Livestock exclosure with consequent vegetation changes alters photo-assimilated carbon cycling in a Kobresia meadow

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Introduction
In the grazing ecosystem, livestock is a major force in the determination of grassland productivity (McNaughton, 1979(McNaughton, , 1983)).Many studies have revealed the positive effects of grazing on grassland productivity with plant compensatory mechanisms (McNaughton, 1985;Kotanen and Jefferies, 1989).Moreover, grazing can increase the plant diversity and indirectly change the competitive relationships among species (Collins, 1987;Denslow, 1980;Knapp et al., 1999).However, the effects of grazing on the grassland ecosystem were shown to be controversial in different ecosystems or with variable intensity (Waser and Price, 1981).In addition to the contribution of palatable plants consumed by livestock, the grassland also helps the ecosystem with C storage.
In grazed grasslands, plants removed by grazing were suggested to decrease the carbon migration into below ground.(Morris and Jensen, 1998).Nevertheless, other works have suggested that grazing has positive effects on the community structure resulting in an increase of C sequestration (Reeder and Schuman, 2002;Derner et al., 2006).
Livestock exclosures have a potential value in assessing the effects of grazing upon vegetation and C sequestration in grasslands (Bock et al., 1984;Cheng et al., 2011;He et al., 2011;Li et al., 2012;Su et al., 2003).Exclosure appears to favor higher community diversity and productivity compared with grazing in arid grasslands (Bock et al., 1984;Cheng et al., 2011).Furthermore, grazing exclosure is widely used as a management practice to restore degraded grasslands (Li et al., 2012;Su et al., 2003).
However, recent studies on a fenced Leymus chinensis grassland in northern China demonstrated that outcomes from exclosure were related to the community types of the grasslands, with responses to litter accumulation dependent on plant density (He et al., 2011).Generally, there were cascade effects on C sequestration in the plant-soil system from variations of the community structure and other factors related to exclosure (Li et al., 2012;Su et al., 2003).Many studies demonstrated that plant diversity and the interactions among different species or plant functional groups had major effects on C sequestration (Fornara and Tilman, 2008;Steinbeiss et al., 2008;De Deyn , 2011).In addition, litter accumulation as a result of lack of grazing was suggested to suppress C cycling in the plant-soil system (Reeder and Schuman, 2002;Schuman et al., 1999).
On the Qinghai-Tibetan Plateau, one-third of the total area is occupied by grasslands at 1.5 × 10 6 km 2 (Sun and Zheng, 1998).The grasslands of the Tibetan Plateau are one of the most extensive grazing systems in the world (Schaller, 1998).Much evidence indicates that grazing has been a widespread land use of the grasslands on the Qinghai-Tibetan plateau since 10 000 yr BP (Qian, 1979;Guo et al., 2006).The plants of the Tibetan Plateau have evolved with grazing.The long history of grazing has had important effects on the community structure and ecosystem function of the grasslands on the Tibetan Plateau (Klein et al., 2004(Klein et al., , 2008)).It has been shown that grazing increases productivity of the grassland (Klein et al., 2007).However, conflicting findings suggest that grazing decreases the productivity of the grassland -especially overgrazing associated with privatization and sedentarization, which leads to land degradation (Zhao and Zhou, 1999;Miller, 1999).Livestock exclosures were widely used as an approach to restore the degraded grassland on the Qinghai-Tibetan plateau (Yeh, 2005).However, the policy of completely eliminating domestic grazing from the grassland may not be suitable for the grasslands with different vegetation types, degrees of degradation and evolutionary histories.Large quantities of carbon are stored in the soil of grasslands, especially on the Qinghai-Tibetan plateau and due to long cold winters, the C sequestration in soil has been shown to be stable (Kuzyakov and Domanski, 2000).The Kobresia pastures are characterized by productive vegetation with a dense root system (Miehe et al., 2008).Its high root/shoot biomass results in 90 % of the carbon assimilation allocated into below ground of the pastures.It has been suggested that the Kobresia pastures may be a moderate C sink as a result of a neutral net ecosystem CO 2 exchange (Ni, 2002;Shi et al., 2006).However, this C sink is vulnerable to the land use and grassland management, which have been suggested to be decisive factors for a C sink and source switch in the Kobresia pastures (Wang et al., 2005).Introduction

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Full In order to investigate how livestock exclosure affects the grassland ecosystem function, we focused on the assessment of vegetation properties and ecosystem services as C storage of a 6 yr grazing exclosure meadow on the Qinghai-Tibetan plateau.Stable C-isotope analysis was carried out to track the newly assimilated carbon for C sequestration.We were interested in (1) the variations of the vegetation community structure after fencing, (2) the effects of exclosure on the cycling of the newly assimilated 13 C in the plant-soil system, and (3) if plant community structure influences 13 C cycling in the plant-soil system.

