
Fax:
+
86 10 6259 9059
Email: swan@ibcas.ac.cn
Received:
Accepted:
6 June 2007
28 July 2007

Introduction
Global warming resulting from rising atmospheric concentrations
of CO
2
and other greenhouse gases has increased global mean
temperature by 0.76
°
C since 1850, and will continue to
increase it up to 1.8–4.0
°
C by the end of the 21st century
(IPCC, 2007). It is expected that there will be concurrent
shifts in global and regional precipitation regimes, with extreme
rainfall events becoming more frequent (Dore, 2005; Groisman
et al ., 2005). Given the limitation of temperature and soil
water availability in plant growth and net primary productivity
(NPP), changes in the Earth’s surface temperature and
precipitation regime could profoundly impact terrestrial carbon
cycling and sequestration, with consequent feedbacks to
global climatic change.
Global warming has well been documented to stimulate
both ecosystem C uptake and release across various terrestrial
biomes (Rustad et al ., 2001; Melillo
2002; Peñuelas et al., 2004; Welker
2005). It is assumed that terrestrial ecosystems may act as a
net C source under global warming scenarios because of the
higher temperature sensitivity of ecosystem C release (auto- and
et al
et al
., 2002; Shaw
., 2004; Wan
et al
et al
.,
.,

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210
heterotrophic respiration) than C uptake (plant photosynthesis,
plant growth and NPP) (Oechel
1995; Illeris et al ., 2004). However, this assumption has been
widely debated, partly because of the ecosystem-specific
responses and intra- and interannual variability of the responses
(Smith & Shugart, 1993; Peñuelas
2005). In arid and semiarid regions, changes in precipitation
may have an even greater effect on ecosystem C fluxes than the
singular effect of rising CO
2
or temperature (Weltzin
2003; Potts et al ., 2006) because water availability dominates
impacts on plant growth and net ecosystem productivity
(Fang et al ., 2001; Knapp et al
Elevated temperature could stimulate evaporation and plant
transpiration, leading to increased water loss from soil (Harte
et al ., 1995; Wan et al., 2002). Unless there is increased soil
wetting, lower soil water availability associated with global
warming will exacerbate water limitations in arid and semi-
arid ecosystems, counteracting any potential direct positive
temperature effects on plant growth and NPP.
Located in arid and semiarid regions, the temperate steppe
in northern China represents one of the typical vegetation
types in the Eurasian content and is predicted to be sensitive
to climatic change (Christensen
changes in both temperature and precipitation have been
reported in this area (Gong et al
predications by Gao et al . (1997) and Piao
shown that ecosystem productivity of the temperate steppe in
northern China is sensitive to the driving forces of global change.
However, direct experimental evidence on the response of eco-
system C cycling to climatic change in this area is still lacking.
A field experiment manipulating temperature and precipi-
tation was conducted, beginning in April 2005, to examine
the potential influence of climatic change on a semiarid tem-
perate steppe in northern China. Given the water limitation
in this area and the possible increase in evapotranspiration
under global warming, we hypothesized that elevated tem-
perature could negatively affect ecosystem C exchange, and
that increasing precipitation/soil water availability could
ameliorate the negative impacts of global warming. The specific
objectives of this study were to evaluate how global warming
influences ecosystem C and water fluxes and whether the
impacts vary with intra- and interannual fluctuations of
precipitation; and the extent to which increased precipitation
stimulates net C exchange and ameliorates the negative
impacts of global warming.
et al ., 1993; Kirschbaum,
et al., 2004; Corradi et al .,
et al .,
., 2002; Weltzin et al., 2003).
