Abstract
The Alxa desert steppe has been strongly degraded by overgrazing, contributing c. 22% of the total springtime dust originating from Asia. Previous work in this region has focused on the impacts of grazer exclusion on restoration of vegetation and soil fertility, yet carbon dynamics are not well known. The effects of 7 years of grazer exclusion on carbon dynamics were studied and related to changes in vegetation and soil properties. Removal of grazing resulted in a significantly greater plant cover and aboveground plant biomass compared with areas that had been subject to grazing, but this had no effects on belowground plant biomass. Removal of grazing resulted in significantly decreased soil bulk density in the 0–10 cm layer, increased soil water content (7% cf. 40%) and greater soil microbial biomass C (6% cf. 73%) compared with soils in the grazed area. Soil organic carbon (SOC) pools were lower and soil inorganic carbon (SIC) pools were higher in areas that were excluded from grazing. After 7 years of grazer exclusion, the total C pool in the plant–soil system was 10% greater (primarily due to 21% greater in SIC) than that in the area that had been grazed over that time period.
Introduction
Motivated by the rapid increase in atmospheric CO2 due to human activities (Amthor Citation1995; Luo et al. Citation2007), much research has focused on ways to mitigate CO2 emissions or remove CO2 from the atmosphere, including sequestering carbon (C) in the terrestrial biosphere (Marland et al. Citation2001; Lal Citation2002, Citation2004; Reeder et al. Citation2004; Li et al. Citation2008; Manning Citation2008; Xie et al. Citation2009). Carbon sequestration in terrestrial ecosystems occurs through physiochemical and biological processes (Schimel et al. Citation1994; Williams et al. Citation2000). The amount of C stored in soils in organic forms is the net result of biological inputs mainly from plant biomass and outputs through leaching or decomposition by both abiotic processes and microbial activity (Post & Kwon Citation2000; Gallo et al. Citation2006). In contrast, C accumulated in soils in an inorganic form is regulated by dissolution and precipitation of carbonate (Lal & Kimble Citation2000; Lal Citation2004). Indeed, while much attention has been given to methods for enhancing organic matter inputs and reducing decomposition outputs to ecosystems as a strategy for C sequestration (Batjes & Sombroek Citation1997; Williams et al. Citation2000), more recently the potential of C sequestration through formation of pedogenic carbonates has gained recognition (Lal & Kimble Citation2000; Monger and Martinez-Rios Citation2001; Reeder et al. Citation2004; Manning Citation2008).
CO2 is produced in soils from respiration by roots and microbes and can reach concentrations hundreds of times greater than atmospheric CO2 (Karberg et al. Citation2005). In the soil solution, CO2 can react with water to form dissolved inorganic C (e.g. 
,
) that can precipitate as a pedogenic carbonate of Ca2 + or Mg2 + and, under pH ranges from 6.4 to 10.3, adequate
is generally available for the precipitation of carbonate (Monger & Martinez-Rios Citation2001). Evapotranspiration exceeds precipitation in dry land ecosystems, leading to wet–dry cycles and alkaline conditions that promote carbonate formation in soils (Lal Citation2004). Because of the vast area of dry land (about 47.2% of Earth's land surface) and its large soil C reservoirs (soil organic carbon (SOC)≈482 Pg and soil inorganic carbon (SIC)>866 Pg (Eswaran et al. Citation2000; Lal Citation2004)), scientists and policy makers are interested in the processes governing the dynamics of SOC and SIC pools and the potential of dry land soils to act as sinks for CO2 (Lal & Kimble Citation2000).
Presently, land degradation and desertification are pervasive in dry land ecosystems, often resulting in increased losses of CO2 into the atmosphere (Lal Citation2004). , Grazing is the predominant land use in dry land ecosystems (Lal Citation2004), and overgrazing leads to desertification by reducing organic matter inputs to soil and decreasing vegetative cover, resulting in erosion-vulnerable soil with poor water-holding capacity and low fertility and microbial activity (Conant & Paustian Citation2002; Su et al. Citation2005; Raiesi & Asadi Citation2006; Zou et al. Citation2007; Steffens et al. Citation2008). Conversely, reducing grazing frequency or intensity leads to vegetation recovery and restoration of soil fertility (Abril & Bucher Citation1999; Lal Citation2004; Su et al. Citation2004, Citation2005; Pei et al. Citation2008). SIC pools may be altered with grazer exclusion due to the strong synergistic interaction between organic matter inputs and precipitation of pedogenic carbonates (Lal Citation2004). For example, the recovery of vegetation after grazing removal may increase soil CO2 concentration through decomposition of organic matter by soil microorganisms, potentially enhancing chemical weathering and thus cation availability, and accelerating the formation of pedogenic carbonates (Lal Citation2004; Karberg et al. Citation2005). However, the extent to which microbial biomass, SOC and SIC are modulated by soil properties with grazer exclusion is unclear (Conant & Paustian Citation2002; Su et al. Citation2005; Chou et al. Citation2008; Pei et al. Citation2008).