Site description
The study was conducted at the Haibei Alpine Meadow Ecosystem Research Station, located in northeast Tibet ( 37• 29 -45 N, 101 At the experimental site, a total area of 100 × 100 m was fenced for ecological research in 2005 to exclude yaks, sheep and goats.The grassland outside the fenced area exposed to moderate grazing in winter was used as the control site in the experiments.

Vegetation structure analysis
We investigated the vegetation structure in the fenced and control grazed sites in late August during the experiment period.Four quadrates were selected randomly in each site.The size of each quadrate was 50 cm × 50 cm.
The point-intercept method was carried out to assess the percentage vegetation coverage of the recorded species (Walk, 1996).In each quadrate, a 50 cm × 50 cm frame with 100 squares divided by nylon strings, each square measuring 5 cm × 5 cm, was placed over the vegetation.We used a short, thin metal rod to vertically insert from the canopy top of the vegetation down to the ground in each square.The species hit by the rod were recorded in the square, and species diversity of each site was the sum of species found in the four quadrates.The ratio of total hits of each species in the quadrates to 100 squares was recognized as the relative coverage of the species.Then, the aboveground biomass was harvested from each quadrate to evaluate the productivity of the grassland.Living and dead material were separated and living species were divided into four functional groups: grasses, sedges, legumes and forbs.The biomass of each functional group was assessed and the samples oven-dried at 70 • C for 48 h before weighing.

13 C pulse labeling
We carried out the 13 C pulse labeling experiment on 22 July in 2011, which was a clear day.Four replicates were selected in the fenced and in the grazed sites.Each plot Figures

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Full replicate was pulse-labeled in a closed chamber consisting of a stainless steel base (1 m × 1 m, 10 cm height) with a channel on the top and a PVC cover (1 m × 1 m, 45 cm height).The bases were installed in the soil at 10 cm depth on the day before the pulse labeling.When the pulse labeling experiment began, the PVC covers were sealed to the bases using air-tight water in the channel on the bases.All the plots were labeled in the 13 CO 2 atmosphere simultaneously between 10:00 and 12:00 LT, with several minutes difference.The 13 CO 2 was released by carefully injecting 10 mL 10 % H 2 SO 4 into the container holding the solution of 2.0 g Na 13 2 CO 2 in each chamber.The containers were connected to the chambers by tubing and were mounted in the center of the plots before the chambers were sealed.The air in each chamber was circulated by the fans mounted on the PVC cover to guarantee a uniform air environment.The chambers were removed after 2 h.Before opening the chambers, the chamber air was injected into 1M NaOH using syringes to absorb the unassimilated 13 CO 2 in the chambers.

Sampling
After pulse labeling, samples were collected at 0 h, 3 h, 6 h, 1 d, 4 d, 11 d, 18 d, 32 d in each replicate pot at the two experiment sites.
At each sampling occasion, shoot samples were harvested in 10 cm×10 cm squares by clipping the aboveground plant parts of all species.The shoot samples were separated into live and dead and the live shoots were oven-dried and ground (< 0.25 mm) for 13 C measurement.
Immediately after shoot sampling, the static alkali absorption method was used to assess the amount of CO 2 , including 13 CO 2 released from soil respiration (Hafner et al., 2012;Singh and Gupta, 1977).Briefly, CO 2 samples of the soil respiration were absorbed in alkali (NaOH) in a closed chamber (10 cm diameter, 10 cm high) on the soil surface where the shoot samples were clipped.At the sampling occasions of 3 h, 6 h, BGD 10, 17633-17661, 2013