et al., 2004). Significant
., 2004; IPCC, 2007). Model
et al . (2003) have
Materials and Methods
Study site
This study was carried out in Duolun county (N42
E116
°
17
′
, 1324 m asl), a semiarid area located in Inner
Mongolia, China. Mean annual temperature is 2.1
monthly mean temperatures of 18.9
°
02
′
,
°
C with
°
C in July and –17.5
°
C
in January. Mean annual precipitation is 385.5 mm with 80%
concentrated from June to September. Soil is chestnut soil
(Chinese classification), Calcis-orthic Aridisol in the US Soil
Taxonomy classification, with 62.75
0.01% silt and 16.95
±
0.01% clay, respectively. Mean soil
bulk density is 1.31 g cm and pH is 6.84
steppe is dominated by Stipa krylovii
Potentilla acaulis , Cleistogenes squarrosa
and Agropyron cristatum, all perennial herbs. The temperate
steppe at our experimental site has relatively low primary
productivity (approx. 100–200 g m
ranging from 0 in early spring and late autumn to nearly 1 in
the mid-growing season (Zhang, 2007).
±
0.04% sand, 20.30
±
–3
±
0.07. The temperate
, Artemisia frigida
, Allium bidentatum
,
–2
yr
–1
), with leaf-area index
Experimental design
The experiment used a paired, nested design with precipitation
as the primary factor and warming as the secondary factor.
There were three pairs of 10
×
pair was assigned as the increased precipitation treatment
and the other as the control. At each precipitation plot, six
sprinklers were arranged evenly into two rows with a distance
of 5 m between any two sprinklers. Each sprinkler covered a
circular area with a diameter of 3 m, so the six sprinklers
covered the 10
×
15-m plot. In July and August, 15 mm of
water was added weekly to the increased precipitation plots.
Therefore a total precipitation amount of 120 mm (approx.
30% of mean annual precipitation in the study site) was
supplied each year (8 wk by 15 mm wk
Within each 10
×
15-m plot, four 3
treated as the warmed and control subplots with two replicates.
The subplots were randomly assigned to the temperature
treatments. The warmed subplots have been heated continu-
ously since 28 April 2005 using 165
infrared radiators (Kalglo Electronics, Bethlehem, PA, USA)
suspended 2.5 m above the ground. The effects of infrared
radiators on soil temperature were spatially uniform within
the warmed plots (Wan
et al ., 2002). In the unwarmed con-
trol subplot, one ‘dummy’ heater with the same shape and size
as the infrared radiator was suspended 2.5 m high to simulate
the shading effects of the heater. Thus there were six replicates
for each treatment (control, warming, increased precipitation,
warming plus increased precipitation).
15-m plots; one plot in each
–1
×
).
4-m subplots were
×
15-cm MSR-2420
Measuring variables
Soil temperature and moisture
of 10 cm was recorded with a CR1000 datalogger (Campbell
Scientific, Logan, UT, USA) at 1-h intervals from 4 June
2005. Soil moisture content (0–10 cm) was measured using a
portable soil moisture device (Diviner 2000, Sentek Pty Ltd,
Balmain, Australia). Volumetric soil water content and ecosystem
gas exchange were both measured once or twice a month over
the growing season of 2005 and 2006.
Soil temperature at the depth

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Measurements of ecosystem gas exchange
two square aluminium frames (0.5
into the soil to a depth of approx. 3 cm at two opposite
corners in each subplot. The distance between the frames and
the subplot edge was 0.3 m. Care was taken to minimize soil
disturbance during installation. The aluminum frames provided
a flat base between the soil surface and the CO
chamber.
Ecosystem gas exchange (CO
ured with a transparent chamber (0. 5
to an infrared gas analyser (IRGA; LI-6400, Li-Cor, Lincoln,
NE, USA) on one of the two frames in each subplot. During
measurements the chamber was sealed with the base surface.
Two small fans ran continuously to mix the air inside the
chamber during measurements. Nine consecutive recordings
of CO
2
and water vapour concentration were taken on each
base at 10-s intervals during a 90-s period. CO
rates were determined from the time-courses of the concen-
trations to calculate net ecosystem exchange (NEE) and
evapotranspiration (ET). The method was similar to that
reported by Steduto et al. (2002). Following the measurements
of NEE, the chamber was vented, replaced on the base, and
covered with an opaque cloth. After the CO
the chamber was steadily increased (usually 0.5 min after the
chamber was covered), the CO
repeated. Because the second set of measurements eliminated
light (and hence photosynthesis), the values obtained repre-
sented ecosystem respiration (ER). Gross ecosystem produc-
tivity (GEP) was then calculated as the difference between
NEE and ER. Ecosystem water-use efficiency (WUE) was
calculated as NEE/ET. Positive and negative NEE values
represent net C uptake by, and release from, the ecosystem,
respectively.