The grasslands of Inner Mongolia, China (c.78.8×106 ha) are among the largest dry land biomes in the world (Steffens et al. Citation2008). Because of deterioration of vegetation and destruction of soil structure by long-term overgrazing over the past few decades, desertification is significant in this region, resulting in frequent severe dust storms that transport 200–300 Tg soil to the North Pacific Ocean each year (Prospero et al. Citation2002; Chen et al. Citation2003; Zhang et al. Citation2003; Wang et al. Citation2004b). The Alxa region is in the west of Inner Mongolia, and livestock pressure in this region has exceeded the local recommended carrying capacity four- to five-fold from 1950 to 2000 (RRIMEC Citation1990). Consequently, productivity and vegetative cover of these grasslands has declined and soil has become degraded (Pei et al. Citation2008), contributing a quarter of the total springtime dust originating from Asia and producing the highest rate of dust emissions in China (Zhang et al. Citation2003). Previous work in this region has focused on the impacts of grazer exclusion on restoration of vegetation and soil fertility (Pei et al. Citation2008). Carbon pools may also be affected by this shift in management, but to date the impacts of grazing exclusion on ecosystem C dynamics are unknown.
This study used the establishment of the 5400 km2 Helan Mountain Reserve as an opportunity to study the consequences of grazer exclusion on SOC and SIC pools within the plant–soil system and to relate differences in soil C to changes in vegetation and soil properties. It was expected that removal of grazing would lead to:
| 1. | increased plant biomass (thereby possibly increasing the SOC pool) | ||||
| 2. | increased soil moisture, possibly enhancing microbial activity and decomposition of Soil Organic Matter (SOM) (thereby increasing soil CO2 concentration for carbonate precipitation, but possibly decreasing the SOC pool) | ||||
| 3. | increased carbonate pools (from enhanced soil moisture, microbial activity and dust capture by recovered vegetation). | ||||
Materials and methods
Study site
This study was performed within and outside the Helan Mountain Natural Reserve (105°50′08′′E, 38°51′30′′N, altitude 2100 m), about 10 km east of Bayanhot city, Alxa county, in the western part of Inner Mongolia in China (Fig. 1). This region belongs to the temperate zone and has a distinct continental arid climate. According to local weather records (averages from 1958 to 2007), the mean annual precipitation (MAP) of the region is 210 mm, with the majority (70%) falling between June and September. Precipitation is quite irregular from one year to another and shows strong seasonal variability (Fig. 2). The mean annual air temperature is 8 °C, while wind speed averages 2.9 m/s. The frost-free period is 120–180 days. The vegetation in this study area is desert steppe dominated by the perennial bunchgrass Stipa breviflora Griseb and the shrub Artemisia frigida Wild. Soils in this region are classified as calcisols according to the soil classification system of the Food and Agriculture Organization (FAO Citation2006). The calcic horizon is below 40 cm in the soil profile and large cobbles are found below 60 cm.
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22 August 2011Figure 1 Map showing the study area and its location in North China. Study sites in/near the Helan Mountain Natural Reserve (105°50′08′′E, 38°51′30′′N, altitude 2100 m).
Figure 1 Map showing the study area and its location in North China. Study sites in/near the Helan Mountain Natural Reserve (105°50′08′′E, 38°51′30′′N, altitude 2100 m).
The Helan Mountain Reserve (180 km×30 km) was established in March 2001, functionally excluding Helan Mountain from grazers. Before it was established, the grassland was heavily grazed by sheep and goats from 1950 to 2000. Herds were put into pens for the night, resulting in limited return of faecal organic material to the soil system. As a result, the regional desert steppe has been severely degraded (Pei et al. Citation2008). Outside the reserve, the grassland has continued to be grazed at an intensity of c. 2 sheep per hectare, four-fold higher than the local recommended carrying capacity (RRIMEC Citation1990). At the time of establishment of the reserve, five large sample plots (100 m×50 m) were randomly located both within (‘Exclosure’) and outside (‘Grazed’) the fenced reserve boundary; one 100 m transect was established within each of the five replicate plots within each treatment for measurement of plant and soil characteristics. At that time, the basic physical–chemical properties of the soil were investigated (sampling method described), which showed soil properties in each treatment were similar at the beginning of the experiment (). The distance between the boundaries of these two treatments (separated by a fence) was 20 m, and the vegetation composition, elevation and geomorphology were similar.
Table 1 The basic physical–chemical properties of the soil before grazer exclusion was performed (standard errors of mean (SEM) in parentheses). There is no significant difference (P ≤0.05) of each parameter between treatments within the same soil depth.
Sampling and measurements
Plant sampling
Plant samples were collected in September 2003, 2005 and 2007 from 1 m×1 m quadrats located at 10 m intervals along each 100 m transect (ten quadrats per transect averaged into one value per replicate plot; n=5 plots each within ‘Grazed and ‘Exclosure’ treatments). Within each quadrat, vegetation cover was estimated by dividing the quadrat into 100 grid points and estimating the percentage cover of vegetation. Plants were clipped to ground level and sorted by species for estimation of aboveground biomass. New 1 m×1 m plots for biomass harvests were chosen at each sampling period to avoid formerly clipped plots. In 2007, belowground biomass in five of the ten clipped quadrats in each transect was sampled to a depth of 60 cm by excavating 20 cm width×20 cm length×60 cm depth clods (Wang & Wang Citation1999). Roots were separated by flotation and hand washing through a 0.5 mm sieve (Wang & Wang Citation1999; Jastrow et al. Citation2000; Pucheta et al. Citation2004). The plant biomass in the 0–5 cm soil layer was divided into two parts: base of the shoot and root. The roots were sorted into living and dead root categories by colour and texture (McNaughton et al. Citation1998).
