Environ Geol (2009) 57:1855–1864 DOI 10.1007/s00254-008-1474-y
ORIGINAL ARTICLE
Abiotic regulators of soil respiration in desert ecosystems Lihua Zhang Æ Yaning Chen Æ Weihong Li Æ Ruifeng Zhao
Received: 15 April 2008 / Accepted: 7 July 2008 / Published online: 29 July 2008 Ó Springer-Verlag 2008
Abstract Soil temperature and soil moisture are the most important environmental factors controlling soil respiration in mesic ecosystems. However, soil respiration and associated abiotic regulators have been poorly studied in desert ecosystems. In this study, soil respiration was measured using an automated CO2 efflux system (LI-COR 8100), and the effects of soil temperature and moisture on the rate of soil respiration were examined in six desert sites [three communities—Haloxylon ammodendron, Halostachys caspica and Anabasis aphylla at high (B) and low (A) vegetation coverage respectively]. It was found that soil respiration was significantly and positively correlated with soil surface temperature. A multi-variable model of soil temperature and soil moisture could explain 61.9% of temporal variation in soil CO2 efflux at a larger scale. There were significantly negative correlations between soil respiration and soil moisture in Haloxylon ammodendron B and Halostachys caspica B sites, which represented the driest and wettest sites, respectively. The results also showed that soil respiration displayed obvious diurnal and seasonal patterns during the growing season. The Q10 values for Haloxylon ammodendron A and B, Halostachys caspica A and B, and Anabasis aphylla A and B sites were 1.3, 1.34, 1.58, 1.65, 1.31 and 1.17, respectively, with a cross-site average of 1.39. The results showed that soil respiration was not positively correlated with soil moisture L. Zhang Y. Chen (&) W. Li R. Zhao Key laboratory of Oasis Ecology and Desert Environment, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China e-mail:
[email protected] L. Zhang R. Zhao Graduate University of Chinese Academy of Sciences, Beijing 100039, China
unlike in most mesic ecosystems. However, soil respiration in desert ecosystems is less sensitive to temperature variation than most mesic ecosystems as indicated by the lower Q10 values possibly due to energy limitation. Keywords Desert ecosystem Q10 Soil moisture Soil respiration Temperature
Introduction Soil provides the second major carbon efflux to the atmosphere (Schlesinger and Andrews 2000) and buffers atmospheric CO2 concentration against seasonal and interannual variations in plant growth (Raich et al. 2002). Soil respiration responds to the on-going climate change and it is likely to significantly impact on the CO2 sink strength of vegetation and future atmospheric CO2 concentrations (David et al. 2006). Among the various environmental parameters controlling soil CO2 efflux, soil temperature and soil moisture are of most importance (Raich and Schlesinger 1992; Davidson et al. 1998, 2000; Fang and Moncrieff 1999, 2001; Joffre et al. 2003; Kirschbaum 2000; Liu et al. 2002). However, how soil respiration responds to soil temperature and moisture in desert ecosystems has seldom been reported. Arid and semiarid areas occupy over two-fifths of the Earth’s total surface (Reynolds 2001), and soil respiration is one of the main processes of C loss from arid and semiarid soils (Conant et al. 2000). Additionally, soil respiration is one of the ecosystem properties most sensitive to the climate change because of the relatively small organic C pools that arid and semiarid soils contain (West et al. 1994). Soil respiration in mesic ecosystems have been extensively studied (Xu and Qi 2001a, 2001b; Irvine and Law