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Full tion was precipitated with a 2 M barium chloride (BaCl 2 ) solution and the NaOH was titrated with 0.1 M hydrochloric acid (HCl) against phenolphthalein indicator (Zibilske, 1994;Werth and Kuzyakov, 2008).10 mL SrCl 2 was added into 10 mL NaOH of each sample to produce SrCO 3 precipitation.The SrCO 3 precipitation in the NaOH solution was neutralized with degassed water and oven-dried for 13 C measurement.
Soil cores of 8 cm in diameter were taken from three layers, 0-5 cm, 5-15 cm and 15-30 cm, immediately after air sampling.All roots and soil in the cores were carefully extracted and sieved with a 2 mm screen.The soil samples that passed through the sieve were air-dried and ground (< 0.15 mm).For 13 C measurement in soil organic C, carbonates were removed from the soil samples by washing in 0.1 M HCl for 24 h (Midwood and Boutton, 1998), the samples neutralized by adding deionized water and dried at 40 • C for 13 C measurement.The roots were carefully washed with river water and rinsed with deionized water through a 0.15 mm screen to remove attached soil and dark-brown/black debris.The roots were further separated into living and dead components based on their color and texture.The living roots were treated in the same way as the living shoots for 13 C measurement.Only data from living roots are mentioned in this work.

Measurement and calculations
Carbon contents in the samples were measured with an elemental analyzer, and natural abundance in samples which was expressed as δ 13 C (% ) was determined with a MAT 253 stable isotope ratio mass spectrometer system coupled to an elemental analyzer.
The isotopic ratio ( 13 C/ 12 C) of each sample R sample was calculated: R PDB = 0.011237 is the isotopic ratio of 13 C/ 12 C in Pee Dee Belemnite.The 13 C in the total C in the samples as 13 C (at %) was calculated: For the existence of natural abundance of δ 13 C (% ) in the unlabeled samples, the isotopic ratio in the total C in the unlabeled samples should be subtracted from that in the samples to assess the 13 C (at %) derived from the pulse labeling in the samples. 13C excess (at %) = 13 C of samples (at %) − 13 C of unlabeled samples (at %).
Finally, the following equation was used to determine the amount of 13 C incorporated in the samples from pulse labeling: ) is the carbon content in samples, which was assumed to be constant in shoots, roots and soil during the chase period.
% of recovered 13 C = 13 C t amount 13 C 0 amount • 100 was calculated to determine the partition (%) of the amount of 13 C incorporated into C pools at a special time t after the labeling. 13C 0 amount represented the weight (mg m −2 ) of 13 C in pools at 0 h after the labeling.

Statistical analyses
All statistical analyses were performed using SPSS 19.0 software.Data were analyzed by ANOVA.The only factor was land use types, fenced and grazed.The statistical analyses of 13 C recovered (%) in carbon pools at each sampling time between the fenced and grazed during the chase period were performed.P < 0.05 was considered statistically significant for treatment means.Introduction

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Vegetation composition
6 yr without grazing has lead to remarkable alterations in the vegetation composition of the Kobresia humilis meadow.More litter (219.00 g m −2 ) and less productivity (351.76 g m −2 ) of the vegetation were estimated in the fenced plot compared with the control grazed plots (litter of 104.08 g m −2 and productivity of 403.35 g m −2 ) (Table 1).The significant differences (P < 0.05) indicated that the fenced grassland had a lower productivity.
In addition, the plant diversity was reduced in the fenced plots (Table 1), with 29 recorded species, while 36 species were represented in the grazed plots.The missing species were legumes and forbs and there was no variation in grasses and sedges.
The biomass variations of the four plant functional groups did not always predict plant productivity.The biomass of legumes (1.57g m −2 ) and forbs (56.59 g m −2 ) in the fenced plot was significantly lower compared with the grazed plot (42.58 g m −2 , 116.29 g m −2 , P < 0.05).As for grasses, the biomass increased from 198.30 g m −2 to 264.43 g m −2 after 6 yr enclosure, and this difference was statically significant (P < 0.05).Sedges showed no significant variations between the two sites, although the biomass of sedges was lower in the fenced sites (29.17 g m −2 vs. 46.18g m −2 , P > 0.05).at each depth, but the differences were not statistically significant compared with the grazed site.Soil had more C distributed at the depth of 5-15 cm with a minimum at 0-5 cm in both sites.C stocks in the soil of 0-5 cm and 15-30 cm were significantly different between the two sites (P < 0.05).Interestingly, we found that exclosure leads to more C distribution in the top-surface of the soil.In addition, there were higher C

Carbon stocks in
stocks in soil at 0-30 cm depth in the fenced site but the difference was not statistically significant (Table 2).

