Spatial Heterogeneity of Soil Nutrients and Respiration in the Desertified Grasslands of Inner Mongolia, China

Pedosphere 20(5): 655–665, 2010 ISSN 1002-0160/CN 32-1315/P c 2010 Soil Science Society of China  Published by Elsevier Limited and Science Press

Spatial Heterogeneity of Soil Nutrients and Respiration in the Desertified Grasslands of Inner Mongolia, China∗1 QI Yu-Chun1 , DONG Yun-She1,∗2 , JIN Zhao2 , PENG Qin1,3 , XIAO Sheng-Sheng1,3 and HE Ya-Ting1,3 1 Institute

of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101 (China) of Earth Environment, Chinese Academy of Sciences, Xi’an 710075 (China) 3 Graduate University of the Chinese Academy of Sciences, Beijing 100049 (China) 2 Institute

(Received December 29, 2009; revised July 14, 2010)

ABSTRACT There is a limited knowledge of spatial heterogeneity in soil nutrients and soil respiration in the semi-arid and arid grasslands of China. This study investigated the spatial differences in soil nutrients and soil respiration among three desertified grasslands and within two shrub-dominated communities on the Ordos Plateau of Inner Mongolia, China in 2006. Both soil organic carbon (SOC) and total nitrogen (TN) were significantly different (P < 0.01) among the three desertified grasslands along a degradation gradient. Within the two shrub-dominated communities, the SOC and TN contents decreased with increasing distance from the main stems of the shrub, and this “fertile island” effect was most pronounced in the surface soil. The total soil respirations during the growing season were 131.26, 95.95, and 118.66 g C m−2 , respectively, for the steppe, shrub, and shrub-perennial grass communities. The coefficient of variability of soil respiration was the highest in the shrub community and lowest in the steppe community. CO 2 effluxes from the soil under the canopy of shrub were significantly higher than those from the soil covered with biological crusts and the bare soil in the interplant spaces in the shrub community. However, soil respiration beneath the shrubs was not different from that of the soil in the inter-shrub of the shrub-perennial grass community. This is probably due to the smaller shrub size. In the two shrub-dominated communities, spatial variability in soil respiration was found to depend on soil water content and C:N ratio. Key Words:

carbon emission, sandy grassland, soil organic carbon, spatial variability, total nitrogen

Citation: Qi, Y. C., Dong, Y. S., Jin, Z., Peng, Q., Xiao, S. S. and He, Y. T. 2010. Spatial heterogeneity of soil nutrients and respiration in the desertified grasslands of Inner Mongolia, China. Pedosphere. 20(5): 655–665.

Soil organic carbon (SOC) and total nitrogen (TN) are crucial indices of soil fertility (Liebig et al., 2006). Soil also plays a vital role in controlling global climate change because it acts as the main source and sink for greenhouse gases (Prentice et al., 2001; Lal, 2004). Arid or semi-arid grasslands are fragile ecosystems in which the organic C and N pools are relatively smaller compared with those in some other ecosystems. As a result, these grassland ecosystems are more sensitive to climate change and the disturbances of human activities (West et al., 1994). Grassland ecosystems in desertified regions are usually characterized by fixed and moving dunes with patchy vegetation dominated by psammophytic shrubs and grass as a result of long-term disturbance by human activities (Li et al., 2008). Shrubs and grass differ significantly in their ability to accumulate and conserve soil nutrients, with most soil nutrients congregating underneath patches of shrubs. This leads to a higher spatial heterogeneity of soil nutrient distribution and the alteration of soil C and N cycle characteristics. The spatial heterogeneity of soil C and N nutrients becomes one of the important indicators to characterize grassland desertification (K´efi et al., 2007). In recent years, there has been a growing concern about the spatial variability of soil nutrients. Many studies have investigated the formation and development of “fertile islands” in arid and semi-arid region, especially in the desert ∗1 Supported

by the National Natural Science Foundation of China (Nos. 40730105, 40501072 and 40973057) and the National “Eleventh Five Years Plan” Key Project on Science and Technology of China (No. 2007BAC03A11). ∗2 Corresponding author. E-mail: [email protected].

