Dynamics of soil physical and chemical properties and vegetation succession characteristics during grassland desertification under sheep grazing in an agro-pastoral transition zone in Northern China

ARTICLE IN PRESS Journal of Arid Environments

Journal of Arid Environments 70 (2007) 120–136

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Dynamics of soil physical and chemical properties and vegetation succession characteristics during grassland desertification under sheep grazing in an agro-pastoral transition zone in Northern China D. Huanga, K. Wanga,, W.L. Wub, a

Department of Grassland Science, China Agricultural University, Beijing 100094, PR China College of Resources and Environment, China Agricultural University, Beijing 100094, PR China

b

Received 20 June 2005; received in revised form 13 December 2006; accepted 14 December 2006 Available online 21 February 2007

Abstract A number of field experiments were conducted from 1986 to 2003 to investigate the dynamics of soil physical and chemical properties as well as vegetation succession characteristics during the grassland desertification process under sheep grazing. Results indicate that fine silt (0.01–0.001 mm) removal and medium sand (0.5–0.25 mm) increase occurred early in the desertification process resulting in coarser surface soil. The fine sand (0.25–0.05 mm) was the major (33.7–68.4%) soil fractions element throughout the process. Changes in soil fractions were associated with a decline in soil fertility as the natural grassland shifted to a desertified landscape. The organic matter concentration decreased significantly by 94%, 89% and 69%, respectively, in the 0–5, 5–10 and 10–30 cm soil layers. The desertification effects on total soil N followed the same trend as the organic matter. Total soil P and K concentrations decreased only slightly and were consistent early in the desertification process. Soil bulk density increased companied with the decline of soil porosity and compaction as the desertification process continued. Species diversity declined both in the plant community and the soil seed bank, and species richness decreased by 56%. Three successional stages were identified, with bunchgrass communities being the first, followed by the growth of rhizomous grasses, and then sandy species and annual plant communities. In conclusion, grassland desertification was accompanied by severe soil erosion, soil nutrition decline and species diversity losses. r 2007 Elsevier Ltd. All rights reserved. Keywords: Annuals; Bulk density; Bunch grasses; Fertility; Sheep; Soil texture

Corresponding authors. Tel./fax: +86 10 62733338.

E-mail address: [email protected] (K. Wang). 0140-1963/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2006.12.009

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1. Introduction China is one of the most severely desertified countries in the world with up to 3.3 million km2 desertified lands (Chen et al., 1996; Decai, 1998; Zha and Gao, 1997; Zhong and Qu, 2003). In recent years, desertification has become a major environmental problem, attracting widespread attention in China, especially, in the arid, semi-arid and dry semi-humid climatic zones. The desertification process generally occurred accompanied by soil and vegetation degradation, water and wind erosion (Dregen, 1998). For the grassland desertification, early research indicated that it is often characterized by vegetation replacement (e.g., perennial herbaceous species are often replaced by higher sandy tolerance shrubs) (Havstad et al., 2000) and soil texture changes (Lauenroth and Milchunas, 1991). At present, 50% of the deserted lands lie in the agro-pastoral transition zone of northern China (Zhao and Masayuki, 1997; Zhu and Cheng, 1994). This desertified transition region serves as the main source of sands that were carried aloft by windstorms and ultimately distributed throughout the country’s eastern regions as heavy layers of dust (Zhang and Shi, 2003). The location of the transitional region determines its great importance as a natural ecological shelterbelt and water conservation reservoir for China’s eastern farming areas (Kang and Dong, 2003; Tang and Zhang, 2003). In addition, the area plays an important role in national ecological policy and environmental protection (Shen, 2001; Zhou, 1992). By investigating the effects of the desertification process on soil properties and vegetation succession in this region, a better understanding of desertification mechanisms will be achieved, and this will lay the ground work for more effective strategies in combating desertification and reducing desertification risk in the future. The transitional region covers a window bounded by 35–501N and 100–1251E (Chen and Li, 2003). To the south of this region is the semi-humid North China Plain, which is an intensive agriculture region. To the north is the nomadic region of steppes along the Mongolian Plateau (Li, 1995; Wang and Gao, 2003). Temporal and spatial variability of climate is one of the most notable features of the transitional region, especially for precipitation and temperature. The climate varies from subhumid to semiarid, with average annual precipitation ranging from 250 to 500 mm. Due to the large variability in climate along with increasing human activity, this region is known as one with high desertification risk (FAO, 1977). In the past century, improper land-use practices severely damaged the transitional region, and soil erosion and grassland desertification were aggravated. According to previous reports, overgrazing is thought to play a major role in increasing desertification (Bahre, 1991; Hodgson and Illius, 1996; Valone and Sauter, 2005; Vavra et al., 1994). Desertified lands caused by overgrazing make up 29% of the total desert lands in China and 31% in the agro-pastoral transition zone of northern China (Zhao and Masayuki, 1997). Most of the research over the past few decades on desertification and its rehabilitation focused on sand transport mechanisms and sand dune movements (Wu, 1965; Zhu et al., 1964; Zhu et al., 1989). More recently, researchers have begun to analyze the effect of climate change and human activities on land desertification (Thomas, 1993), with more effort on desertification trends and possibilities in China (Yong and Jay, 1997). These efforts have helped promote the national ecology-protection project, but most were conducted in north-western China with few efforts on desertification processes in the transitional region. Of those studies that did address desertification processes in the