13 C allocation and dynamics in the plant-soil system
At the beginning of the chase period, 495.43 mg and 370.36 mg 13 C were labeled as total amounts of 13 C in the plant-soil systems of the fenced and control grazed plots, respectively.The allocation of 13 C in shoots was 53 % in the fenced plots and 42 % in the grazed plots, a statistically significant difference (P < 0.05).Then, the amount of 13 C decreased following exponential decay within the chase period in both plots (Fig. 2a).The recovery of 13 C in shoots declined from 53 % to 43 % (P < 0.05) during the first 24 h in the fenced plots; this was lower than that observed in the grazed grassland site where we observed a decrease of 16 % in 13 C recovery in shoots.A slower rate of 13 C decay in shoots was observed in the first 4 d after labeling in the fenced plots compared with the grazed plots.This was followed by a nonsignificant decrease until the end of the chase period in the grazed plots (Fig. 2a).The decreasing trend of 13 C recovered with no plateau period was apparently due to shoots in the fenced plots during the chase period.
A larger recovery of 13 C was detected in soil during the chase period in both land types, with a significantly lower value in the fenced plots (Fig. 2c).However, the allocation of 13 C in roots was lower during the chase period (Fig. 2b).The allocation of 13 C in roots was effected by the import of 13 C in shoots, the export of 13 C into soil, and CO 2 in soil respiration.There were no significant differences in 13 C found in roots between the fenced and grazed grassland types at any sampling time during the chase period, probably as a result of the higher variability among the plots (Fig. 2b).It was obvious Figures

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Full that less 13 C migrated to the soil through roots in the fenced plots immediately after labeling (44 %, P < 0.05).A higher rate of 13 C decline in soil was found in the fenced plots during the first 24 h after labeling, while there were nonsignificant variations at the grazed site.Thereafter (from 24 h to the end of the chase period), the allocation of 13 C in the soil of the fenced plots reached stable values with nonsignificant differences.
In the grazed plots, a minimum was reached 18 d after labeling.The results correlated with the 13 C variations in shoots and the 13 CO 2 efflux rates from soil respiration.During the first 24 h after labeling, there was less 13 C loss from shoots (10 %), with a lower rate at the fenced site.Coupled with a high rate of 13 C loss from soil respiration, this resulted in the significant decrease observed in the amounts of 13 C in soil at the fenced grassland (Fig. 2a and d).A significant reduction in 13 C amounts in shoots was observed in the grazed plots.However, the 13 C variations in soil were not significant during the first 24 h, indicative of significantly higher rates of 13 CO 2 efflux from soil respiration than what occurred in fenced site (Fig. 2d).
At the end of the chase period (32 d after labeling), significantly less 13 C (14 %) was left in shoots in the fenced plots and 18 % was left in the grazed plots (P < 0.05).However, the allocation of 13 C into below ground at the fenced sites (53 %) was significantly lower than that of the grazed plots (61 %, P < 0.05).The reduced distribution of 13 C into below ground at the fenced plots resulted from the significantly lower allocation of 13 C both in soil (37 %, P < 0.05) and soil respiration (5 %, P < 0.05).These values were 47 % and 8 % for 13 C allocated in soil and soil respiration at the grazed plots, respectively.We found a greater allocation of 13 C in roots at the fenced plots in comparison with the grazed sites.However, this did not influence the unequal below ground 13 C allocation observed in the two land types.As shown in Fig. 3, the allocation of 13 C in roots was not significantly different between the two sites.Introduction