Seasonal gas exchange was usually measured once or twice
a month at the middle of the two precipitation treatment
events. Diurnal patterns of gas exchange were measured at a
3-h interval on 29 July 2005 and 13 July 2006.
In April 2005,
×
0.5 m) were inserted
2
sampling
2
and water fluxes) was meas-
×
0.5
×
0.5 m) attached
2
and H
2
O flux
2
concentration in
2
exchange measurements were
Leaf gas-exchange measurement
(Li-6400; Li-Cor) with a 6-cm
used to measure the leaf photosynthesis and transpiration
An open gas-exchange system
2
clamp-on leaf cuvette was
rate of the dominant plant,
expanded leaf of one individual in
measurement on 31 August 2005. Leaf gas exchange was
measured between 09:00 and 11:00 h (local time) in the
morning. Leaves were illuminated at 1500 mol m
the LED light system. The maximum photosynthetic rate
(P
max
), transpiration rate (E ), stomatal conductance (
vapour pressure deficit (VPD) were recorded.
A. cristatum. At each subplot, one
A. cristatum was selected for
–2
s
–1
using
g
s
), and
Statistical analyses
examine warming and precipitation effects on soil microclimate,
ecosystem C and water fluxes over the growing season in 2005
and 2006. Between-subject effects were evaluated as warming
or precipitation treatment; within-subject effects were time-
of-season. One-way
ANOVA
was used to examine the statistical
difference in seasonal averages for measuring variables among
the four treatments. All statistical analyses were conducted
with
SPSS
software (
SPSS
11.0 for
Repeated-measures
ANOVA
was used to
WINDOWS
).
Results
Microclimate changes induced by warming and
precipitation treatments
In comparison with the long-term averages (1953–2005) of
mean annual temperature (MAT, 2.1
and 2006 (2.8
°
C) had higher MAT. However, 2005 had
lower annual precipitation (322.8 mm) whereas 2006 had
higher annual precipitation (407.7 mm) than the long-term
mean annual precipitation (MAP, 385.5 mm; Fig. 1).
Experimental warming manipulated with infrared radiators
significantly elevated soil temperature (
whole experimental period, mean soil temperature at 10 cm
depth was 1.39 and 1.02°C higher under warming than the
control in the natural and increased precipitation plots,
respectively. Volumetric soil moisture at 0–10 cm depth fluc-
tuated greatly over the growing season (Fig. 2a,b). Warming
decreased seasonal mean volumetric soil moisture by 1.72
(v/v%) in 2005 and 1.34 (v/v%) in 2006 (all P < 0.05) across
the increased and natural precipitation plots. Increased pre-
cipitation significantly elevated seasonal mean soil moisture
°
C), both 2005 (2.6
°
C)
P
< 0.05). Across the
Fig. 1 Daily precipitation (bars) and daily
mean air temperature (line) in 2005 and
2006. Data for 2005 are from the
Meteorological Station of Duolun County,
Inner Mongolia, 26 km from the study
site. Data in 2006 are from the eddy tower
adjacent (approx. 100 m) to the experimental
plots.

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212
by 1.37 and 3.18 (v/v%) in 2005 and 2006 in the unwarmed
subplots (P < 0.05; Fig. 2a,b), respectively, but insignificantly
increased soil moisture by 0.30–0.65 (v/v%) in the warmed
subplots in both years (P > 0.05).
