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J. Plant Nutr. Soil Sci. 2012, 000, 1–9 DOI: 10.1002/jpln.201100327 1

Short-term degradation of semiarid grasslands—results from a controlled-grazing experiment in Northern ChinaMartin Wiesmeier1*, Olivia Kreyling1, Markus Steffens1, Philipp Schoenbach2, Hongwei Wan2, Martin Gierus2,Friedhelm Taube2, Angelika Kölbl1, and Ingrid Kögel-Knabner1

1 Lehrstuhl für Bodenkunde, Department für Ökologie und Ökosystemmanagement, Wissenschaftszentrum Weihenstephan für Ernährung,Landnutzung und Umwelt, Technische Universität München, Emil-Ramann-Straße 2, 85350 Freising-Weihenstephan, Germany

2 Institute of Crop Science and Plant Breeding–Grass and Forage Science/Organic Agriculture, Christian-Albrechts-Universität zu Kiel,24118 Kiel, Germany

AbstractThe assessment of grassland degradation due to overgrazing is a global challenge in semiaridenvironments. In particular, investigations of beginning steppe degradation after a change orintensification of the land use are needed in order to detect and adjust detrimental land-use man-agement rapidly and thus prevent severe damages in these sensitive ecosystems. A controlled-grazing experiment was established in Inner Mongolia (China) in 2005 that included ungrazed(UG) and heavily grazed plots with grazing intensities of 4.5 (HG4.5) and 7.5 (HG7.5) sheep perhectare. Several soil and vegetation parameters were investigated at all sites before the start ofthe experiment. Topsoil samples were analyzed for soil organic C (SOC), total N (Ntot), total S(Stot), and bulk density (BD). As vegetation parameters, aboveground net primary productivity(ANPP), tiller density (TD), and leaf-area index (LAI) were determined. After 3 y of the grazingexperiment, BD increased and SOC, Ntot, Stot, ANPP, and LAI significantly decreased withincreasing grazing intensity. These sensitive parameters can be regarded as early-warning indi-cators for degradation of semiarid grasslands. Vegetation parameters were, however, more sen-sitive not only to grazing but also to temporal variation of precipitation between 2006 and 2008.Contrary, soil parameters were primarily affected by grazing and resistant against climatic varia-tions. The assessment of starting conditions in the study area and the application of definedgrazing intensities is essential for the investigation of short-term degradation in semiaridenvironments.

Key words: steppe / desertification / soil organic carbon (SOC) / overgrazing / Inner Mongolia

Accepted December 17, 2011

1 Introduction

Degradation of semiarid and arid environments, also referredto as desertification, is a global environmental problem, parti-cularly in N China, where 2.1–2.6 million km2 are affected bydesertification (Yang et al., 2005). In some regions of InnerMongolia, up to 72% of the total area are classified asdegraded steppe (Tong et al., 2004). The main cause ofgrassland degradation in N China is overgrazing as a resultof increasing stocking rates and a static grazing managementduring the last 50 y (Akiyama et al., 2007). Beside well-estab-lished methods and concepts to assess desertification in anadvanced state (Imeson and Cammeraat, 2000; Ludwig et al.,2000; Mouat et al., 1997; Rubio and Bochet, 1998; Tongwayand Hindley, 2000), many authors point out that it is importantto investigate beginning degradation of semiarid grasslandsat an early stage (Lin et al., 2010; Yang et al., 2005). In emer-ging nations like China with a large increase of the popula-tion, agricultural use further expands in the boundary areas ofsemiarid steppe regions where intensification of grazing canlead to a severe deterioration of this ecologically sensitiveenvironment (Christensen et al., 2004; Wiesmeier et al.,2011, 2012). In order to prevent grasslands from consider-

able ecological and economical damages and to establish asustainable land-use management a rapid detection of begin-ning steppe degradation is essential. Until now, only fewstudies are available that investigated the early stages ofdegradation in semiarid ecosystems (de Soyza et al., 1998;Lin et al., 2010). Moreover, many studies failed to include anassessment of the starting conditions and a defined intensityof the land use in the study area, e.g., stocking rates of graz-ing animals (Su et al., 2005; Veron et al., 2006; Xie andWittig, 2004). This is necessary, as effects of an intensifica-tion or change of the land use on soil and vegetation para-meters are probably decelerated and difficult to detect aftershort periods of time. Possible changes of such parameterscan be influenced and superimposed by frequent climaticvariations, particularly droughts, in semiarid ecosystems(Ellis et al., 2002; Yang et al., 2005).