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2002; Janssens and Pilegaard 2003; Li et al. 2004; Tang et al. 2006). Soil respiration is highly correlated with changes in soil temperature when water is not limited (Drewitt et al. 2002; Curiel Yuste et al. 2003; Jassal et al. 2005). Temperature may explain up to 72–96% of the variation in soil respiration in temperate forests (Keith et al. 1997; Rey et al. 2002; Subke et al. 2003). However, the seasonal dependence of soil respiration on soil moisture is still poorly understood because the variations in soil temperature and moisture are often correlated and the independent effect of each variable is hard to detect (Davidson et al. 1998). Considerable uncertainty remains as to how these factors interact to control soil respiration in xeric ecosystems (Conant et al. 2000; Casals et al. 2000; Frank et al. 2002). In Colorado Plateau temperature and moisture were found to be the dominant abiotic controls of soil respiration, with the highest fluxes occurring in spring when temperature and moisture were favorable (Fernandez et al. 2006). Whereas in semiarid Mediterranean forest seasonal variations in soil CO2 efflux were found to relate mainly to changes in soil temperature, and high respiration rates were observed in mid summer in spite of drought (Casals et al. 2000). To better understand the dynamics of global C cycles and the feedback of terrestrial C sequestration to future climate change, it is critical to study the changes of C pools in desert ecosystems to temperature and soil moisture. In this study, the soil respiration of Haloxylon ammodendron, Halostachys caspica and Anabasis aphylla communities, which were typical plant communities in this desert, were examined in 2005 in Junggar Basin, Northwestern China. The specific objectives of this research are (1) to evaluate the effect of soil temperature and moisture on soil respiration, and (2) to determine the effect of soil nutrient and salinity on soil respiration.
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Three plant communities were selected, each with a high or low coverage (Fig. 1). Six sites were distributed from northwest to southwest. The topography of all sites was flat. The first community was dominated by Haloxylon ammodendron, with Anabasis aphylla, Nitraria sibirica Pall, Lycium ruthenicum, shrubs and Peganum harmala, Halogeton glomeratus and Salsola spp., grasses also present. The height of plants was between 100 and 250 cm with over 50% coverage in high coverage site (HAH, N45°23.9720 , E84°51.2740 ). The average height was 65 cm with coverage of 20% in the low coverage site (HAL, N45°24.6690 , E84°50.4890 ). The distance between two sites was 1,500 m. The second community was dominated by Halostachys caspica, with Reaumuria soongorica, N. sibirica Pall, Halocnemum strobilaceum, Kalidium foliatum, Anabasis aphylla, shrubs and Limonium suffruticosum, S. foliosa, grasses also present. The Halostachys caspica community with high coverage (HCH), located at N45°23.0800 , E84°51.4120 , had a plant height of 80 cm and coverage of 25%. The height was about 40 cm with the coverage of 15% in the low coverage site in Halostachys caspica community (HCL, N45°22.9990 , E84°51.5050 ). The distance between two sites was 300 m. The third community was dominated by Anabasis aphylla, with R. soongorica, N. sibirica Pall, shrubs and Aeluropus pungens, Limonium suffruticosum, grasses also present. The height of plants reached 50–70 cm with coverage of 20% in the high coverage site of Anabasis aphylla community (AAH, N45°23.8490 , E84°51.4310 ). The plants were between 20 and 30 cm in height in the low coverage site (AAL, N45°23.8740 , E84°51.4680 ) with coverage of 5%. The distance between two sites was 300 m. Field measurement
Materials and methods Site description The study area is located in the periphery of a newly cultivated oasis in Kelamayi in the western Junggar Basin, adjacent to Zhayier Mountain in the north and the alluvion plain in the south. The altitude of the southwest is 273– 280 m, and that of the northeast is 258–260 m. The main soil types are relic bog soils, relic solonchaks and desert aeolian soils (Qian et al. 2004). The area is characterized by a continental arid-desert climate, which is hot in summer, cold and without stable snow cover in winter and windy in spring. It has a mean annual temperature of 8°C, a mean annual precipitation of 105.3 mm and a mean annual evaporative potential of 3,545 mm.