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Vertical-spatial variations of belowground 13 C allocation
The 13 C recovery in roots varied according to depths at both land use types.It decreased gradually from the highest in the surface layer to the lowest values obtained in the deepest layer (Fig. 4a).The 13 C in roots at 0-30 cm was mainly allocated in the shallow depth of soil (0-5 cm) and the recovery of 13 C at 0-5 cm determined the amounts of 13 C in roots for the whole surface layer (0-30 cm), as shown in Fig. 4a.
The 13 C allocated in the soil was not consistent with the rank order of the depth.In both land use types, more 13 C was recovered in the depths of 5-15 cm and 15-30 cm with the lowest in the shallow layer (Fig. 4b).After being fenced for 6 yr, there was lower 13 C recovery in the exclosure soil at 5-15 cm and 15-30 cm than that observed in the grazed site.At the end of the chase period, significantly lower 13 C was allocated into the depths of 5-15 cm and 15-30 cm in the fenced plots.However, the 13 C allocation at 0-5 cm showed a nonsignificant difference between the two sites (Fig. 4b).

Discussion
Livestock exclosure alters structure of grassland communities as well as the cycling of materials in the grassland ecosystem (Morris and Jensen, 1998;Reeder and Schuman, 2002;Derner et al., 2006;Altesor et al., 2005).Some field studies have proposed exclosure as an effective approach for restoring vegetation and improving C storage of the grassland (Li et al., 2012;Su et al., 2003), while others have demonstrated that exclosure has had negative effects on C sequestration in grasslands (Reeder and Schuman, 2002).This apparent contradiction can be explained by the different evolutionary processes, degrees of degradation and grazing history in these grassland communities.Indeed, the effects of exclosure on the grassland are community-specific.Introduction

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Effect of exclosure on vegetation structure
In the Kobresia meadow, the peak biomass and plant coverage was assessed (Table 1).
The results indicated that elimination of livestock from grassland lead to more accumulation of litter (219.00 g m −2 ) than what was observed in the grassland with grazing (104.08 g m −2 ).In contrast, the productivity of live plants in the fenced grassland (351.76 g m −2 ) was significantly lower than that in the grazed grassland (403.35g m −2 ).These results are not consistent with several works that have demonstrated exclosure has had positive effects on the growth of the vegetation (Morris and Jensen, 1998;Derner et al., 2006;Bock et al., 1984;Cheng et al., 2011).However, other studies have shown that exclosure is detrimental to the vegetation because of the lack of livestock grazing (McNaughton, 1983;Knapp and Seastedt, 1986).The Kobresia humilis meadow is productive and is well known to evolve with the involved grazing (Qian, 1979;Guo et al., 2006).For a winter grazing pasture in our research, elimination of livestock from grassland caused more accumulation of aboveground litter, leading to a decrease of the bare ground area for the extension and reproduction of the vegetation in spring.
Furthermore and importantly, the litter on the ground may alter the productivity by reducing the absorption efficiency of radiation due to self-shading (Altesor et al., 2005;Knapp and Seastedt, 1986).We showed that the composition of the four functional groups was changed after fencing of the grassland (Table 1), with livestock exclosure promoting the growth of grasses while suppressing legumes and forbs.In addition, exclosure reduced the plant diversity, especially that of forbs (Table 1).The different characteristics of the functional groups and the aboveground accumulated litter may be important factors affecting the composition of the functional groups (Klein et al., 2004).Grasses are palatable for livestock and it was shown that grasses had a stronger competition for light than the other functional groups in the grassland community (Kull and Aan, 1997).Many studies have demonstrated that light is one of the most important resources that determine plant production (Altesor et al., 2005;Knapp and Seastedt, 1986 1996).After suppression of livestock feeding, the increased growth of grasses and the accumulated litter had negative effects on the productivity and diversity of legumes and forbs through light limitation.The Kobresia humilis meadow was therefore found to be nutrient-limited.Thus, nutrient level may be another important factor (in addition to light) cooperating with grazing to affect the community composition of the grassland.