Soil sampling
Soil sampling was performed in March 2001 and September 2005 and 2007. Ten samples, located at 10 m intervals along each 100 m transect, were taken each time. In 2001, soil samples were collected with a soil auger (4 cm diameter×25 cm length) at two depths (0–20 and 20–40 cm). In 2005 and 2007, soil samples were collected with a soil auger at three depths (0–10, 10–20 and 20–40 cm). In 2007, soil samples at 40–60 cm depths (in the calcic horizon) were collected by excavation with a small scoop. Soil samples from the same depth in the same transect were homogenised to form one composite sample per transect for each soil layer (n=5 soil samples per depth per treatment). Soil samples were placed in sealed plastic bags and transported in coolers to the laboratory for processing. Additionally, soils in five of the ten clipped quadrats in each transect were excavated for measurement of bulk density (per depth interval) and soil properties (for the calcic horizon at 40–60 cm depth, in 2007 only). Soil bulk density was determined by the core method using a steel cylinder (100 cm3) of diameter 5 cm and height 5.1 cm (Feng et al. Citation2002b).
Laboratory measurements
All plant biomass samples were dried in an oven at 60°C for 72 h and weighed to calculate the total dry biomass. For measurement of soil properties in the laboratory, all the soil samples were quickly passed through a 2 mm sieve to remove plant crowns and debris and then half of each soil sample was stored at 4 °C. Field-moist soil sub-samples were dried at 105 °C for 12 h to determine gravimetric water content (ISSCAS Citation1978) and the remaining soils were air-dried.
Soil microbial biomass C (MBC) and microbial biomass nitrogen (MBN) were measured using the fumigation–extraction method, using field-moist soils that were allowed to warm to room temperature after having been stored at 4 °C for 10 days (Brookes et al. Citation1985; Vance et al. Citation1987; Williams et al. Citation2000; Silvan et al. Citation2003). MBC and MBN were estimated as the difference between C or N extracted with 0.5 M K2SO4 from chloroform-fumigated and non-fumigated soil samples using a conversion factor of 0.38 (for C) and 0.45 (for N), respectively. Extractable C was measured by the dichromate oxidation method and extractable N by the Kjeldahl method (Lovell et al. Citation1995). Air-dried soil samples were analysed for pH using a 1:1 ratio of soil to distilled water (g/g). Additionally, sub-samples of air-dried soil were finely ground to pass a 0.5 mm sieve and analysed for SOC using the dichromate oxidation method (Pei et al. Citation2008; Sinha et al. Citation2009). Soil total carbon (STC) was analysed by combustion of a 150 mg soil sample in an elemental analyser (Elementar-LiquiTOC, Germany). SIC was determined by subtracting SOC from STC. Total nitrogen (Nt) and plant-available soil nitrogen (Nava) were analysed by the Kjeldahl method and distillation method (alkaline solution diffusion method), respectively (ISSCAS Citation1978). Plant C content (% of dry mass) was analysed using a Thermo CHNS/O analyser (FlashEA1112, USA).
Data analyses
Total C in the plant–soil system described here includes the sum of all plant and soil C pools to 60 cm depth. Total plant C was estimated as the sum of aboveground and belowground biomass C pools, calculated by multiplying the C content of plant tissue by the corresponding plant biomass. Total soil C pools (in kg/m2) were estimated using a depth-weighted average of total organic C plus total inorganic C (Schlesinger Citation1990).
All statistical tests were run using SPSS (v. 11.0; Chicago, USA). Differences between treatments and year were analysed using analysis of variance (ANOVA). For plant aboveground biomass and plant cover, data collected over multiple years were analysed using two-factor ANOVA with year and grazing treatment as fixed factors followed by simple effects analysis for comparison between treatments within the same year and comparison between years within the same treatment (Field 2005). For the soil parameters (except 40–60 cm), data collected over multiple years were analysed using three-factor ANOVA analyses with year, grazing treatment and soil depth as fixed factors followed by simple effects analysis for the comparison of grazing treatment within each soil depth and the comparison between years. For belowground biomass, total soil C in the plant–soil system and soil parameters in the 40–60 cm layer (all estimated only once), the effect of grazing treatment was assessed using independent-sample t-tests. The original data were transformed as necessary to satisfy assumptions of normality and equal variances before statistical analysis. All the comparisons were considered significantly different at P≤0.05.
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22 August 2011Figure 2 Monthly precipitation over the years of sampling in 2003, 2005 and 2007; also shown is the long-term average (1958 to 2007).
Figure 2 Monthly precipitation over the years of sampling in 2003, 2005 and 2007; also shown is the long-term average (1958 to 2007).
Results
Vegetation response
The effects of grazer exclusion on vegetation characteristics are presented in . Total aboveground biomass and plant cover were significantly higher within the exclosure than on the grazed site, but the strength of these responses depended on year (treatment×year, P<0.05). Removal of grazing significantly decreased belowground biomass at the base of plant shoots but there were no significant differences between treatments in total root biomass (including living and dead roots). As a result, no significant difference in plant belowground biomass was found between the treatments. Differences in plant community composition were also observed between treatments (). Removal of grazing increased A. frigida dominance in the exclosure, whereas S. brevifora was dominant in the grazed site. In 2005, the percentage of A. frigida of total biomass in the exclosure reached 64%; in contrast, the biomass contribution of S. brevifora was 69% in the grazed treatments.
Table 2 Plant biomass and plant cover as affected by long-term grazer exclusion (SEM in parentheses). Uppercase letter indicates significant difference (P≤0.05) of each parameter between treatments within the same year; lowercase letter indicates significant difference (P≤0.05) of each parameter between the years within the same treatment.