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Six sites were established, each one was 30 m 9 30 m. There were five sampling points for soil respiration measurements at each site. A soil collar, with a height of 10 cm and a diameter of 10 cm, was inserted into the soil for 7 cm at each sampling point 1 day prior to the first measurements. All soil collars were left on site for the entire study period. Soil respiration was measured by an automated soil CO2 efflux measurement system (LI-8100, LI-COR, Lincoln, NE, USA). Soil respiration measurements were conducted once every month around the 20th day on each month from May to October 2005, 1 day on each site. The measurements were made every 1 h from 8:00 to 20:00. For each site, six to seven measurements were conducted in each day. Air temperature (at 50 cm in height) and soil temperature (every 5 cm from 0 to 50 cm depth) were monitored at
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Fig. 1 Location of study area and sample scheme of different communities and coverage types. 1. HAL, 2. HAH, 3. AAL, 4. AAH, 5. HCL, 6. HCH
three points adjacent to the chamber using digital thermometer (WMY-01C, Huachen Medical Instrument, Inc., Shanghai, China) in each site. Temperature measurements were made every 1 h from 8:00 to 20:00 in each day. Gravimetric soil moisture at 0–5 cm, 5–15 cm, 15–30 cm and 30–50 cm depths was measured at three points, with oven-drying method at 105°C for 48 h. Soil samples for the soil layers of 0–5 cm, 5–15 cm, 15–30 cm and 30–50 cm were collected for soil physical and chemical properties analysis in June 2005. Soil nutrients (including soil organic matter, organic carbon, total and available N, P and K) and soil salts contents (including total salt, pH, electrical conductivity, Cl-, SO42-, CO32-, HCO3-, Ca2+, Mg2+, Na+ and K+) were measured using routine methods (Nanjing Institute of Petrology 1978).
from 8:00 to 20:00 through June to October in each site (n = 35), and the data in May were not used due to incomplete soil temperature measurement. Regression analysis between soil respiration and soil moisture in each site used daily mean soil respiration data in accordance with soil moisture measurement points from May to October, except several missing values (n = 16–18). Stepwise regression analysis between soil respiration and soil moisture, soil temperature for the six sites together used the daily mean soil respiration and soil temperature data from June to October (n = 30). Pearson correlation analysis was also used to test the relationship between soil CO2 efflux and soil temperature, soil moisture and chemical soil properties among six sites (n = 6). All statistical analyses were performed using SPSS 11.5 (SPSS for Windows, Version 11.5, Chicago, IL, USA).
Data analysis Analysis of variance (ANOVA) was used to test the difference in soil CO2 efflux among the six sites and regression analysis to examine the relationship between soil CO2 efflux and environmental factors. Exponential regression analysis between soil respiration and temperature used the data of daily soil respiration and temperature
Results Effect of temperature on soil respiration Soil respiration showed a typical daily pattern that followed surface temperature, with maximums occurring
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during 12:00*14:00 and minimums at 8:00 or 20:00 at all six sites (Fig. 2a–f). The maximum and minimum soil respiration rate were 1.82 and -0.24 lmol CO2m-2 s-1, with the maximum occurring in July at HAH site and the minimum in October at HCL site. The diurnal range was normally less than 1 lmol CO2m-2 s-1 at six sites. Soil respiration increased gradually from May to June or July and then decreased to October, except for several cloudy days (in June and August at HAH site, in August at HCH site, in July at AAL and AAH sites) and rainy day (in June at AAH site). The soil respiration rate in summer (June, July and August) was higher than that in autumn (September and October), with the minimum in spring (May). However, soil respiration did not exactly follow the trend of temperature in seasonal variation. Soil respiration did not reach the minimum in October while temperature was the lowest. In addition, the rates of soil respiration were lower than 0 in early morning (at 8:00) in September or October when soil temperature at 0 cm was around 1°C. The soil respiration got to positive at 10:00 with temperature increase. ANOVA showed that only the Haloxylon