Effect of exclosure on 13 C dynamic and allocation
To investigate the effects of exclosure on C cycling in the ecosystem, stable C-isotope analysis was used to track carbon movement in the fenced grassland as well as the grazed sites (Fig. 1).The chase period to determine the cycling of 13 C newly incorporated by photosynthetic was approximately 32 d in our study (Hafner et al., 2012;Wu et al., 2009).However, a different chase period has been reported as well due to the steady state of 13 C in the plant-soil system after labeling in their experiments (Wang et al., 2007).Four replications were used to evaluate the cycling and allocation of 13 C in each land use type.The carbon stocks (g m −2 ) of the different pools in the plant-soil systems were assumed to be constant during the chase period of 32 days (Wu et al., 2009).We used an average δ 13 C of the four replications to assess the dynamics and allocation of 13 C in the plant-soil system during the chase period (Wang et al., 2007).
Immediately after pulse labeling, 53 % of 13 C assessed in the plant-soil system was recovered in shoots in the fenced plots, which was significantly higher than the 42 % obtained in the grazed plots.During the first 24 h after labeling, 19 % of 13 C was estimated to loss or export from shoots in the fenced plots, and this was twice lower than in the grazed plots (38 %) (Fig. 2a).These results suggest that 13 C is lost or exported from shoots at a lower rate in the grassland after exclosure.The lower rate of 13 C migrating to the soil in the fenced plots during the first 24 h confirmed the restricted 13 C dynamics from shoots into below ground in the grassland under exclosure (Fig. 2c).This is probably due to the variations in vegetation structure.It is known that plant diversity has positive effects on C accumulation in soil (Steinbeiss et al., 2008;De Deyn et al., 2011).This was confirmed by our results with reduced plant richness in the fenced

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Full grassland.Indeed, the loss of legumes in the fenced grassland may be an important factor affecting C sequestration (Fornara and Tilman, 2008) (Table 1).The 13 C in shoots continually declined during the chase period in the fenced plots, while in the grazed plots, the recovered 13 C (%) diminished to a constant value during the first days, followed by no significant variations from 4 days after labeling until the end of the chase period (Fig. 2a).These finding suggest that a part of 13 C was transformed into more stable structural carbon in shoots in the grazed plots, in accordance with previous works (Wu et al., 2009).The data were consistent with the lower peak biomass of vegetation as lower ANPP (annual aboveground net primary productivity) was obtained in the grassland under exclosure (Table 1).Less 13 C was allocated to shoots for growth in the fenced plots.The lower 13 C allocation in shoots at the end of the chase period in the fenced plots (Fig. 3) is likely caused by lower productivity due to the light or nutrient limitation.Generally, roots are recognized as a major sink in the plant-soil system, and more C migrate in soil by means of roots than exudates (Wu et al., 2009).However, our results showed that 13 C recovered (%) in living roots was much less than that in soil at both land use types (Fig. 2b and c), consistent with another study using stable C labeling (Hafner et al., 2012).It has been suggested that the plant development stage during the chase period influences the 13 C allocation in roots (Kuzyakov et al., 1999;Palta and Gregory, 1997).In an earlier Kobresia humilis meadow study, many plants such as Kobresia humilis, Elymus nutans, and many forbs species were in the state of flowering and fruit-bearing (August) (Shi et al., 1988).It is possible that less 13 C was found in roots because the plants first attributed 13 C to produce assimilates for generation instead of root growth during the chase period in our experiments (Hafner et al., 2012).There were no significant differences of 13 C recovered (%) in roots between the two land use types at any sampling time.However, the 13 C recovered in roots tended to increase during the chase period in the fenced plots (Fig. 2b).Lower productivity of shoots in the fenced grassland favored more 13 C migration to roots, although higher spatial variability obscured the differences of 13 C in roots between the two sites.Fur-Figures

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Full thermore, the evidence from root C stocks in the two land use types confirmed these results.Indeed, higher but not statistically significant C stocks were assessed in roots in the fenced grassland in comparison with the grazed site (Table 2).
Our data suggest that at low levels, 13 C is firstly shifted into soil by means of exudates from roots.This process took place immediately after labeling.Indeed, less 13 C was allocated in soil in the grassland under exclosure, in addition to significantly lower 13 CO 2 efflux rate of soil respiration (Fig. 2c and d).It is likely that the plant diversity of the vegetation influences the 13 C dynamic migration from plant to soil with major influences on the microorganism activity in our study (Steinbeiss et al., 2008;Wardle et al., 1999;Hooper et al., 2000;Stephan et al., 2000).The reduced rate of CO 2 efflux in the fenced grassland may indicate a decrease of microorganism activity in soil compared with the grazed site.There have been complementarities effects that promote the nutrient cycling existing in the grassland with high plant diversity (Oelmann et al., 2007;Temperton et al., 2007).Additionally, the composition of the functional groups, especially the variations in legumes, has been suggested to influence 13 C allocation in soil (Fornara and Tilman, 2008;De Deyn, 2011).We found that exclosure management has negative effects on 13 C allocated in deeper layers (Fig. 4b).Less 13 C migrated from roots to soil at deeper soil of the fenced plots compared with the grazed grassland during the chase period.Probably the characteristics of different root types and structures affected the vertical distributions of 13 C input into soil.Indeed, legumes and forbs species have deep root systems.The suppressed growth of legumes and forbs in the fenced grassland had therefore negative effects on 13 C exudation into deeper soil.In addition, a weak leaching effect in the fenced grassland as a result of accumulated litter on the above ground constitutes another factor influencing the vertical distribution of