Investigations at long-term experimental sites with differentgrazing intensities in Inner Mongolia revealed a severe dete-rioration of several soil and vegetation parameters. Evidencefor sensitivity to long-term grazing was found for soil organic

* Correspondence: Dr. M. Wiesmeier;e-mail: wiesmeier@wzw.tum.de

C (SOC), total N (Ntot), total S (Stot) of the bulk soil, bulk den-sity (BD), aggregate stability, and quantity and quality of dif-ferent soil organic matter (SOM) fractions (Steffens et al.,2008, 2009; Wiesmeier et al., 2009). Furthermore, consider-able changes were found for several vegetation parameters,e.g., above- and belowground net primary productivity(ANPP, BNPP), vegetation cover, canopy height, communitylevel tiller density (TD), and leaf area index (LAI) (Gao et al.,2008; Li et al., 2008; Zhang et al., 2004; Zhao et al., 2009).

In order to determine short-term effects of different grazingintensities on semiarid grasslands, a controlled-grazingexperiment was established in the Xilin River Catchment(Inner Mongolia) in 2005. The aims of this study were to iden-tify soil and vegetation parameters which are sensitive toshort-term intensive grazing and to quantify short-termchanges of these parameters. We investigated SOC, Ntot,

Stot, and BD as potentially sensitive soil parameters andANPP, TD, and LAI as vegetation parameters. We deter-mined starting conditions of all parameters at the beginningof the grazing experiment in 2005 and again after 3 y of con-trolled grazing in 2008.

2 Materials and methods

2.1 Study area

The study site is located in the Xilin River Basin in the autono-mous province Inner Mongolia, P.R. China, nearby the InnerMongolia Grassland Ecosystem Research Station (IMGERS,Chinese Academy of Sciences, 43°38′ N, 116°42′ E, 1270 masl; Fig. 1). The region is part of the continental semiaridgrasslands of the Central Asian steppe ecosystem with a dry

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Figure 1: Map of the Xilin River catchment in Inner Mongolia,N China, showing the location of the Inner MongoliaGrassland Experimental Research Station (IMGERS) andthe study sites of the grazing experiment (UG = ungrazed,HG4.5 = heavily grazed with 4.5 sheep units ha–1, HG7.5 =heavily grazed with 7.5 sheep units ha–1).

2 Wiesmeier et al. J. Plant Nutr. Soil Sci. 2012, 000, 1–9

and cold middle-latitude climate (Kawamura et al., 2005).Mean annual temperature at IMGERS is 0.7°C, and meanannual precipitation is ≈ 350 mm with highest values in thesummer from June to August. The vegetation period fromMay to September is relatively short (< 150 d). The zonalvegetation type is grassland-dominated by the perennialrhizome grass Leymus chinensis and the perennial bunch-grass Stipa grandis (Chen et al., 2005; Xiao et al., 1995).Soils were classified as Calcic Chernozems (IUSS WorkingGroup WRB, 2006) that developed from aeolian sedimentsdeposited on a pleistocene basalt plateau. They are charac-terized by a dark, carbonate-free Ah horizon followed by anAch horizon with secondary Ca carbonate nodules. Furtherinformation on the study area is presented by Kawamuraet al. (2005), Kölbl et al. (2011), Wiesmeier et al. (2011), andXiao et al. (1995).