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ecosystem of North America (Titus et al., 2002; Thompson et al., 2006; Ruiz et al., 2008; Butterfield and Briggs, 2009) and in the African savannas (Dean et al., 1999; Mlambo et al., 2005; Okin et al., 2008; Wang et al., 2009). The spatial heterogeneity of surface soil properties and its influencing factors have also been discussed, respectively, using a traditional statistics method (Titus et al., 2002; Schade and Hobbie, 2005; Housman et al., 2007; Abril et al., 2009) or a geostatistics method (Schlesinger et al., 1996; Maestre and Cortina, 2002; McGrath and Zhang, 2003; Chen et al., 2006; Don et al., 2007). In addition, as the main mechanism of C transfer from the soil to the atmosphere, soil respiration is known to have high variation in space. Spatial coefficients of variation (CVs) for soil respiration within most ecosystems are above 10%, even up to 150% in some ecosystems (Stoyan et al., 2000), and are frequently higher than those of other soil factors (Fang et al., 1998). A number of factors have been studied to explain the spatial variations of soil respiration within a site and between sites in recent years, including root biomass (Han et al., 2007; Rodeghiero and Cescatti, 2008), microbial biomass (Xu and Qi, 2001; Scott-Denton et al., 2003), litter amount (Rayment and Jarvis, 2000; Saiz et al., 2006), quantity and quality of organic matter (Epron et al., 2004; Petrone et al., 2008), vegetation characteristics (Longdoz et al., 2000; Khomik et al., 2006), and soil texture or soil porosity (Dilustro et al., 2005), but the spatial variability in soil respiration is still poorly documented compared with temporal variability, with most data based on forest ecosystems and only a few from the dry lands. Biological soil crusts, composed primarily of cyanobacteria, algae, lichens, mosses, soil particles, and organic matter, are found in all deserts around the world and can occupy up to 70% of intercanopy space (Belnap and Lange, 2003). They have been proven to play an important role in fixing organic C and N through the activity of microorganisms and photosynthesis (Lange et al., 1997), effectively preventing soil erosion, directly or indirectly altering water infiltration, decomposition, and movement of organic matter and soil fines, and accordingly changing the biogeochemical cycle of ecosystem nutrients and CO2 exchange significantly (Thompson et al., 2005; Thomas et al., 2008). China is seriously threatened by desertification, with approximately 1.35 million km2 of desertified grasslands nationwide (Ma et al., 2003). To improve the understanding of nutrient spatial dynamics and to estimate accurately the C budget, three grassland communities along a degradation gradient (two of them are shrub-dominated communities) on the Ordos Plateau of Inner Mongolia were chosen for investigation and comparison of the variability of SOC, TN, and soil respiration at different scales. The objectives of this study were as follows: (1) to compare the SOC and TN contents and soil respiration rates along a degraded gradient in the three desertified grasslands, (2) to describe the effects of shrub on the spatial distribution of soil nutrients and soil respiration rates quantitatively in two shrub-dominated grasslands, and (3) to evaluate the contribution of environmental factors and soil crusts on the spatial variability of soil respiration. MATERIALS AND METHODS Site description The experimental sites were located in the Mu Us Sand Land of the Ordos Plateau in Inner Mongolia, China, and within the vicinity of the Ordos Sandy Grassland Research Station (39◦ 29 N, 110◦ 11 E, 1 335 m above sea level). The region, where desertification is identified as a serious problem, is considered to have an ecotone between grassland and shrubland. The area has a typically semi-arid continental climate with pronounced seasonal and diurnal temperature variations and low rainfall. Mean precipitation is 345.2 mm annually (93% falling between April and October), with a mean evaporation of 2 535 mm, and mean annual temperature is 6.7 ◦ C (Zheng et al., 2005). The soils are mainly light chestnut (Kastanozems, FAO/UNESCO), with a texture of approximately 300 to 400 g kg−1 clay and 600 to 700 g kg−1 sand, and aeolian sandy soil, with a texture of approximately 100 to 200 g kg−1 clay and 800 to 900 g kg−1 sand. Artemisia ordosica Krasch (A. ordosica) is one of the most prevalent semi-shrub species in the