Interannual differences in C and water fluxes caused
by precipitation
The temporal dynamics of NEE, ER, GEP and ET followed
the seasonal patterns of air temperature in both years, which
were higher in summer and lower in spring and autumn
(Fig. 3). In addition, there were substantial interannual
variations in ecosystem CO2 and water fluxes (P < 0.001). In
the unwarmed control subplots under natural precipitation,
seasonal mean NEE (3.71 µmol m–2 s–1), GEP (5.99 µmol m–2 s–1)
and ET (1.93 mmol m–2 s–1) in 2005 were 25.3, 14.5 and
36.3% greater than in 2006 (2.77 µmol m–2 s–1, 5.12 µmol m–2 s–1,
1.23 mmol m–2 s–1), respectively. By contrast, ER (–2.28
µmol m–2 s–1) and WUE (1.86 µmol CO2 mmol–1 H2O)
in 2005 were 3.1 and 31.7% lower than in 2006 (–2.35
µmol m–2 s–1 and 2.45 µmol CO2 mmol–1 H2O), respectively.
Monthly mean NEE in the growing season changed greatly
between the two years. The difference in monthly mean NEE
between years coincided with the difference in monthly mean
soil moisture, but was contrary to the difference in monthly
mean soil temperature (Fig. 4), suggesting controls of soil
water content over the interannual variability of NEE.
Negative warming effects on C and water fluxes varied
with year and water availability
The main effect of warming was statistically significant on
NEE and GEP in 2005 (Table 1). Under natural precipitation,
NEE and GEP were, on average, 12.3 and 11.1% lower
(P < 0.05), respectively, in the warmed than in the control
subplots over the 2005 growing season (Fig. 3a,e). Warming-
induced reductions in the seasonal mean NEE and GEP in
2005 were 4.9 and 3.6%, respectively, in the increased
precipitation plots. Experimental warming slightly, but
insignificantly, decreased ET (5.7%) and WUE (5.4%) in
2005. In 2006, marginal reductions (3.7–11.4%) in all measured
variables were observed under the warming treatment.
Fig. 2 Effects of experimental warming and increased precipitation
on volumetric soil water content at 0–10 cm depth in (a) 2005;
(b) 2006. C, control; W, warming; P, increased precipitation; WP,
warming plus increased precipitation.
Table 1 Results (F values) of repeated-measurement ANOVA on the effects of warming (W), increased precipitation (P), measuring date (D), and
their interactions on net ecosystem CO2 exchange (NEE), gross ecosystem productivity (GEP), ecosystem respiration (ER), ecosystem
evapotranspiration (ET) and ecosystem water-use efficiency (WUE)
20052006
NEEERGEPETWUENEE ER GEPETWUE
D
D × P
D × W
D × P × W
P
W
P × W
193***
1.57
0.89
2.15*
2.25
9.99**
1.59
187***
2.16*
0.57
0.71
4.18^
1.34
0.02
320***
1.69^
1.45
2.41*
8.86**
20***
2.30
178***
2.92**
0.52
0.20
7.30*
1.94
0.01
18***
1.25
0.25
0.44
0.08
0.00
1.21
73***
13***
2.36*
1.44
42***
1.78
0.01
96***
5.98***
1.19
0.78
12**
0.10
0.97
149***
18***
2.55*
1.71^
36***
0.80
0.02
26***
2.84**
1.95^
1.97^
4.03^
2.95
2.27
78***
2.34*
1.62
2.62*
5.22*
2.60
0.03
Significance: ^, P < 0.10; *, P < 0.05; **, P < 0.01; ***, P < 0.001.

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Fig. 3 Seasonal dynamics and means (mean ± 1 SE) of (a,b) net ecosystem CO2 exchange (NEE); (c,d) ecosystem respiration (ER); (e,f) gross
ecosystem productivity (GEP); (g,h) ecosystem evapotranspiration (ET); (i,j) ecosystem water-use efficiency (WUE) in 2005 (left) and 2006
(right). Different letters in insets indicate significant difference (P < 0.05) in seasonal averages among treatments. C, control; W, warming;
P, increased precipitation; WP, warming plus increased precipitation.