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transitional region, most focused on the effects of climatic and geographical variation on ecosystem production and vegetation distribution (Wei and Xin, 2003; Xin et al., 2004). In this study, we examined the dynamics of soil properties and vegetation succession characteristics during the grassland desertification process caused by overgrazing in agropastoral transition zone of northern China. It was hoped that this study might contribute new information to better understand the grassland desertification process, and prove valuable in addressing national ecological and environmental protection issues. 2. Materials and methods 2.1. Study site The study was conducted at the Key Grassland Experiment Station of the Ministry of Agriculture (1151400 E, 411460 N, 1460 m a.s.l), which lies in the typical agro-pastoral transition region in Hebei Province in northern China. The research site is located at the southern end of Hunshandak Sandy Land and is considered as one of the potential desertified areas (Fig. 1). It is characterized by a continental, semi-arid, monsoon climate in the temperate zone, with windy and dry winters and springs (Fig. 2), and warm and comparatively rain-rich summers followed by short and cool falls. Annual precipitation ranges from 350 to 450 mm, about 80% of which falls between June and September (Fig. 3). The annual potential evaporation is 1700–2300 mm and the annual mean temperature is 1 1C. The landscape has the physical appearance of typical Inner Mongolian steppes with Stipa krylovii, Stipa grandis, Aneurolepidium chinensis and Ulmus pumila being predominant. The soil belongs to the Cambids. 2.2. Experimental design and treatments The research was conducted from 1986 to 2003 on the experimental field under yearround, heavy grazing by Mongolia sheep. Before the study carried out, the experimental field was in natural steppe conditions for 2 years grazing elimination, and the vegetation canopy coverage were similar in all plots. Four control plots (300  300 m) were randomly selected and fenced to prevent grazing, and four parallel plots (300  300 m) were established outside the fenced areas for grazing. Grazing intensity was determined according to 80% biomass (forage aboveground dry matter) was ingested by sheep. The aboveground forage biomass was measured using cutting method in the sampling plots (1 m2) in the grazing field. Stocking density were 6.7, 4.4 and 3.3 sheep unit (a 50 kg sheep mother+lamb) per ha, respectively, in 1986–1991, 1992–1999 and 2000–2003 period. The sheep grazing regime were conducted according to the traditional ways, and grazed from 6:00 to 10:00 and 15:00 to 18:00 in summer (June to September), and from 9:00 to 15:00 during autumn to spring (October to May) period. The distance between plots was 50 m, minimizing the influence of microclimate (e.g., precipitation and air temperature) and soil conditions. 2.3. Soil sample collection and analysis Thirty random quadrats were used for soil sample collection in each plot each year (from 1986 to 2003). One soil sample was collected using soil auger (30 mm in diameter) within

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Fig. 1. Sketch map of the study site and the location of Agro-Pastoral Transition Zone of Northern China (APTZNC).