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sandy grasslands of the Ordos Plateau. According to the differences in utilization intensity, the A. ordosica community probably develops in two directions. One is the replacement by Stipa bungeana Trin. (S. bungeana), a progressive succession caused by the condition of reduced utilization intensity or enclosure by fencing. The other is retrogressive succession in which the Cynanchum komarovii Al. (C. komarovii) partly substitutes for the A. ordosica, forming a mixed species community of A. ordosica plus C. komarovii and sometimes even a single dominant C. komarovii community. C. komarovii is a perennial grass and a strong xerophyte whose presence indicates serious desertification (Cheng et al., 2007). In the present study, three typical communities in the study area representing a degraded gradient in the process of grassland desertification were selected for sampling: (1) an S. bungeana steppe community, (2) an A. ordosica shrub community, and (3) an A. ordosica-C. komarovii shrub-perennial grass community. The S. bungeana site (steppe community) lies at 109◦ 11 37 E and 39◦ 23 43 N with 1 355 m above sea level. The major species of the community are S. bungeana, Cleistogenes songorica (Roshev.) Ohwi, Lespedeza dahurica (Laxim.) Schindl, Thymus serpyllum L. var. mongolicus Ronn, and Allium mongolicum Regel. The vegetation coverage is about 70%, with an average height of 20 cm. The A. ordosica sampling site (shrub community) is located at 110◦ 11 56 E and 39◦ 29 47 N with 1 335 m above sea level. The underlying surface is a fixed sandy land with widely distributed biological soil crusts, 2 to 6 mm in thickness. This community is dominated by A. ordosica, a xerophytic semi-shrub. Hedysarum fruticosum Pall., Pennisetum centrasiaticum Tzvel., Agropyron desertorum (Fisch.) Schult, Agropyron fragile (Roth) Candargy, and Astragalus melilotoides Pall. also coexisted in the community. The coverage of A. ordosica, with an average height of 69 cm and a canopy diameter of 71 cm, is about 53.56%. The A. ordosica-C. komarovii site (110◦ 11 02 E and 39◦ 29 54 N, 1 333 m above sea level), a shrub-perennial grass community with an underlying surface of semi-fixed sand land, lies about 500 m northeast of the A. ordosica sampling site. The dominant species of the community are A. ordosica and C. komarovii. Cleistogenes squarrosa (Trin.) Keng is the chief companion species. A. ordosica has an average height of 47 cm and a canopy diameter of 59 cm, with a 30.55% vegetation coverage, while the height of C. komarovii is about 22 cm, with a 5.4% vegetation coverage. Sampling and analyses To compare soil spatial heterogeneity among the three different grassland types, soils at each site were sampled at depths of 0–5, 5–10, 10–20, and 20–30 cm using a handheld auger. The soil was randomly sampled at the end of each month from May to October in 2006. Each sample was a mixture of 10 samples from the same layer. To assess the effects of individual plants on soil spatial heterogeneity, soils were sampled only in the shrub and shrub-perennial grass communities, where the semi-shrub A. ordosica is well developed. In these two communities, five A. ordosica individuals were selected randomly in May (to represent the early growing season) and in October (to represent the late growing season). To minimize potential shrub-shrub interactions, the shrubs chosen were those whose canopies were separated from the nearest neighbors by a gap of at least 100 cm. Starting at the main stem of the selected A. ordosica, six transects were drawn on the soil surface, separated by about 60◦ (Fig. 1). On every transect, soil cores (3.3 cm in diameter) were taken from the following: i) beneath the canopy and next to the main stem of A. ordosica (location 1, L1 ), ii) beneath the canopy and a half canopy diameter away from the main stem (location 2, L2 ), iii) at the edge of the canopy (location 3, L3 ), and iv) a half canopy diameter outside the edge of the canopy (location 4, L4 ). At each location soil cores were sampled at depths of 0–5, 5–10, 10–20, 20–30, and 30–40 cm, and six cores of the same layer collected in the same location (L1 , L2 , L3 and L4 ) of the different transects were mixed to reduce error. The same sampling procedure was carried out in the A. ordosica-C. komarovii community, but because the individuals of A. ordosica in this community were relatively poorly developed, the sampling locations were limited to L1 , L3 , and L4 . The samples were stored in plastic bags and immediately transported to the laboratory, where they were air-dried and later hand-sieved through a 2-mm screen to remove

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roots and other debris. Afterwards, the samples were pulverized in a grinder and passed through a 0.15-mm sieve. Sub-samples were analyzed for organic C using the K2 Cr2 O7 oxidation method (Tiessen and Moir, 1993; Su et al., 2006) and TN using the micro-Kjeldahl method (Bremner, 1996).