Soil response
Analysis of variance showed significant differences in water content, bulk density and pH between the grazed and exclosure treatments and that these differences depended on depth and year (). At the time of sampling, soil water content was significantly higher in the exclosure compared with the grazed treatment at all depths in all years except the 0–10 cm category in 2005. Moreover, the effect of soil depth on water content depended on the year of sampling: soils were drier in both treatments at 0–10 cm in 2007 relative to 2005, but they remained moist at 20–60 cm. Removal of grazing decreased bulk density only at the soil surface (0–10 cm). Soil pH at all depths was generally higher in the exclosure than the grazed site. Soil pH increased in the 10–20 cm and 20–40 cm soil layers in the grazing treatment from 2005 to 2007, but exclosure soils were always more alkaline than grazed soils.
Table 3 Soil water content, bulk density and pH in the soil profile as affected by long-term grazer exclusion (SEM in parentheses). A indicates significant difference (P≤0.05) of each parameter between treatments within the same soil depth and year; a indicates significant difference (P≤0.05) of each parameter between the years within the same soil depth and treatment.
Total and plant available soil N decreased with soil depth (). However, there were no significant differences in Nt and Nava between the exclosure and grazed treatments (). Nava increased from 2005 to 2007 in both treatments; these increases were significant for the 0–10 cm depth in both treatments and for the 20–40 cm depth in the exclosure. The ratio of SOC/Nt increased with soil depth to 40 cm. However, the effect of the grazing exclosure was significant only in 2007 and at depths below 20 cm.
Table 4 Soil total nitrogen (Nt), plant available nitrogen (Nava) and SOC/Nt ratio in the soil profile as affected by long-term grazer exclusion (SEM in parentheses). A indicates significant difference (P≤0.05) of each parameter between treatments within the same soil depth and year; a indicates significant difference (P≤0.05) of each parameter between the years within the same soil depth and treatment.
Analysis of variance showed that removal of grazing increased MBC, MBN and the MBC/SOC ratio relative to the grazed treatment and that these effects depended on both depth and year (). Soil MBC was significantly higher in the exclosure only in the subsoil in 2005 but in both surface and subsurface soils in 2007. Similarly, MBN was higher in the exclosure but only at depth in 2007. MBC and MBN in both soil depths increased in the exclosure but not the grazed site over time. In contrast, MBC at both soil depths in the grazed site decreased over time. The fraction of SOC made up of microbial biomass (MBC/SOC ratio) was significantly higher in the exclosure than the grazed site in 2007. Similarly, while the MBC/SOC ratio declined in the grazed site in both soil depths from 2005 to 2007, the ratio remained the same for surface soils in the exclosure and increased in subsurface soils over this 2 year period.
Table 5 Soil MBC and MBN as affected by long-term grazer exclusion (SEM in parentheses). A indicates significant difference (P≤0.05) of each parameter between treatments within the same soil depth and year; a indicates significant difference (P≤0.05) of each parameter between the years within the same soil depth and treatment.
Removal of grazing increased SOC content at 0–10 cm depth but decreased SOC content below 20 cm depth in the soil in 2007, 7 years after exclusion began (). SOC content increased in the soil surface (0–10 cm) over time in the exclosure treatment, but declined at 20–40 cm depth. In contrast, subsurface SIC content (10–40 cm) was higher in the exclosure than in the grazed soils and it increased over time. Overall, removal of grazing increased STC at 10–40 cm depths relative to the grazed site.
Table 6 Soil organic carbon (SOC), soil inorganic carbon (SIC) and soil total carbon (STC) in the soil profile as affected by long-term grazer exclusion (SEM in parentheses). A indicates significant difference (P≤0.05) of each parameter between treatments within the same soil depth and year; a indicates significant difference (P≤0.05) of each parameter between the years within the same soil depth and treatment.
Total C in the plant–soil system
Total C pools in the 0–60 cm plant–soil system within the grazed and exclosure sites are shown in . Although the effect was relatively small, grazing exclusion significantly increased aboveground biomass C (0.031±0.005(SD) kg/m2 exclosure; 0.021±0.001 kg/m2 grazed), but had no effect on belowground biomass C. Grazing exclusion also decreased SOC pools (7.23±0.16(SD) kg/m2 exclosure; 8.17±0.19 kg/m2 grazed). However, the largest effect of grazing removal was evident in the SIC pool. SIC content was 20.22±0.91(SD) kg/m2 in the exclosure compared to 16.67±1.45 kg/m2 in the grazed site. Assuming soils were similar to one another prior to the exclosure treatment being instigated, grazing exclusion for 7 years increased the 0–60 cm SIC and total soil C pools by 21.3% and 10.5% respectively. Overall, total C in the plant–soil system to 60 cm depth increased by 10.2% with grazing removal, from 25.2±1.37(SD) to 27.8±0.83 kg/m2.
Table 7 Total C pools in the plant–soil system (0–60 cm soil depth) after 7 years of grazing exclusion (SEM in parentheses). A indicates significant difference (P≤0.05) of each parameter between treatments.