2.2 Design and sampling of the controlled-grazingexperiment

The controlled-grazing experiment was established in 2005on a grazed typical steppe next to a farm (Schönbach et al.,2009, 2010). Until 2003, grazing intensity was moderate(≈ 1.2 sheep units ha–1) and thereafter, the area recoveredfrom grazing for 2 y before the experiment started in June2005. An area of ≈ 160 ha was fenced and divided into plotsof 2 ha with different stocking rates and land-use manage-ment. In this study, ungrazed control plots (UG) were com-pared with heavily grazed plots with stocking rates of 4.5(HG4.5) and 7.5 (HG7.5) sheep units ha–1, respectively(Fig. 1). Grazing animals were 2-y-old female sheep, aver-aging 35 kg initial live-weight, and were kept continuously onthe plots throughout the grazing season from June toSeptember.

Soil samples from the upper 4 cm of the soil were taken fromeight UG plots, two HG4.5 plots, and two HG7.5 plots in June2005 before the grazing experiment started and again afterthree grazing seasons in June 2008. At each plot, threeundisturbed soil samples were taken from five randomlyselected locations using steel cylinders with a volume of100 cm3 and bulked to obtain a composite sample. Soil mate-rial was dried at 105°C for 24 h, weighed, sieved (2 mm), andanalyzed for texture, pH, BD, SOC, Ntot, and Stot.

Aboveground biomass was successively sampled monthly atthe beginning of June, July, August, and September in 2005and again in 2007 for the third grazing season of the experi-ment at UG, HG4.5, and HG7.5 from two plots, respectively.Before grazing started, three representative sampling areaswere chosen within each plot to reflect the sward composi-tion. Moveable exclosure cages (2 m × 3 m) were used tomeasure the productivity of grazed plots (McNaughton et al.,1996). Directly after finishing the first sampling at the begin-ning of June, exclosures were set up at each sampling pointand sheep were allocated to the plots. At each following sam-pling, exclosures were moved to new locations. Litter wascombed out before clipping standing aboveground herbagemass to 1 cm within rectangular 0.5 m2 frames (0.25 m × 2 m)inside and outside the exclosures. The collected material was

dried at 60°C for 24 h, and the dry matter (DM) was weighedfor determination of ANPP. The herbage samples comprisedliving and standing dead material. Yields presented in thisstudy were corrected for residual water at 105°C.

Leaf-area index (LAI) and TD were measured at the begin-ning of July and September in 2005 and 2008 at UG and HGplots. At each plot, three representative sampling points werechosen and tillers from the most dominant species Leymuschinensis, Stipa grandis, and Cleistogenes squarrosa werecounted and clipped within squares of 20 cm × 20 cm. Tillerswere packed in cooling boxes and transferred to IMGERS forleaf area determination.

2.3 Determination of soil properties

For the analyses of texture, soil samples were oxidized withH2O2 to remove OM. The remaining material was dispersedwith Na4P2O7 and shaken for at least 16 h, followed by wetsieving to isolate sand fractions of 2000 to 630 lm, 630 to200 lm, and 200 to 63 lm. To determine silt and clay frac-tions ≈ 3 g of the < 63 lm fraction was suspended in deio-nized water with Na4P2O7 and an ultra-sonication for 3 minwith 75 J mL–1 was conducted. Afterwards the distribution of63 to 20 lm, 20 to 6.3 lm, 6.3 to 2 lm, and < 2 lm fractionswere obtained by measuring the X-ray absorption of thesoil–water suspension during sedimentation of the soilparticles with a Micromeritics Sedigraph 5100 (Micromeritics,Norcross, USA).

Bulk density was quantified with the mass of the oven-dry soil(105°C) divided by the core volume (Hartge and Horn, 1989).Soil pH values were measured in 0.01 M CaCl2 at a soil-to-solution ratio of 1 : 2.5 at room temperature.