Fig. 1

Sampling locations for the spatial heterogeneity investigation of soil nutrients in two shrub-dominated communities.

The CO2 gas samples were mainly collected through a static closed opaque chamber during the growing season (May–October) of 2006. The use of this opaque chamber eliminated the influence of plant photosynthesis and the overly rapid rising chamber temperature during the measurements; its effectiveness for measuring CO2 efflux has been reported previously (Dong et al., 2000; Zou et al., 2004). In this study, the chamber, measuring 50 cm (length) × 50 cm (width) × 40 cm (height), was made of 8 mm thick of black acrylic material with a tinfoil reflecting film attached to the external surface. The lid of the chamber held a 12 V battery-operated fan for air circulation, a highly precise temperature sensor (with a resolution of 0.1 ◦ C) connected to a digital thermometer, a PVC tube gas channel, a silica gel pipe connected to a 200 mL syringe, and a three-way stopcock for gathering gas. The sketch map of the chamber set-up can be seen in the paper of Qi et al. (2007). In the steppe community of S. bungeana with relative uniform vegetation, six chambers were placed on soil where grass was cut to ground level one week before sampling. The results from the six chamber measurements were taken to determine the CV of soil respiration and later averaged to calculate the total soil respiration during the growing season in the steppe community. Due to the presence of patchy shrub and biological crusts in the A. ordosica shrub community, three different measurements were made: i) on bare soil; ii) on soils with biological crusts within interplant space; and iii) on soil beneath shrubs (above-ground vegetation was cut to ground level prior to sampling). In addition, three replications were conducted for each measurement. Six plots were also set in the A. ordosica-C. komarovii community. Three plots were positioned on the soil beneath the shrubs and three on the soil beneath the perennial grasses (above-ground vegetation was cut to ground level one week before sampling). Gases were collected every 10 days from May to October. The samplings were done at a relatively uniform time of day, which was around 9:00 to 11:00 local standard time in the morning, because respiration rates measured during this time are regarded to be representative of the daily average efflux (Kessavalou et al., 1998; Du et al., 2001, Xu and Qi, 2001). During the course of the measurements, the sampling chamber was placed into a groove of a stainless steel frame and sealed with distilled water, and the stainless steel frame was inserted 5 cm into the soil. Gas samples were extracted from the chamber at 0, 10, 20, and 30 min after closing the chamber. Each time about 200 mL of gas was extracted from the chamber and collected into polyethylene-coated aluminum gas bags. The CO2 concentrations were measured in the laboratory shortly after the sampling using an LI-6252 infrared CO2 analyzer (LICOR Inc., Lincoln, NE, USA). During each gas sampling, air temperature, soil temperature, soil water content, and the internal temperature of the chamber were recorded simultaneously. Temperature in the chamber was measured with a temperature sensor; air temperature was measured with a DHM2 mechanical ventilated ther-

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mometer; and soil temperature at depths of 0, 5, and 10 cm were measured with a SN2202 digital thermo detector (Sinan Instruments Plant of Beijing Normal University). An oven-drying method was used to determine the soil water content. Data calculation and statistical analysis The CO2 efflux was calculated using Eq. 1 below: F =

Δm V Δm D = hc D Δt A Δt

(1)

where F refers to CO2 efflux (mg m−2 h−1 ); V is the volume of the sampling chamber; A is the land area covered by the chamber; Δm/Δt denotes the linear slope of the concentration change with time over the measurement period; hc represents the height of the sampling chamber; and D is the gas density of the chamber (mol m−3 ), expressed as D = P/RT , where P is the air pressure; T is the temperature inside the chamber; and R is the gas constant. The enrichment factor (EF) reflecting the accumulative effect of shrub on soil C and N nutrients was calculated using the following Eq. 2: EFi = Ai /A4