Wind speed (m . s-1)

8

6

4

2

0 Jan

Feb

Mar

Apr

Jun

Jul Aug Month

Oct

Nov

Dec

Feb

Fig. 2. Daily mean wind speed during the experiment period (1986–2003).

each quadrat and separated according to depth (0–5, 5–10 and 10–30 cm). After air-drying, all samples were brought back to laboratory for physical and chemical analyses. The soil samples were analyzed using standard methods as described in the Soil Analysis Book

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600

15

500

10

90

5 0

60

-5 -10

30 0 Jan Feb mar Apr May Jun Jul AugSep Oct NovDec Months Precipitation (nm) Temperature (°C)

4 3

400 300

2

200 1

-15

100

-20

0

Temperature (°C)

20 Temperature (°C)

Precipitation (nm)

120

Precipitation (nm)

124

0 1986

1990

1994

1998

2002

Year Annual Precipitation Annual temperature June-September Precipitation

Fig. 3. Changes of the monthly (a) and annual (b) mean precipitation and temperature in the study site (from 1986 to 2003).

(Nanjing Institute of Soil Science, 1980) and Methods of Soil Analysis (Soil Science Society of America, 1982). Soil particle size was classified into coarse sand (1–0.5 mm), medium sand (0.5–0.25 mm), fine sand (0.25–0.05 mm), coarse silt (0.05–0.01 mm) and fine silt (0.01–0.001 mm). Soil particle size distribution was determined for the 0–5, 5–10 and 10–30 cm soil layers using a Kohn-type pipette analyzer. Soil bulk density and total porosity were measured for the 0–10 cm soil layer. Soil bulk density was measured by the core method (100 mm in diameter). Soil particle density was determined using a liquid displacement method, and was used to calculate the total porosity together with bulk densities. Soil compaction was measured using the soil compaction tester (Agridry Rimik Pty. Ltd., Australia) in the 0–10 cm soil layer, and the soil surface spectral reflectivity was determined using a portable spectrometer (Nanjing Soil Apparatus Company, China). The soil chemical properties were determined for the 0–5, 5–10 and 10–30 cm layers. Soil samples were pretreated with hydrogen peroxide prior to digestion test for soil organic matter (SOM) concentration. The SOM extraction processes were conducted as following: 10 g of air-dried soil was put into a 200-ml propylene flask, 100 ml of extractant (0.5 N NaOH) was added and the air in the flask was displaced by N2, and shook for 24 h at room temperature, then the dark-colored supernatant solution was separated from the residual soil by centrifugation (10,000 rpm for 10 min), suspended the soil residue in 50 ml of distilled water, and separated the phases by centrifugation as before, the washings were added to the supernatant. Total soil N and P concentration were determined on sub-samples using an Alpkem autoanalyzer (AA2, TECHNICON Company, USA). Total soil N was analyzed using the standard Kjeldahl acid-digestion method. Total soil P concentration was analyzed using acid digestion method, mixed 2.0 g air-dried soil (o2 mm) with 30 ml of 60% HCLO4 in a 250-ml volumetric flask, and continue heated the mixture at the boiling temperature 20 min longer, cooled the mixture, and add distilled water to obtain a volume of 250 ml. Total soil K concentration was analyzed using the digestion techniques (HF-HCLO4), after extracted from the soil, it was determined by atomic absorption spectrophotometer (AA-670, SHIMADZU Company, Japan).