Fig. 2 Daytime changes of soil respiration (solid symbols) and soil temperature at 0 cm (open symbols) from 8:00 to 20:00 in HAL, HAH, HCL, HCH, AAL and AAH sites in growing season (a–f represent the six sites respectively). Error bars represent means ± SE (n = 5 for soil respiration; n = 3 for soil temperature at 0 cm)
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ammodendron site (B) was significantly different from other sites (P \ 0.05). Exponential equations were used to model the relationship between soil respiration and temperature for each site using air temperature, soil temperature at 0 cm and mean soil temperature at 0–20 cm and 20–50 cm depth (Table 1). Soil temperature at 0 cm fitted best with soil respiration. Soil respiration was better fitted with mean soil temperature at 0–20 cm depth than 20–50 cm depth. Q10 for soil temperature at 0 cm were 1.30, 1.34, 1.58, 1.65, 1.31 and 1.17, respectively, for HAL, HAH, HCL, HCH, AAL and AAH site. Cross-site mean of the Q10 values was 1.39 with a coefficient of variation of 13.21%. Effect of soil moisture on soil respiration Through correlation analysis between soil moisture and daily mean soil respiration for each site, it was found that soil respiration was significantly and negatively correlated with soil moisture at 30–50 cm depth in the HAH site (r = -0.63, n = 16, P = 0.02). Soil respiration and soil
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Table 1 Exponential equations, amounts of variation explained and Q10 values of soil respiration for temperature from HAL, HAH, HCL, HCH, AAL and AAH sites Site
Temperature
n
Temperature (°C)
Regression equation
Q10
F-value
R2
HAL
Air temperature
35
30.18
Y = 0.187e0.042X
1.52
14.88
0.32
0.026X
1.30
15.70
0.32
1.27
4.23
0.12
HAH
HCL
HCH
AAL
AAH
0 cm
35
41.23
Y = 0.242e
0–20 cm
35
27.32
Y = 0.306e0.024X
20–50 cm
35
21.06
Not pass F-test
Air temperature
35
29.73
Y = 0.297e0.036X
1.43
12.30
0.28
0 cm
35
38.62
Y = 0.326e0.029X
1.34
14.59
0.31
0.026X
0–20 cm 20–50 cm
35 35
26.83 22.11
Y = 0.401e Not pass F-test
1.30
4.32
0.12
Air temperature
35
30.29
Y = 0.074e0.071X
2.03
24.50
0.45
0 cm
35
42.79
Y = 0.112e0.046X
1.58
36.84
0.54
0–20 cm
35
27.45
Y = 0.102e0.065X
1.92
12.92
0.30
0.075X
20–50 cm
35
21.48
Y = 0.107e
2.12
6.17
0.17
Air temperature
35
28.19
Y = 0.095e0.061X
1.84
25.22
0.46
0 cm
35
32.92
Y = 0.110e0.050X
1.65
42.54
0.59
0–20 cm
35
23.95
Y = 0.121e0.058X
1.79
16.82
0.36
20–50 cm
35
20.73
Y = 0.107e0.070X
2.01
10.99
0.27
Air temperature
35
31.56
Y = 0.235e0.030X
1.35
11.84
0.29
0.027X
1.31
21.23
0.42
0 cm
35
42.63
Y = 0.220e
0–20 cm
35
27.73
Not pass F-test
20–50 cm
35
21.60
Not pass F-test
Air temperature
35
30.17
Y = 0.346e0.017X
1.19
4.14
0.13
0 cm 0–20 cm
35 35
39.66 26.91
Y = 0.331e0.016X Y = 0.086e0.066X
1.17 1.94
8.11 7.14
0.22 0.20
20–50 cm
35
21.42
Y = 0.069e0.085X
2.34
4.84
0.14
Temperature (°C) is the average temperature at 14:00 on observation days through June to October