(2)

where EFi refers to the enrichment factors of different locations surrounding the shrub, and Ai is the content of SOC or TN for sampling location i (i is from 1 to 4). The more EF differs from 1, the more the soil nutrient in location i differs from that in location 4. All the statistical analyses were conducted using the SPSS 11.0 software package (SPSS Inc., 2001). The one-way analysis of variance (ANOVA) was used to test the differences in soil respiration among the three communities along a degraded gradient and among the different locations within shrub and shrub-perennial grass communities, while the two-way ANOVA was performed on the data of SOC and TN from different layers of the three desertified grasslands. The multiple stepwise regression method was used to determine the contributions of environmental factors to the spatial difference in soil respiration among different locations within the two shrub-dominated steppes. The index of EF was estimated to analyze the “fertile island” effect of shrubs. RESULTS SOC, TN, and soil respiration among the three grassland communities along a degraded gradient Among the three communities along a degraded gradient of grassland vegetation (Fig. 2), the levels of both SOC and TN were the highest in the S. bungeana steppe community, intermediate in the A. ordosica shrub community, and lowest in the A. ordosica-C. komarovii shrub-perennial grass community. The SOC and TN in the three communities all decreased progressively with soil depth, in accordance with the vertical distribution of plant residues and roots. However, an exception was found at 10–20 cm depth in the steppe community, that is, SOC and TN levels were higher than those at 5–10 cm. Significant differences were found in both SOC and TN of all the sampling layers among the three sites (P < 0.05, two-way ANOVA). In descending order, the C:N ratios were the shrub community > the shrub-perennial grass community > the steppe community. Soil respiration of the three sampling sites is shown in Table I. Over the growing season, the native and lightly degraded steppe community, the moderately degraded shrub community, and the seriously degraded shrub-perennial grass community showed a total respiration of 131.26, 95.95, and 118.66 g C m−2 , respectively, which is inconsistent with a degraded gradient. The differences in total soil respiration during the growing season between the steppe and the shrub communities reached a significant level of 0.05, but those between the steppe and shrub-perennial grass communities or between the shrub and the shrub-perennial grass communities were not statistically significant.

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Fig. 2 Comparison of soil organic carbon (SOC), total N, and C:N ratio among the three communities along a degraded gradient on the Ordos Plateau of China. The data shown are based on the averages from May to October in 2006. TABLE I Comparison of soil respiration among the three communities along a degradation gradient on the Ordos Plateau in China Community type

Maximum soil respiration rate

Minimum soil respiration rate

Total soil respiration of growing season

40.18 36.31 59.43

g C m−2 131.26aa) 95.95b 118.66ab

mg m−2 h−1 Steppe Shrub Shrub-perennial grass a) Means

246.27 407.10 293.79

followed by the same letter(s) in the same column are not significantly different at P = 0.05.

Spatial distribution of SOC and TN surrounding shrubs and soil respiration within the three communities The horizontal and vertical distributions of SOC and TN are shown in Fig. 3. In the shrub community, the distributions of SOC and TN had stronger spatial heterogeneity, and the contents of SOC and TN in L1 were significantly higher than those in the other locations (Fig. 3a, b). The further away from the main stem, the lower SOC and TN contents became. The EF values of different locations, in decreasing order, were EF1 > EF2 > EF3 (Table II). In the shrub-perennial grass community, the EF values were smaller than those of the same locations in the shrub community (Fig. 3c, d). The spatial differences of SOC and TN in the shrub-perennial grass community were both smaller than those in the shrub community. In addition, the SOC and TN contents of the soils from different locations in the shrub community both attenuated markedly with the soil depth, with the highest found in the soil layer of 0–5 cm (Fig. 3a, b). The vertical variations of SOC and TN in the shrub-perennial grass community were more complicated (Fig. 3c, d), but the EF values decreased with the soil depth except in some separate layers (Table II). Similar to the horizontal heterogeneity of SOC and TN, the vertical variation of the shrub-perennial grass community was also smaller than that of the shrub community. Taking the L1 for example, the EF at the 0–5 cm soil depth for SOC was 1.74–2.08 times higher than those of the corresponding deeper soil layers and was 1.47–2.63 times greater for TN in the shrub community. In

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Fig. 3 The spatial variability of soil organic carbon (SOC) and total N among different locations in the shrub (a and b) and shrub-perennial grass communities (c and d) of the Ordos Plateau. TABLE II The enrichment factors (EFs) of soil organic carbon (SOC) and total N for the different locations in the shrub and shrub-perennial grass communities of the Ordos Plateau Item