Soil organic C, Ntot, and Stot were determined in duplicates bydry combustion on a Vario Max CNS elemental analyser (Ele-mentar, Hanau, Germany). The measured C concentrationsof the samples, that were free of carbonate, represent SOCconcentrations. As grazing normally increases bulk densityand consequently affects calculation of elemental stocks, weused the following equation, referred to as “equivalent-stockcalculation” (Ellert and Bettany, 1995), to estimate SOC, totalN, and total S stocks of the upper 4 cm for 1 m2:

ES = EM · EC · 10–3, (1)

where ES is the elemental stock (kg m–2), EM the equivalentmass (kg m–2), and EC the elemental concentration (mg g–1).The mean soil masses of the upper 4 cm of one square meterof UG, HG4.5, and HG7.5 sites from 2005 were calculatedand used as normalized masses for stock calculations.

2.4 Determination of vegetation parameters

For calculation of ANPP at HG4.5 and HG7.5 sites the initialherbage mass, that is available for grazing sheep at thebeginning of June, was determined outside the exclosuresand summed up by the monthly herbage growth sampledinside the ungrazed exclosures (Schönbach et al., 2009):

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J. Plant Nutr. Soil Sci. 2012, 000, 1–9 Short-term degradation of semiarid grasslands 3

ANPP =Wout1 + (Win2 – Wout1) + (Win3 – Wout2) + (Win4 – Wout3), (2)

where Wout and Win are the DM weights measured outsideand inside the exclosures at sample time ti (i = 1, 2, 3, 4:beginning of June, July, August, and September). For UGsites, ANPP was equated with the peak herbage masssampled in August.

For determination of LAI, leafs were separated from stemsand scanned with a leaf-area meter LI-3000A (LI-COR, Lin-coln, USA). Leaf-area index was calculated by scanned totalleaf area divided by the investigated ground area. Tiller den-sity was calculated by the number of tillers of all investigatedspecies divided by the sampled ground area.

2.5 Statistical methods

All statistical analyses were carried out using SPSS 15.0(SPSS Inc., Chicago, USA). Descriptive statistical methodswere used to analyze the data set for maximum and minimumvalues, mean, median, standard deviation, skewness, kurto-sis, variance, and coefficient of variation. To test the signifi-cance of grazing effects on the examined parameters a one-way analysis of variance (ANOVA) combined with post-hoctests (LSD, Tukey, and Scheffé) and a Student’s T-test wasapplied. The significance of correlations between variousparameters was expressed by Pearsons’s correlation coeffi-cient.

3 Results

3.1 Soil parameters

Basic soil properties of the studied samples are given inTab. 1. Soil textures of UG, HG4.5, and HG7.5 were compar-able in 2005 with 55%–59% sand, 27%–30% silt, and15%–16% clay. The pH values were similar ranging from 6.7to 6.9.

In 2005, elemental concentrations and BD were similar at allsites with 20 mg g–1 SOC, 2.1 mg g–1 Ntot, 0.3 mg g–1 Stot,and a BD of 1.27 g cm–3 (Fig. 2). In 2008, after 3 y ofgrazing exclusion, UG plots showed significantly (p < 0.05)increasing SOC, Ntot, and Stot concentrations that were14%–16% higher compared with 2005. Bulk density was sig-nificantly lower (p < 0.05) and decreased to 1.15 g cm–3.

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Table 1: Soil texture and pH values of the topsoil (0–4 cm) fromungrazed (UG) and heavily grazed (HG4.5, HG7.5) sites before thebeginning of the grazing experiment in 2005 (standard deviation inparentheses, n = 40 for UG, n = 10 for HG4.5 and HG7.5).

Site Sand/ mg g–1

Silt/ mg g–1

Clay/ mg g–1

pH(CaCl2)

UG 574 (± 8) 281 (± 6) 145 (± 2) 6.9 (± 0.3)

HG4.5 547 (± 6) 298 (± 4) 156 (± 2) 6.7 (± 0.4)

HG7.5 594 (± 8) 267 (± 5) 139 (± 2) 6.7 (± 0.3)

1.00

1.05

1.10

1.15

1.20

1.25

1.30

1.35

1.40

BD

/ g

cm–3

UG HG4.5 HG7.5

2005 2008

0

5

10

15

20

25

30

35

SOC

/ m

g g–1

UG HG4.5 HG7.50.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Nto

t / m

g g–1

UG HG4.5 HG7.5

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

S tot /

mg

g–1

UG HG4.5 HG7.5

2005 20082005 2008

2005 2008

Figure 2: Concentrations of SOC,Ntot, Stot, and BD determined in 2005and 2008 from the topsoil (0–4 cm)of ungrazed (UG) and heavilygrazed (HG4.5, HG7.5) sites. Errorbars represent standard deviation(n = 40 for UG, n = 10 for HG4.5and HG7.5).