moisture at 0–5 cm and 15–30 cm depth was highly correlated in the HCH site (r = -0.60, -0.65, P = 0.01, 0.005, n = 17) whereas the significant correlation was not found in other sites. During experimental period, soil moisture at 30–50 cm depth in the HAH site were lowest among six sites, and varied from 5.39 to 13.35% with least fluctuation. Soil moisture at 0–5 cm and 15–30 cm depth were largest in the HCH site given the same soil layer, which fluctuated in the range of 5.23–27.40% and 17.35– 28.22% respectively. The location of the HCH site is relatively lower than the other sites. The relationship between soil respiration and soil moisture in the HAH and HCH site was empirically fitted (Fig. 3). The seasonal variation in soil respiration followed the trend of soil temperature at 0 cm in most sites, and the maximum occurrence was consistent with soil temperature at 0 cm (in June or July) while soil moisture did not reach the minimum or maximum in the experimental period (Fig. 4). The difference in mean soil respiration, soil temperature and soil moisture for six sites was shown in Fig. 5. The soil
respiration rate was largest in the HAH site where soil moisture was the lowest. Simultaneously, soil respiration rate was the lowest in the HCH site with the highest soil moisture. Soil respiration was significantly and negatively correlated with soil moisture, and the correlations with soil moisture at 5–15 cm, 15–30 cm and 30–50 cm depth were the followings respectively: r = -0.84, -0.82, -0.91, P = 0.04, 0.046, 0.01 (n = 6). The correlation between soil respiration and soil temperature at 0 cm was not significant (P [ 0.1). The Q10 value was not correlated with soil moisture (P [ 0.1, n = 6). Effects of soil temperature and moisture on soil respiration Stepwise linear regression analysis was used to model the relationship between soil respiration and soil moisture, soil temperature at 0 cm, air temperature for the six sites together (Table 2). Furthermore, regression analysis was applied to detect the relationship between soil respiration and soil temperature at 0 cm, soil moisture at 0–5 cm and
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Fig. 3 Regression between soil respiration and soil moisture in sites of HAH and HCH
Fig. 4 Seasonal variation of soil respiration, soil temperature at 0 cm and soil moisture at 0–50 cm depth at HAL, HAH, HCL, HCH, AAL and AAH sites (a–f represent the six sites respectively). Error bars represent means ± SE (n = 7 for soil respiration and soil temperature; n = 3 for soil moisture)
30–50 cm depth respectively (Table 2). It was concluded that the multivariable model of soil respiration–temperature–soil moisture explained 61.9% of the temporal variation in large spatial scale and was better than a univariable model.
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Soil respiration and soil properties Average soil properties values at 0–50 cm and the correlation between soil respiration and variables describing soil properties for six sites are summarized in Table 3. Soil
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Fig. 5 Mean soil respiration, soil surface temperature and soil moisture in growing season for six sites. 1. HAL, 2. HAH, 3. HCL, 4. HCH, 5. AAL, 6. AAH. Error bars represent means ± SE (n = 6 for soil respiration, soil temperature at 0 cm and soil moisture)
Table 2 Amount of variation explained, F-values and regression equations relating soil respiration rates (Y) with different independent variables at six sites Variable
n
R2
F-value
Regression equation
Soil temperature at 0 cm
30
0.49
26.65
Y = 0.578T00.085
Soil moisture at 0–5 cm depth
30
0.26
9.70
Y = 0.829e-0.034W
Soil moisture at 30–50 cm depth
30
0.31
12.30
Y = 0.423 + 3.167/W
Temperature–soil moisture
30
0.62
14.08
Y = 0.324 + 0.015T0 - 0.019W30*50 + 0.020W0–5
respiration was positively correlated with available K, pH, CO32- and HCO3-, negatively with Ca2+ but not significantly. Strong correlation was not found between soil respiration and soil organic carbon, available N, P. Soil nutrients and salts content at 0–5 cm varied largely in six sites. Mean soil organic carbon and organic matter content at 0–50 cm was 3.34 (±0.24) (Table 3) and 5.76 (±0. 41) g/kg. Cl-, SO42- and Na+ had high percentage of soil salt ions. The content of available K was largest in HCH site, and that of available P was largest in HAH site. Soil pH, CO32- and HCO3- content were all largest in HCH site.