SOC

Total N

Soil depth cm 0–5 5–10 10–20 20–30 30–40 0–5 5–10 10–20 20–30 30–40

Shrub community

Shrub-perennial grass community

EF1 a)

EF2

EF3

EF1

EF3

2.429 1.396 1.361 1.335 1.170 2.580 1.756 1.671 1.267 0.980

1.423 1.067 1.258 1.168 1.074 1.449 1.602 1.175 1.083 0.872

1.116 1.006 1.033 1.143 1.014 1.238 1.242 1.021 0.966 0.528

1.823 1.333 1.172 1.212 1.064 1.590 1.287 1.491 1.425 1.307

1.018 1.102 1.040 1.049 0.989 1.066 0.983 1.301 1.292 1.181

a) EF , 1

EF2 , and EF3 reflect the accumulated effects of shrub on SOC and TN for the sampling locations 1, 2 and 3, respectively.

contrast, the value for SOC was 1.37–1.71 times higher in the shrub-perennial grass community, while the value for TN was 1.07–1.24 times higher. In this study, the CVs for soil respiration rates within the three communities were calculated. CV was the highest in the shrub community (13.26%–52.78%) where shrubs were well developed, while it was the smallest in the steppe community (6.48%–27.29%), which mainly comprised of herbage. The average CVs of the growing season for the steppe, shrub-perennial grass, and shrub communities were 12.22%, 15.89%, and 36.71%, respectively. The soil respiration of different locations in the shrub community was in the decreasing order of soil beneath shrubs > soil covered with biological crusts > bare soil in the interplant spaces (Table III), which was in agreement with spatial distribution of SOC. Significant differences (P < 0.05) in soil respiration were found between soil beneath the shrubs and the other two treatments, but the difference was insignificant between the soil with and without crusts. In the

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TABLE III The comparison of soil respiration rates among the different locations in the two shrub-dominated grasslands of the Ordos Plateau Community type

Shrub

Shrub-perennial grass

Sampling location

Soil beneath shrubs Soil with biological crusts Bare soil in interplant spaces Soil beneath shrubs Soil beneath perennial grasses

Maximum soil respiration 407.10 217.85 144.73 293.79 190.56

Minimum soil respiration

mg m−2 h−1 69.69 36.02 36.31 62.37 59.43

Total soil respiration of growing season g C m−2 132.50aa) 85.80b 69.55b 130.16a 107.17a

a) Means followed by the same letter(s) within each column for the same community are not significantly different at P = 0.05.

shrub-perennial grass community, the soil respiration rates beneath shrubs were also larger than those of the interplant spaces, but this difference was not statistically significant (P = 0.431). DISCUSSION Effect of shrub on the spatial distribution of soil nutrients and soil respiration Shrub encroachment has a strong influence on the spatial distribution of soil nutrients and soil respiration in arid ecosystems (Schlesinger and Pilmanis, 1998; Cross and Schlesinger, 1999; Hibbard et al., 2001). Schlesinger and Pilmanis (1998) summarized the experimental results from desert shrublands and pointed out that the distributions of soil N, P, and K were strongly associated with the presence of shrubs in desert habitats. The biotic process leading to the formation and development of “fertile islands” includes plant uptake of essential nutrients, followed by the deposition of litter in the localized areas beneath the shrubs. By increasing soil moisture and protecting the understory soils from the effects of high temperature, shrubs also help to retain soil N, increase soil organic matter, and create local microsites for microorganisms (Cross and Schlesinger, 1999). The experiment conducted in the Negev Desert of Israel suggested that shrubs played an important role in determining the activity of microorganisms as moderators of abiotic variables, and that CO2 effluxes from the soil under the canopy of Hammada scoparia shrub were significantly higher than those from the bare soils (Berg and Steinberger, 2008). In this study, A. ordoscia individuals in the two shrub-dominated communities showed strong but discrepant “fertile island” effects; these could be caused by the differences in canopy diameter and plant density of A. ordoscia observed in the two communities. A. ordoscia individuals were taller with larger canopy diameters in the shrub community than in the shrub-perennial grass community; thus, they could trap more wind-blown fine soil particles and dust. Furthermore, the well-developed root system of A. ordoscia in the shrub community could have provided more root exudates and rhizosphere litter, making nutrients more concentrated under the shrub canopy (Li et al., 2008; Diedhiou et al., 2009). In addition, there were higher contents of SOC and TN as well as better water conditions under the canopy than in the other locations, which would enhance the activities of microbes in accelerating the decomposition and mineralization of the SOC. This is probably the reason why significantly higher (P < 0.05) soil respiration rates were observed under the shrub canopies. Effect of biological crusts on soil respiration In the shrub community where the biological crust is well developed, the respiration rates from the soils with crusts were insignificantly higher than those from bare soils. This is probably due to the dual effects of soil crusts on soil respiration. On one hand, higher SOC contents and microbial