4 Wiesmeier et al. J. Plant Nutr. Soil Sci. 2012, 000, 1–9

After three seasons of heavy grazing in 2008, elemental con-centrations tended to decrease while BD remained almostunchanged at HG4.5 and HG7.5 sites. Soil organic C, Ntot, andStot were 10%–18% lower compared with the results of 2005,but differences were only significant for HG4.5 (p < 0.05) andnot for HG7.5 (p = 0.06–0.22). Although, differences betweenUG and HG sites were significant (p < 0.05) in 2008 after 3 y ofgrazing but not in 2005 before the start of the grazing experi-ment.

Elemental stocks calculated for the first 4 cm of the topsoilfrom all plots were comparable in 2005 with amounts of1.0 kg m–2 for SOC, 0.1 kg m–2 for Ntot, and 0.015 kg m–2 forStot (Tab. 2). In 2008, UG plots revealed significantly (p < 0.05)higher SOC, Ntot, and Stot stocks whereas HG4.5 and HG7.5sites tended to show lower elemental stocks, but differenceswere not significant.

Correlations between elemental concentrations and BD andSOC were calculated for all plots (Fig. 3). For Ntot and SOC,Stot and SOC, and Stot and Ntot, a strong positive correlationwas found at all plots for both years. Between BD and SOC,a negative correlation was determined.

3.2 Vegetation parameters

In the first year of the grazing experiment, all sites revealedsimilar amounts of ANPP of 119–130 g DM m–2 (Fig. 4). In2008, both ungrazed and grazed plots showed higheramounts of ANPP than in 2005. While accumulation of her-bage was 36% higher at UG sites in 2008 compared with2005, the increase at HG4.5 and HG7.5 was less pronouncedwith 24% and 14% higher values, respectively. However, after3 y of grazing exclusion, ANPP was significantly higher(p < 0.01) at UG compared with HG4.5 and HG7.5 sites.

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Table 2: Stocks of SOC, Ntot, and Stot determined in 2005 and 2008for the first 4 cm of the topsoil of ungrazed (UG) and heavily grazed(HG4.5, HG7.5) sites (standard deviation in parentheses, n = 40 forUG, n = 10 for HG4.5 and HG7.5).

Site/year SOC/ kg m–2

Ntot/ kg m–2

Stot/ kg m–2

UG/2005 1.04 (± 0.23) 0.107 (± 0.020) 0.015 (± 0.002)

UG/2008 1.11 (± 0.24) 0.113 (± 0.022) 0.016 (± 0.003)

HG4.5/2005 1.03 (± 0.14) 0.106 (± 0.010) 0.015 (± 0.002)

HG4.5/2008 0.88 (± 0.05) 0.092 (± 0.006) 0.013 (± 0.001)

HG7.5/2005 1.01 (± 0.19) 0.104 (± 0.017) 0.015 (± 0.003)

HG7.5/2008 0.81 (± 0.09) 0.090 (± 0.010) 0.013 (± 0.002)

0.6

0.8

1.0

1.2

1.4

1.6

BD

/ g

cm–3

10 15 20 25 30 35 40

SOC / mg g-1

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Nto

t / m

g g–1

10 15 20 25 30 35 40

SOC / mg g-1

UG 2005UG 2008

HG4.5 2005HG4.5 2008

HG7.5 2005HG7.5 2008

0.0

0.1

0.2

0.3

0.4

0.5

0.6

10 15 20 25 30 35 40

SOC / mg g-1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

1.0 1.5 2.0 2.5 3.0 3.5 4.0

Ntot / mg g-1

y = 0.088x + 0.324R² = 0.8885, p<0.01

y = 0.0096x + 0.101R² = 0.6597, p<0.01

y = 0.112x + 0.061R² = 0.7663, p<0.01

y = –0.012x + 0.472R² = 0.5458, p<0.01

(a) (b)