Conclusions
Our results demonstrated that livestock exclosure changed the vegetation community structure.We also found that exclosure decreased the 13 C dynamic rate in the plant-soil system, suggesting that more 13 C is allocated into roots in the fenced grassland.However there was less 13 C migration into soil under exclosure.There were relations between the variations of vegetation community structure and C cycling.The decreased productivity of legumes, forbs and the accumulated aboveground litter after fencing may be responsible for the difference observed in C cycling in the short-term.However, long-term experiments should be carried out in order to better understand the effects of exclosure on the Kobresia humilis meadow.In this study, considering the negative effects of exclosure, we found evidence that livestock exclosure was detrimental to the Kobresia humilis meadow, which has evolved with a long history of grazing.Given different evolutionary processes, degrees of degradation and grazing histories of different grassland ecosystems, the application of exclosure practice to the grassland management should be community specific.Introduction

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Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 3 Results the plant-soil system during the 13 C chase period During the chase period, C stocks in the aboveground and belowground C pools of the plant-soil systems were significantly different (P < 0.05) between the fenced and grazed sites in the shoot and soil pools at 0-5 cm, 15-30 cm.C stock in the aboveground shoot pools of the fenced site was 1.46 Mg ha −1 , significantly lower (P < 0.05) than that found in the grazed site (2.12 Mg ha −1 ), as shown in Table2.In the root pool, the pool sizes indicated a rank order of the depth ≈ 0-5 cm > 5-15 cm > 15-50 cm.C stocks were greater in the root pool of the fenced site Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ; Semmartin and Oesterheld, Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Fig. 1 .Fig. 2 .Fig. 4 .
Fig. 1.Experimental chamber setup for 13 C pulse labeling.The steel base (a) was installed in the soil before labeling.At the beginning of labeling, a PVC cover was sealed to the base to form a chamber (b).The plant in the chamber was labeled by13 C through photosynthesis with 13 CO 2 .The 13 CO 2 was released by injecting H 2 SO 4 into the container holding Na 2 CO 3 in the chamber.
Kobresia pygmaea meadow, and Kobresia tibetica swamp meadow.Plants grow from May to September.The experiments were carried out at the alpine Kobresia humilis meadow.The soil of the Kobresia humilis meadow is classified as Mat Cry-gelic Cambisols according to the Chinese National Soil Survey and Classification System (Chinese Soil Taxonomy Figures Research Group in Institute of Soil Science of CAS, 2001).The vegetation is mainly dominated by Kobresia humilus, Stipa aliena, Festuca ovina, and so on.
• 12 -23 E) at an altitude of 3250 m.The station has a continental monsoon type climate, characterized by long, cold winters and short, cool summers.The annual average temperature is −2 • C, with the coldest monthly temperature of −18 • C recorded in January, and the warmest of 10 • C in July.The mean annual precipitation ranges from 426 to 860 mm, with more than 80 % precipitation occurring in the short summer from May to September.The annual average sunlight is 2462.7 h, 60 % of which is available for plants to grow.This provides advantages for the photosynthesis of herbage.The research area is dominated by four most important vegetation communities, Kobresia humilis meadow, Dasiphora fruticosa shrub,

Table 1 .
Species diversity, mean (± SD) vegetation aboveground biomass, and vegetation cover in the fenced and control grazed meadow.

Table 2 .
Mean (± SD) C stocks (Mg ha −1 ) during the chase period in aboveground and belowground C pools of the fenced and grazed meadow.