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activity of biological crust, as well as the additional dark respiration caused by crusts, should markedly increase CO2 efflux of the soils with crusts. On the other hand, the crust has a compact structure and seals the soil surface (Li et al., 2008), thereby decreasing water infiltration to the subsoil. The heterotrophic bacterial activities of the subsoil will be suppressed due to the lower soil water content, resulting in minimal CO2 production in the subsoil layer (Thomas et al., 2008). Result obtained by Cable and Huxman (2004) indicated that over 80% of the efflux originated from surface crusts during small wetting events, whereas after a large precipitation pulse, 98% of efflux originated from the subsoil. In this study, soil respiration of subsoil contributed little to the whole CO2 efflux in the treatment with crusts because CO2 effluxes were mostly measured during fine days. Furthermore, the compacted structure of crusts would also impede the diffusion of CO2 out of soils to a certain extent (Maestre and Cortina, 2003). Contribution of environmental factors to the spatial variability of soil respiration The reasons for the spatial variation of soil respiration can vary depending on the type of community and site for sampling. Han et al. (2007) reported that the interactions among soil temperature, soil moisture, root biomass, and net primary productivity largely controlled the spatial variation in soil respiration. Stoyan et al. (2000) indicated that there was a highly significant correlation between soil respiration and moisture in poplar plots, but the correlation between soil respiration and root weight was the highest in wheat plots. The variation of soil moisture and root weight can help to model soil respiration and explain some of the reasons behind the extreme heterogeneity in two sites. Xu and Qi (2001) found that soil temperature and moisture together explained less than 34% of the spatial variation of soil CO2 efflux and that microbial biomass, fine root biomass, soil nitrogen, soil organic matter, and soil magnesium were positively correlated with soil CO2 efflux, whereas bulk density and pH value had a negative correlation with CO2 efflux. In this study, the soil moisture and C:N ratio were found to contribute more to the spatial difference of soil respiration in different locations within shrub-dominated communities (Table IV). Soil moisture appeared to be the main limiting factor for the vegetation growth and nutrient cycles in semi-arid and arid grasslands, while the C:N ratio was a combined response of root biomass, microbial biomass and activity, and the water-heat conditions, all of which were consistent with the previous studies. TABLE IV Contribution of environmental factorsa) to the spatial differences in soil respiration among different locations within two shrub-dominated steppes Community type

Regression equation

F value

P

R2

Shrub Shrub-perennial grass

Y = 1.1433X5 − 2.288X9 Y = 1.1392X7

2 169.4 642.1

< 0.05 < 0.05

0.999 0.997

a) Environmental factors involved in the regression process include: X , surface ground temperature (◦ C); X and X , 1 2 3 soil temperatures at 5 and 10 cm depths (◦ C), respectively; X4 , temperature inside sampling chamber (◦ C); X5 and X6 , 0–10 and 10–20 cm soil water contents (g kg−1 ), respectively; X7 –X10 , C:N ratios of 0–5, 5–10, 10–20, and 20–30 cm soil layers, respectively; and Y , soil respiration rate (mg m−2 h−1 ).

ACKNOWLEDGEMENTS The authors are grateful to Dr. CHU Yu and Mr. JIANG Bin from the Ordos Sandy Grassland Research Station, Chinese Academy of Sciences, for their field assistance. Special thanks are also given to anonymous referees for their helpful reviews and constructive suggestions. REFERENCES Abril, A., Villagra, P. and Noe, L. 2009. Spatiotemporal heterogeneity of soil fertility in the Central Monte desert (Argentina). J. Arid Environ. 73: 901–906.

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