(c) (d)

UG 2005UG 2008

HG4.5 2005HG4.5 2008

HG7.5 2005HG7.5 2008

UG 2005UG 2008

HG4.5 2005HG4.5 2008

HG7.5 2005HG7.5 2008

UG 2005UG 2008

HG4.5 2005HG4.5 2008

HG7.5 2005HG7.5 2008

S tot

/ m

g g–1

S tot

/ m

g g–1

Figure 3: Correlations between (a) Ntot and SOC, (b) Stot and SOC, (c) Stot and Ntot, (d) BD and SOC of ungrazed (UG) and heavily grazed(HG4.5, HG7.5) sites.

J. Plant Nutr. Soil Sci. 2012, 000, 1–9 Short-term degradation of semiarid grasslands 5

Figure 5 shows the results for TD and LAI determined at UG,HG4.5, and HG7.5 sites in July 2005 and 2008. At the begin-ning of the experiment, all plots revealed similar values forTD of ≈ 2630 tillers m–2. In July 2008, 3 y after the beginningof the grazing experiment, TD slightly but not significantlydecreased to 2345 tillers m–2 on average at all sites.

Results for LAI show similar initial values for all experimentalsites ranging from 0.33 to 0.41. Three years of grazing exclu-sion resulted in a significant (p < 0.05) increase of LAI of62% at UG sites. At HG4.5 and HG7.5 sites, LAI also tendedto show higher values but differences were not significant.Apart from that, LAI was significantly (p < 0.05) higher at UGcompared with HG4.5 and HG7.5 after 3 y of the grazingexperiment.

4 Discussion

4.1 Recovery and degradation of soils in a short-term grazing experiment

Results from the initial soil sampling in 2005 showed that thestarting conditions for all studied plots of the grazing experi-ment were comparable in terms of physical and chemical soilproperties (Tabs. 1 and 2, Fig. 2). Grazing exclusion for 3 y

resulted in a considerable increase of SOM associated with adecrease in soil compaction, indicated by significantly higherconcentrations and stocks of SOC, Ntot, and Stot and signifi-cantly lower BD (Fig. 3). Obviously, the recovery of continu-ously grazed steppe soils after cessation of grazing takesplace in short time periods. A short-term improvement ofgrassland soils after grazing exclusion was also indicated forthe Alxa steppe in Inner Mongolia, where SOC and N con-tents were 22% and 14% higher after 6 y of grazing cessation(Pei et al., 2008). Other investigations in semiarid grasslandsof N China also reported a significant increase of SOM and adecrease of BD when grazing was excluded for 3–5 y, but evi-dence for these findings is missing as the starting conditionswere not investigated (Su et al., 2003, 2004; Wu et al., 2008).The relatively rapid increase of SOM in grazed steppe soilsafter grazing exclusion is probably accompanied by an in-creased soil aggregation. In adjacent long-term ungrazedsites, considerably higher aggregate stabilities, higher con-tributions of aggregate-occluded OM, and a lower OM miner-alization were found than at grazed sites that indicates animproved physical protection of SOM after grazing exclusion(Wiesmeier et al., 2012; Wu et al., 2011). However, there areindications that stabilized SOM at ungrazed sites is verysusceptible to a resumption of grazing pressure (Wiesmeieret al., 2012).

In contrast to the positive effects at UG sites, heavy grazingfor 3 y resulted in a considerable reduction of SOM in the top-soil of HG4.5 and HG7.5 plots, although the differencescompared with 2005 were only significant for HG4.5 plots.The processes leading to degradation of topsoils of long-termgrazed grasslands in Inner Mongolia are well documentedand understood (e.g., Steffens et al., 2008). Nevertheless,the observed short-term changes 3 y after intensified grazingpressure were not expected. Zhou et al. (2008) investigateda degraded grassland area with different desertification gradi-ents in N China and assumed that most of SOC and Ntot islost in early stages of the desertification process but noresults are available from controlled-grazing experimentswith known starting conditions.