Discussion
Grassland restoration following grazer removal
Comparison of plant primary production and plant cover between managed and reference ecosystems is often used to evaluate the degree of ecosystem degradation (de Soyza et al. Citation1998). In agreement with observations by Pei et al. (Citation2008), the data presented here show that removal of grazing significantly increased aboveground plant biomass and plant cover. The lower aboveground plant biomass and plant cover in the exclosure were probably due to grass intake by the livestock. Additionally, in this area, herds were put into pens for the night, which also precluded the return of faecal organic material to the soil system, and further limited plant primary production. The results thus support the viewpoint that grazer exclusion is an effective way to restore vegetation and attenuate soil loss by wind (Lal Citation2004; Su et al. Citation2004). In addition, the significant treatment-by-year interaction in this study suggests that interannual climatic variation is another important factor affecting responses of aboveground plant biomass and cover. For example, even after only 3 years of grazing exclusion, aboveground biomass and cover was highest in the exclosure site in 2003, which was a wet year relative to the long-term average (328 mm rainfall in 2003 compared with 210 mm long-term average (Fig. 2). Despite significant changes in aboveground biomass, however, removal of grazing did not alter belowground plant biomass, possibly due to the relatively short duration of the exclosure treatment. For example, Su et al. (Citation2005) found that plant belowground biomass increased only after 10 years of grazing exclusion in a degraded sandy grassland in northern China.
Vegetation community composition was also strongly affected by grazing removal in this work. A. frigida became dominant within the exclosure over the 7 year period since establishment of Helan Mountain Reserve, while S. brevifora remained dominant in the grazed site outside the reserve fence (). However, these species responses may not be generalisable to other sites. For example, Liu et al. (Citation2002) found the opposite pattern in a S. brevifora desert steppe in northern China, where A. frigida became dominant with heavy grazing over time. In another study in China, Wang et al. (Citation1999) showed that A. frigida increased during the first 5 years after grazer removal but declined thereafter. The underlying reasons for these different responses are unclear.
In addition to biotic changes, grazing exclusion had clear impacts on soil physical and chemical properties in the desert steppe ecosystem (). Soil moisture was higher in the exclosure relative to the grazed site, despite two-fold greater plant biomass. A. frigida is a species with pubescent leaves and high water use efficiency (Zhou & Zhao Citation2002), which may lead to less transpiration than expected based on biomass alone. More importantly, however, increased plant cover in the exclosure likely shaded soil surfaces, leading to lower soil temperatures and reduced evaporation. Additionally, surface soils in the exclosure site were less dense due to decreased trampling by livestock, which likely increased water infiltration to depth (Stavi et al. Citation2008). The results of the current study are supported by other research carried out in a desert steppe of Western China (Pei et al. Citation2008) and Chaco savannah in Argentina (Abril and Bucher Citation1999).
Other studies in grassland systems have shown that heavy grazing decreases total N stocks in soil, which recover after grazing is stopped (Pei et al. Citation2008; Steffens et al. Citation2008). Soil N depletion with grazing is likely due to N removal with the actual livestock themselves (Reeder et al. Citation2004), especially where herds are put into pens at night resulting in limited N return with faecal organic material to the soil system. In our study, no significant differences in soil N were found as a function of grazing removal (), possibly because any excess N was absorbed by plants to support higher plant aboveground biomass in the exclosure (Berg et al. Citation1997). Su et al. (Citation2005) also reported no significant change in soil total N after 10 years of grazing exclusion in a degraded sandy grassland. The increase in Nava in both treatments in 2007 relative to that in 2005 may reflect higher precipitation in 2007, which may have enhanced mineralisation of soil organic N.
Effects of grazer exclusion on soil C pools and distribution
Soil water content is fundamental to dry land ecosystem functioning because it not only drives plant productivity but also regulates biogeochemical processes in soils (Williams et al. Citation2000; Stavi et al. Citation2008). Among other factors, soil microbes regulate SOM decomposition and thus influence the accumulation of SOC (Raiesi & Asadi Citation2006). In dry lands, enhanced soil moisture could increase microbial activity, which could accelerate the decomposition of SOM (Li et al. Citation2004; Stavi et al. Citation2008). Despite higher soil moisture in the exclosure, grazing removal had no impact on total SOC content (), perhaps due to the counteracting effects of increased aboveground inputs and increased microbial activity in the moister exclosure soils (Kieft Citation1994; Williams et al. Citation2000; Shrestha & Stahl Citation2008). In support of this hypothesis, removal of grazing increased MBC (), similar to patterns found in other locations (Raiesi & Asadi Citation2006; Shrestha & Stahl Citation2008). Additionally, the results of this study show that the fraction of SOC made up of microbial biomass (MBC/SOC) was higher in the exclosure, suggesting that grazing removal altered the quality of organic matter (Sparling Citation1992; Kieft Citation1994; Shrestha & Stahl Citation2008), possibly leading to increased SOM decomposition. Consistent with this pattern, Jia et al. (Citation2006) reported that SOC/N ratios of 5.6-11.3 enhanced N mineralization and significantly increased microbial biomass in a semi-arid agro-ecosystem. In the current study, the range of SOC/N was 7.30–9.31, supporting the premise that grazing removal may have accelerated SOM decomposition.
While grazing exclusion had little impact on SOC content, removal of grazing significantly increased SIC content at depth (). This pattern could be caused by changes in the carbonate precipitation process in the plant–soil system (e.g. OM→SOC→CO2→
→CaCO3) (Lal Citation2004; Manning Citation2008; Schlesinger Citation1990). In soils, the C source for SIC precipitation is CO2, mainly from root respiration and decomposition of SOM. Enhanced soil water content within the exclosure may have stimulated root respiration and soil microbial activity, which in turn may have increased soil CO2 concentration and accelerated the translocation of cations (e.g. Ca2+) from primary calcium-bearing minerals (carbonate, silicate minerals). 90% of the belowground biomass in the study site was found in the top 40 cm of the soil, similar to the depths of increased SIC content. In this case, precipitation of pedogenic carbonates may have occurred (Karberg et al. Citation2005). Pan & Guo (Citation2000) showed that SOC and SIC were negatively related in arid soils, supporting a mechanism linking SOC to the precipitation of pedogenic carbonates.