Discussion Effect of temperature on soil respiration In this study the seasonal variation of soil respiration followed the trend of soil temperature at 0 cm in most sites (Fig. 4), but the explained value of surface temperature to soil respiration was above 50% in three sites only.
Moreover, the mean Q10 value for soil respiration in six sites was 1.39, lower than 1.5 across the global range (except for wetland) based on air temperature (Raich and Potter 1995). In this study the mean soil temperature at 0 cm measured was 23–46°C from May to September except 15°C in October. Many studies found that the Q10 value of soil respiration was higher at low temperatures than at high temperatures (Schleser 1982; Raich and Schlesinger 1992; Kirschbaum 1995, 2000). Furthermore, the mean soil organic carbon content across six sites is 3.34 g/kg, and litter input of desert plant is relatively smaller than other vegetation types, which might result in low microbial biomass and activity due to inadequate substrate. Substrate quality and quantity could regulate responses of soil respiration to temperature and potentially resulted in acclimatization (Luo et al. 2001). Respiratory acclimatization possibly resulted from changes in the composition of the microbial community and/or other physiological and ecological adjustments in response to a limited substrate supply, which reduced the respiratory capacity in soil (Atkin et al. 2000). Thus, energy limitation
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Table 3 Summary statistics of soil properties and coefficients of correlation among themselves and between them and soil CO2 efflux for the six sites pH
CO32(g/kg)
HCO3(g/kg)
Ca2+ (g/kg)
Organic C (g/kg)
Available N (mg/kg)
Available P (mg/kg)
Available K (mg/kg)
Mean
8.40
0.01
0.25
0.32
3.34
15.46
3.41
148.67
Standard deviation
0.38
0.03
0.14
0.21
0.24
6.03
1.36
45.63
pH CO32-
1 0.94**
1
HCO3-
0.96**
0.98**
1
Ca2+
-0.90*
-0.76
-0.82*
1
Organic C
-0.58
-0.43
-0.61
0.61
1
Available N
-0.38
-0.31
-0.37
0.40
0.64
1
Available P
-0.10
0.04
-0.06
-0.06
0.17
-0.47
1
Available K
0.87*
0.98**
-0.95**
-0.65
-0.41
-0.27
0.05
June
0.19
0.11
0.02
-0.12
0.53
0.52
-0.34
1
July
0.77
0.90**
0.80
-0.55
-0.14
-0.38
0.32
August
0.32
0.002
0.09
-0.48
-0.15
0.22
-0.47
September
0.94**
0.99**
0.99**
-0.77
-0.53
-0.33
-0.02
October
0.88*
0.93**
0.93**
-0.69
-0.42
-0.02
-0.26
0.93**
Mean flux (June–October)
0.91*
0.91*
0.87*
-0.74
-0.29
-0.13
-0.11
0.85*
0.02 0.89* -0.16 0.98**
n=6 *Significant at 0.05 confidence, **Significant at 0.01 confidence
and limited substrate supply might result in low temperature sensitivity of soil respiration in spite of high temperature in study area. The low values of Q10 than the global average in this study basically indicates that this desert ecosystem is energy limited for microbial activity as found in other ecosystems under dry climate (Xiao et al. 2007). It was found that soil respiration reached the minimum in May not in October when temperature was the lowest, possibly suggesting energy limitation of soil respiration in spring and lack of it in autumn. In addition, the depth of soil temperature was an important factor which could influence temperature relationship with Q10 (Davidson et al. 2000; Xu and Qi 2001b). Borken et al. (2002) assumed that the most appropriate soil depth for measuring temperature was the one explaining largest variation in soil respiration, and deduced that the highest CO2 production within the soil profile occurred at this depth. Soil temperature at 0 cm explained relatively large variation of soil respiration (Table 1), which might be due to plants increased root respiration to survive under dry and hot environment. However, the depth within the soil profile at which highest CO2 produces requires further research. The author found that soil respiration rate was negative in early morning (at 8:00) in September or October and got positive at 10:00 measurement in this study. Soil temperature at 0 cm was around 1°C, and air temperature was -1–4°C