The loss of SOM can be ascribed to a limited input of above-and belowground biomass, destruction of soil aggregatesdue to enhanced animal trampling associated with minerali-zation of formerly protected OM and enhanced soil erosion,

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050

100150200250300350400450

AN

PP /

g D

M m

–2

UG HG4.5 HG7.5

20052008

Figure 4: ANPP for 2005 and 2008 of ungrazed (UG) and heavilygrazed (HG4.5, HG7.5) sites. Error bars represent standard deviation(n = 2).

0

500

1000

1500

2000

2500

3000

35004000

4500

TD /

m–2

UG HG4.5 HG7.5

2005 2008

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

LAI

UG HG4.5 HG7.5

20052008

Figure 5: TD and LAI for 2005 and2008 of ungrazed (UG) and heavilygrazed (HG4.5, HG7.5) sites. Errorbars represent standard deviation(n = 6).

6 Wiesmeier et al. J. Plant Nutr. Soil Sci. 2012, 000, 1–9

particularly of SOM-rich fine material (Hoffmann et al., 2008;Steffens et al., 2009). In general, a negative correlation ofSOC and BD was found at all plots (Fig. 3). However, HG4.5and HG7.5 sites showed almost unchanged BD but decreas-ing SOC contents in the third year of the grazing experiment.Continuous grazing for several decades has probably causeda relatively high precompression level (Krümmelbein et al.,2006), and the number of additional sheep at HG4.5 andHG7.5 sites did not result in further topsoil compaction. Theobserved rapid decrease of SOM after short-term intensivegrazing reveals that semiarid grasslands are very sensitivetowards land-use changes and intensifications.

4.2 Vegetation parameters as affected by grazingand climatic variations

For both grazed and ungrazed sites similar values for TD weredetected 3 y after the start of the grazing experiment. Highstandard deviations in both years at all sites indicated that thereis a high spatial and temporal variability of TD (Fig. 5). Particu-larly in species-rich grasslands as in Inner Mongolia, differentresistances and tolerances of the plants to grazing probablycontribute to the variability of TD. Thus, community level TDcannot be regarded as sensitive parameter for grazing-induced short-term degradation of semiarid steppes. Never-theless, determination of TD from selected species that aresupposed to be sensitive to short-term grazing could be apromising approach to assess grazing-induced degradation.

Determination of ANPP in 2005 showed that accumulation ofaboveground biomass was comparable between all studiedsites in the first year of the grazing experiment (Fig. 4). Althoughproductivity was higher at both ungrazed and grazed sites in2008, there was a pronounced decrease of ANPP with increas-ing grazing intensity. UG showed significantly higher values ofANPP compared with HG4.5 and HG7.5 in 2008. For LAI, signif-icantly higher values at UG compared with HG4.5 and HG7.5were found 3 y after the grazing experiment, although valuesslightly increased at HG sites (Fig. 5).

The observed increases of ANPP and LAI through yearswere expected for UG plots to be a result of both precipitationincrease and grazing exclusion. Slightly higher values of

ANPP and LAI at HG plots in 2008 can be attributed to higherprecipitation rates during the period of the experiment. In2005, when the experiment started, annual precipitationaccounted for 166 mm pointing towards a pronounced dryyear. For 2007 and 2008, precipitation was much higher with240 mm and 362 mm, respectively (Fig. 6). As water is thelimiting factor in this semiarid environment and controls netprimary productivity (Fynn and O’Connor, 2000), the impactof grazing is superimposed by precipitation-induced plantgrowth. There is evidence for other semiarid ecosystems,that biomass production is primarily controlled by climatic var-iations and grazing plays only a secondary role (Biondiniet al., 1998; Ellis and Swift, 1988; Zhang, 1998). A multipleregression analysis on a worldwide 236-site data set compiledfrom studies that compared grazed with ungrazed plots showedthat ANPP was more affected by environmental variables thangrazing intensities (Milchunas and Lauenroth, 1993).