The availability of cations (e.g. Ca2+) is often the limiting step in the formation of pedogenic carbonates in arid ecosystems (Manning Citation2008). Plants often play a role as ‘ion pumps’ of elements in a plant–soil system. In other words, plants absorb mineral elements from the soil through roots in both lateral and vertical directions to support aboveground production; upon senescence, they release these elements to surface soils underneath plant canopies, creating ‘islands of fertility’ (Schlesinger & Pilmanis Citation1998). Preliminary data from the study site suggest that tissues of A. frigida are more concentrated in Ca and Mg (Ca, 0.36%; Mg, 0.68%) than S. brevifora (Ca, 0.13%; Mg, 0.25%). Thus, the increase of A. frigida biomass in the exclosure may have contributed to increased Ca or Mg availability in the upper soil profile (above 20 cm), enhancing pedogenic carbonate precipitation. Moreover, with increasing plant cover in the exclosure, mineral elements might become more available from trapped aeolian dust (Semb et al. Citation1995; Feng et al. Citation2002a; Ridgwell Citation2002; Cao et al. Citation2005; Luo et al. Citation2007), resulting in a net gain of cations to the plant–soil system. In this case, net C sequestration could occur, leading to increased atmospheric CO2 storage in inorganic C pools. Altogether, increased root and microbial activity and addition or redistribution of cations in the soil profile from increased water availability could explain larger SIC contents between 10 and 40 cm depth in exclosure soils (Reeder et al. Citation2004).
Removal of grazing increased the total size of the C pool in the plant–soil system to 60 cm depth (). Assuming SOC content was similar between the grazed and exclosure soils prior to the beginning of the treatment, over the almost 7 year restoration period, the pool of SOC decreased by 11.5% in exclosure soils (0.94 kg/m2) compared with the grazed site (explained by a decrease in surface soil bulk density (Berg et al. Citation1997) but aboveground biomass C increased by 44.8% (0.01 kg/m2) and the SIC pool increased by 21.3% (3.55 kg/m2). Combined, grazing exclusion led to an increase in C within the plant–soil system of 10.2% (2.56 kg/m2). With regard to the C lost with the biomass harvested by grazing animals, it can be roughly calculated based on the grazing intensity (c. two sheep per hectare) and dry matter intake (0.4 kg dry forage per sheep per day−1 (RRIMEC Citation1990)). This equates to 204 g/m2 plant biomass taken out from the grazed ecosystem with grass intake by the animals over 7 years, compared with 90 g C lost from the grazed treatment. However, it cannot offset the total C difference between exclosure and grazed treatments. Because soil is the largest pool of C in desert grasslands (about 72–96 times higher than plant biomass in the study site), the increase of total C in this system is mainly attributable to the increase in subsurface SIC pools. SIC in the top 60 cm of soil increased by 507±245(SD) g C/m2 per year over the 7 years of grazing removal, which is equivalent to a rate of C increase in the plant–soil system of 366±229(SD) g C/m2 per year. Other studies have reported high rates of C uptake: 102–127 g C/m2 per year in the Mojave Desert (Jasoni et al. Citation2005; Wohlfahrt et al. Citation2008) and 62–622 g C/m2 per year in saline/alkaline desert soils in China (Xie et al. Citation2009). However, these rates of C increase are unusually large compared with some other studies (29 g C/m2 per year (Reeder et al. Citation2004) and 0.12–0.42 g C/m2 per year (Schlesinger et al. Citation2009)), which are comparable to the rates of C uptake in forests (Waring et al. Citation1998).
As the plots in the current study were not excavated to bedrock, it is possible that a significant fraction of the calculated SIC increase resulted from upward migration of carbonates from deeper soil layers as a result of increased water infiltration and soil moisture in the exclosure soils compared with those that were grazed (Reeder et al. Citation2004). On the other hand, increased carbonate in exclosure soils may at least partially reflect increased trapping of aeolian carbonate materials by recovered vegetation (Eswaran et al. Citation2000). This region of China receives extremely high levels of dust input (c. 500 g dust/m2 per year according to a local weather station), 1.5–10 times higher than levels observed in other regions (Vallack & Shillito Citation1998). The carbonate content of dust in northwestern China ranges from 7.4 to 12.8% (Wang et al. Citation2005), including approximately 5% Ca and 2% Mg (Feng et al. Citation2002a; Wang et al. Citation2004a). Thus, while our results suggest that grazer exclusion has the potential to increase total C pools, further studies that combine deep soil measurements with assessments of soil respiration are required to elucidate the C storage potential of grazing reduction in arid regions. Additionally, the results presented here are only short term in nature and the measurements taken are ‘snapshots’. Caution should thus be applied when considering long-term ecological responses. To better understand this, details of seasonal and interannual variation in C pools must also be studied.