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when soil respiration rate were lower than 0. The negative soil respiration was not found at other measurement time. Thus, the occurrence of negative soil respiration rate might be related to lower temperature. Xie et al. (2008) also found the negative soil CO2 fluxes during nocturnal hours at saline desert in northwest of China. They considered that alkaline and saline soils apparently absorb CO2 under natural conditions. The report on the negative soil respiration rate has been few by far, it worths wide attention and further research to explain the phenomenon. Effect of soil moisture on soil respiration Soil respiration was significantly and negatively related to soil moisture in several sites (Fig. 3). Simultaneously, regression analysis between seasonal variation of soil respiration and soil moisture for six sites together showed negative relationship (Table 2). However, the maximum soil respiration was not inhibited by highest or lowest soil moisture in hot summer (Fig. 4). This result did not agree with that of Maestre and Cortina (2003), who reported a decrease in soil CO2 efflux at the highest temperatures when soil moisture values were lowest. It was also different from that highest fluxes occurred in spring when temperature and moisture were favorable in a cold desert (Fernandez et al. 2006). However, it supported previous studies where high respiration rates were observed in mid
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summer in spite of drought (Casals et al. 2000; Frank et al. 2002). Higher soil CO2 flux in summer may have been due to enhanced root respiration resulting from active root growth and increased microbial respiration from organic carbon transformation by higher microbial activity, both associated with high soil temperature (around 30°C) (Lou et al. 2003). In addition, roots were better able to maintain turgor than shoots under dry conditions and therefore gradually became the preferred sink for soluble carbohydrates of plants (Huck and Hillel 1983). As the canopy began to experience water stress at lower soil water contents, the fraction of soluble carbohydrates allocated to root growth increased, resulting in higher root respiration (Casals et al. 2000). Here that soil moisture did not limit soil respiration may reflect strong ability to endure drought for plants in desert. Furthermore, the mean organic carbon content is 3.34 g/kg in study sites, and litter is relatively smaller than other vegetation types, which might result in low microbial biomass and respiration due to inadequate substrate. In a word the influence of soil moisture on soil respiration might not be as strong as others reported in arid ecosystem. Soil respiration and soil properties Soil respiration was positively correlated with available K, while significant correlation was not found between soil respiration and soil organic carbon, available N, P in this study. Soil in study area suffered under low organic carbon and available N, P, only total P content was high and stable, and the content of available K variated in large range (Qian et al. 2003). Maestre and Cortina (2003) also found that soil respiration rates were not significantly correlated to soil organic matter in a semiarid steppe. It is probably noted that the nutrient content is small in desert soil, and microbial decay of soil organic matter is consequently small, so the relationships between soil respiration and organic carbon, N and P were not clear. Soil respiration was positively and significantly correlated with pH, CO32- and HCO3- in this study. Soil pH, CO32- and HCO3- content were all largest in HAH site (9.09, 0.07 g/L, 0.53 g/L, respectively). It was also found that the changes in soil CO2 respiration were in high correlation with that of the content of CaCO3 (r = 0.93, n = 12) in the southern Gurbantunggut Desert, China (Zhu et al. 2008). In the arid zone CaCO3 formed the calcium stratum easily (Duan et al. 1999). The reverse reaction of dissolution and re-precipitation processes would occur between CaCO3 and Ca (HCO3)2, which changed the motion state of CO2 gas in soil and influenced soil CO2 respiration (Emmerich 2003). Thus, carbonate releasing or absorbing CO2 might influence soil respiration rate in study area.
1863 Acknowledgments This study was supported by the importance directional project of the Knowledge Innovation Program of Chinese Academy of Sciences (No. KZCX2-YW-127), National Natural Science Foundation of China (No. 40671014) and National Science and Technology support plan (No. 2006BAC01A03). The authors thank Dr Su-fen Yuan, Ms Qian Liu, Li-ping Zhang and Hong-yan Pu for their assistance and help in the field work.
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