Thus, the informative value of LAI and ANPP in short-termstudies in semiarid environments is bound on the availabilityof meteorological data and adequate reference sites whenanalyzing interannual variations, because potential detri-mental effects of grazing on vegetation can be influenced byclimatic variations. Therefore, it should be differentiatedbetween within-year and between-year differences of graz-ing-induced effects on vegetation parameters. A more stablevegetation parameter for the investigation of grazing-inducedchanges in semiarid grasslands would be plant-species diver-sity (Sanderson et al., 2004), which is largely resistant to short-term climatic variations. However, an analysis of the plant-species composition did not show significant differences inthe first 3 y of the grazing experiment and may be determinedonly after several years of intensified grazing due to the rela-tively short grazing season of 4 months per year (Christensenet al., 2003; Schönbach et al., 2010; Wan et al., 2010)

4.3 Short-term degradation of semiarid grasslands

The comparison of grazed with ungrazed sites revealed a con-siderable decrease of SOM at HG sites and a strong decline ofANPP 3 y after the start of the experiment. However, on closerexamination of the individual sites, SOM only slightly decreasedwhereas ANPP actually increased compared with the initial

2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.plant-soil.com

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J FMAM J J A S OND J FMAM J J A S OND J FMAM J J A S OND J FMAM J J A S OND

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Figure 6: Mean temperatureand amount of precipitation permonth at the Inner MongoliaGrassland Ecosystem ResearchStation (IMGERS) between2005 and 2008.

J. Plant Nutr. Soil Sci. 2012, 000, 1–9 Short-term degradation of semiarid grasslands 7

situation at the start of the experiment due to the wet years2007 and 2008. Thus, both grazed sites showed a decreaseof SOM although ANPP and associated input of OM into thesoil increased. Apparently, the decline of SOM at HG4.5 andHG7.5 can be completely explained by a destruction of soilaggregates that is associated with a mineralization of for-merly protected OM (Wiesmeier et al., 2012) and not by areduced input of OM. As ANPP and consequently the input ofOM into the soil were even higher at grazed sites, theprocess of aggregate destruction seems to be the dominantfactor for the initial loss of SOM after an increase of the graz-ing intensity. However, with ongoing grazing pressure adecreased input of OM will contribute to a continuous declineof SOM as it was indicated in adjacent long-term grazed sites(Steffens et al., 2009; Wiesmeier et al., 2009).

5 Conclusions

Three years after the start of a controlled-grazing experiment,a clear deterioration of soil and vegetation properties in termsof decreasing SOM and ANPP was observed at sites with in-creased grazing intensity. In contrast, short-term grazingexclusion resulted in a notable recovery of the sites. Weconclude that semiarid grasslands of N China are very sus-ceptible to increased changes and intensifications of the landuse in this region. Due to significant differences already 3 yafter intensified grazing the investigated soil parameterscould function as early-warning indicators for degradation ofthe steppes. Vegetation parameters except TD are also gen-erally applicable parameters indicating beginning desertifica-tion. However, as ANPP and LAI revealed a distinct sensitiv-ity to increased precipitation during the grazing experiment,grazing-induced differences might be rather detected by soilparameters instead of vegetation parameters. For futureinvestigations of short-term degradation in arid and semiaridenvironments we recommend the assessment of startingconditions of the studied sites together with an application ofdefined grazing intensities.

Acknowledgments

The authors would like to thank Qimei Lin, Tao Ma, andYuandi Zhu (Department of Soil and Water Science, ChinaAgricultural University) for logistic handling. We thank XingGuo Han, Yong Fei Bai, and the Institute of Botany (ChineseAcademy of Sciences) for the opportunity to work atIMGERS. This study was funded by the Deutsche For-schungsgemeinschaft (DFG) within the MAGIM project(Forschergruppe 536 MAGIM–Matter fluxes in grasslands ofInner Mongolia as influenced by stocking rate).

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