Conclusions
Removal of grazing is an effective way of restoring vegetation and altering soil C pools. Grazing removal increased aboveground biomass and soil moisture, both of which increased soil microbial biomass and activity. Subsurface SIC pools also increased following grazing exclusion, possibly due to the enhanced formation of pedogenic carbonates, due to higher soil water content or increased carbonate capture in dust by recovering vegetation. Soil SOC pools were lower in exclosure soils than the grazed site, which was mainly explained by the decrease in surface soil bulk density. Combined, after 7 years of grazer exclusion, the total C pool in the plant–soil system was 10% greater (primarily due to 21% greater in SIC) than that in the area that had been grazed over the same time period. These findings are potentially important because the Inner Mongolia grassland is the largest in the world and its degradation under heavy grazing is a source of dust storms that have major regional and global impacts.
| Depth (cm) | Treatment | Water content (g/100g) | Bulk density (g/cm3) | Nt (g/kg) | STC (g/kg) | pH (H2O) 1:1 |
|---|---|---|---|---|---|---|
| 0–20 | Grazed | 16.11(0.41) | 1.20(0.03) | 2.01(0.06) | 17.72(0.77) | 8.00(0.02) |
| Exclosure | 15.77(0.58) | 1.17(0.03) | 1.95(0.05) | 18.42(0.27) | 8.12(0.09) | |
| 20–40 | Grazed | 19.35(0.07) | 1.26(0.01) | 1.44(0.12) | 29.30(1.11) | 8.42(0.08) |
| Exclosure | 18.98(1.03) | 1.24(0.01) | 1.38(0.14) | 30.00(1.81) | 8.24(0.05) |
| 2003 | 2005 | 2007 | ||||
|---|---|---|---|---|---|---|
| Grazed | Exclosure | Grazed | Exclosure | Grazed | Exclosure | |
| Aboveground biomass (g/m2) | ||||||
| Stipa brevifora Griseb | 22.7 (2.4) | 41.6 (2.3)Aa | 16.4 (3.1) | 19.7 (5.0)b | 21.6 (2.5) | 9.7 (0.8)Ac |
| Artemisia frigida Wild | 17.2 (3.5)a | 49.5 (7.5)A | 3.6 (0.9)b | 46.8 (7.0)A | 11.7 (1.7)a | 38.8 (3.8)A |
| Others | 7.2 (2.0)b | 8.4 (1.2)b | 3.6 (1.2)b | 6.5 (2.6)b | 14.9 (3.6)a | 21.4 (2.9)a |
| Total aboveground biomass1 | 47.1 (2.5)a | 99.5 (9.5)Aa | 23.6 (2.9)b | 73.0 (9.8)Ab | 48.2 (1.4)a | 70.0 (5.1)Ab |
| Plant cover (%)1 | 54.0 (3.5)a | 87.2 (1.1)Aa | 45.9 (2.0)b | 75.3 (2.4)Ab | 58.0 (1.0)a | 66.1 (2.8)Ac |
| Belowground biomass (g/m2) | ||||||
| Base of shoot | 169.2 (26.7) | 69.8 (11.7)A | ||||
| Living root | 397.1 (41.2) | 395.5 (28.3) | ||||
| Dead root | 291.5 (66.1) | 236.3 (28.3) | ||||
| Total root | 688.6 (92.5) | 631.8 (52.8) | ||||
| Total belowground biomass | 857.8 (106.7) | 701.6 (43.9) | ||||
| Total biomass (g/m2) | 905.9 (108.1) | 771.6 (48.8) | ||||
| 1Two-factor ANOVA analysis. For total plant aboveground biomass: P (treatment) < 0.001; P (year) < 0.001; P (treatment×year) = 0.011. For plant cover: P (treatment) < 0.001, P (year) < 0.001, P (treatment×year) < 0.001. | ||||||
| Water content (g/100g) | Bulk density (g/cm3) | PH (H2O) 1:1 | |||||
|---|---|---|---|---|---|---|---|
| Soil depth (cm) | Treatment | 2005 | 2007 | 2005 | 2007 | 2005 | 2007 |
| 0–10 | Grazed | 7.73 (0.28) | 6.03 (0.14)a | 1.23 (0.01) | 1.25 (0.01) | 8.22 (0.04) | 8.05 (0.02)a |
| Exclosure | 8.29 (0.26) | 7.10 (0.24)Aa | 1.15 (0.02)A | 1.15 (0.01)A | 8.38 (0.11)A | 8.31 (0.02)A | |
| 10–20 | Grazed | 8.01 (0.20) | 7.82 (0.19) | 1.23 (0.01) | 1.24 (0.02) | 8.15 (0.04) | 8.45 (0.04)a |
| Exclosure | 8.97 (0.28)A | 8.87 (0.22)A | 1.22 (0.03) | 1.22 (0.01) | 8.55 (0.06)A | 8.52 (0.03) | |
| 20–40 | Grazed | 10.06 (0.30) | 12.63 (0.48)a | 1.30 (0.02) | 1.33 (0.02) | 8.39 (0.03) | 8.50 (0.01)a |
| Exclosure | 11.67 (0.43)A | 16.44 (0.59)Aa | 1.30 (0.01) | 1.33 (0.00) | 8.66 (0.03)A | 8.69 (0.03)A | |
| 40–60 | Grazed | 11.34 (0.90) | 1.37 (0.04) | 8.65 (0.02) | |||
| Exclosure | 15.90 (0.37)A | 1.36 (0.05) | 8.74 (0.01)A | ||||
| Nt (g/kg) | Nava (mg/kg) | SOC/Nt | |||||
|---|---|---|---|---|---|---|---|
| Soil depth (cm) | Treatment | 2005 | 2007 | 2005 | 2007 | 2005 | 2007 |
| 0–10 | Grazed | 2.10 (0.06) | 2.03 (0.04) | 38.46 (1.46) | 49.48 (4.17)a | 7.42 (0.18) | 7.97 (0.14) |
| Exclosure | 2.11 (0.07) | 2.15 (0.07) | 33.83 (0.57) | 44.66 (1.22)a | 7.75 (0.28) | 8.06 (0.13) | |
| 10–20 | Grazed | 2.09 (0.07) | 1.93 (0.03)a | 30.42 (2.50) | 34.55 (2.05) | 7.54 (0.22) | 8.27 (0.12)a |
| Exclosure | 2.12 (0.06) | 1.96 (0.05)a | 29.88 (1.14) | 32.49 (2.57) | 7.54 (0.20) | 8.04 (0.10) | |
| 20–40 | Grazed | 1.19 (0.04) | 1.26 (0.05) | 15.50 (0.70) | 21.47 (1.17) | 9.31 (0.30) | 8.86 (0.30) |
| Exclosure | 1.17 (0.02) | 1.18 (0.06) | 13.20 (0.70) | 18.48 (1.69)a | 8.97 (0.23) | 7.30 (0.46)Aa | |
| 40–60 | Grazed | 0.64 (0.02) | 8.61 (0.18) | 6.70 (0.16) | |||
| Exclosure | 0.60 (0.02) | 8.04 (0.36) | 6.56 (0.20)A | ||||
| MBC (mg/kg) | MBN (mg/kg) | MBC/SOC (%) | |||||
|---|---|---|---|---|---|---|---|
| Soil depth (cm) | Treatment | 2005 | 2007 | 2005 | 2007 | 2005 | 2007 |
| 0–10 | Grazed | 532.62 (10.44) | 406.14 (15.01)a | 139.02 (1.13) | 135.66 (2.03) | 3.42 (0.07) | 2.52 (0.15)a |
| Exclosure | 565.09 (18.08) | 610.47(18.09)Aa | 128.67 (4.88) | 149.04 (3.99)Aa | 3.47 (0.10) | 3.55 (0.20)A | |
| 10–20 | Grazed | 335.23 (10.57) | 246.05 (0.29)a | 59.32 (2.62) | 54.44 (1.29) | 2.14 (0.05) | 1.54 (0.02)a |
| Exclosure | 369.28 (8.13)A | 426.17 (2.75)Aa | 65.70 (1.28) | 80.69 (0.57)Aa | 2.32 (0.08) | 2.71 (0.05)Aa | |
| SOC (g/kg) | SIC (g/kg) | STC (g/kg) | |||||
|---|---|---|---|---|---|---|---|
| Soil depth (cm) | Treatment | 2005 | 2007 | 2005 | 2007 | 2005 | 2007 |
| 0–10 | Grazed | 15.58 (0.25) | 16.17 (0.33) | 2.45 (0.38) | 3.69 (0.58) | 18.03 (0.48) | 19.85 (0.47)a |
| Exclosure | 16.29 (0.10) | 17.31 (0.50)Aa | 2.66 (0.18) | 3.80 (0.72) | 18.95 (0.18) | 21.11 (0.31)a | |
| 10–20 | Grazed | 15.70 (0.40) | 15.96 (0.20) | 7.36 (0.69) | 8.24 (0.78) | 23.05 (0.90) | 24.20 (0.89) |
| Exclosure | 15.98 (0.30) | 15.75 (0.20) | 11.49 (0.68)A | 15.89 (0.86)Aa | 27.47 (0.47)A | 31.64 (0.84)Aa | |
| 20–40 | Grazed | 11.10 (0.58) | 11.14 (0.24) | 19.79 (0.65) | 20.22 (2.10) | 30.89 (0.82) | 31.36 (0.12) |
| Exclosure | 10.50 (0.43) | 8.50 (0.26)Aa | 28.43 (1.53)A | 29.37 (1.09)A | 38.94 (1.25)A | 37.87 (0.91)A | |
| 40–60 | Grazed | 4.40 (0.13) | 35.76 (1.90) | 40.16 (1.87) | |||
| Exclosure | 3.91 (0.08)A | 37.00 (1.20) | 40.91 (1.21) | ||||
| Biomass carbon (kg/m2) | Soil carbon (kg/m2) | ||||
|---|---|---|---|---|---|
| Treatment | Aboveground | Belowground | SOC | SIC | Total carbon (kg/m2) |
| Grazed | 0.02 (0.001) | 0.35 (0.044) | 8.17 (0.087) | 16.67 (0.648) | 25.22 (0.612) |
| Exclosure | 0.03 (0.002)A | 0.29 (0.018) | 7.23 (0.073)A | 20.22 (0.407)A | 27.78 (0.372)A |
Related Research Data
Acknowledgements
The authors thank Zhuxin Mao, Yigong Zhang, Haiyan Wen, Yanfei Zhou, Keshun Wu and Yi Yang for their assistance in the collection and analyses of soil and plant samples. This research was supported by grants to Hua Fu from the National Support Project for Science and Technology in China (2008BAD95B03) and the National Natural Science Foundation of China (No. 90711002 and 31070412), the ‘Strategic Priority Research Program - Climate Change: Carbon Budget and Related Issues’ of the Chinese Academy of Sciences, Grant No. XDA05050406.
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