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2.4. Vegetation sampling Vegetation surveys were conducted during the growing season (June to August) each year. From the center of each plot, four transects were established along east (E), south (S), west (W) and north (N) directions, respectively. Fifteen quadrats (1  1 m) were randomly placed along each transect. Each plant was individually separated and counted (including ramets), and the plants were separated according to different grass species, so, the plant density could be calculated. Plant canopy coverage (C) of each species was measured based on visual estimation of the percentage plant cover. Each species above-ground biomass (dry matter) (W) was conducted using cutting method. The relative canopy coverage (C0 ), plant density (D0 ) and biomass (W0 ) were calculated using the ratio of one individual species surveyed data to all species. Plant species diversity was analyzed, including the Simpson index (D), Shannon–Wiener index (H0 ), importance value (V), species richness and species evenness (E) (Department of Biology, Inner Mongolia University, 1986; Sun et al., 1993). The following formulas were used: D¼1

s X

ðpi Þ2 ,

i¼1

H0 ¼ 

S X

pi ln ðpi Þ,

i¼1

pi ¼ N i =N, E¼

s X

ðDi  Dmin Þ=ðDmax  Dmin Þ,

i¼1

Dmin ¼ ½ðS  1Þð2G  SÞ=½GðG  1Þ, Dmax ¼ ½GðS  1Þ=½SðG  1Þ, V ¼ C 0 þ D0 þ W 0 , where S is of the number of species, G the samples number, Ni the individual number of species i, N the total individuals, number of all the species, and Pi the ratio of species i, s the individual number in all species. C0 , D0 and W0 refer to the relative canopy coverage, density and biomass of the respective species. 2.5. Soil seed bank The soil seed bank research was conducted in different desertification intensity years, potential (1986), slight (1991), medium (1995), severe (1998) and extremely severe (2003). Germination method was used to determine the characteristics of soil seed bank. Sixty soil samples (1  1 m  5 cm deep) were collected at randomly selected positions within each plot. Samples were brought to laboratory and spread evenly in seed trays that had been prepared with a 30 cm layer of sterilized vermiculite. Trays were placed on benches in an open greenhouse under natural sunlight conditions (temperature 10–25 1C) for germination test. Emerging seedlings were identified every 2 weeks. Seedlings were removed after

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identification species. This study lasted until there were no new seedling emergences within 3 weeks. 2.6. Statistical analysis The experimental design was a repeated measure with several dependent variables (i.e., soil nutrients and vegetation characteristics), consisting of two treatments with four replications. The treatments were two factors with independent variables of grazing vs. no grazing and different desertification years. All data were calculated for each quadrat, then used the mean (4 replicates) for analysis. Two-way ANOVA (grazing vs. no grazing and years) at p ¼ 0.05 level were used according to least significant difference (LSD) test. SPSS 10.0 for Windows software package was used in statistical analyses. 3. Results 3.1. Soil properties 3.1.1. Soil particle size distributions In the control plots, soil particle size distribution (%) were maintained at relative consistent levels in each soil layer throughout the experiment period, with only fine sand (0.25–0.05 mm) showing a slightly increasing trend, and the percentage of coarse sand (1–0.5 mm) decreased gradually. However, the soil particle size distribution in the grazing plots changed significantly overtime (Fig. 4). The percentage of fine silt (0.01–0.001 mm) significantly decreased in each soil layer, which was decreased by 91%, 78% and 81%, respectively, to the 0–5, 5–10 and 10–30 cm soil layers at the end of the experiment. The percentage of coarser silt (0.05–0.01 mm) and sand (1–0.5 mm) in the 0–5 cm soil layer increased in the early experiment period and reached the maximum in 1994, and then gradually decreased, and in the 5–10 and 10–30 cm soil layers mostly of them were under 10%. Meanwhile, the fine sand (0.25–0.05 mm) content maintained at the relatively consistent levels in the 0–5 and 5–10 cm soil layers throughout the experiment period. The changes of the soil particle composition suggesting that the silt was selectively removed, which aggravated the surface soil coarsening process. Changes in coarse silt and fine silt contents indicate a gradient of grassland desertification process. The dominant soil particles were fine (33.7–68.4%) and medium (10.6–32.1%) sands. However, the dynamics of medium sand distribution showed considerable differences among the 0–5, 5–10, and 10–30 cm soil layers during the grassland desertification process. In the top 0–5 cm soil layer, it gradually increased and reached a maximum at the middle experimental stage, and then decreased. It was peaking around 1995. In the 5–10 and 10–30 cm soil layer, the medium sand percentage significantly changed overtime, but showed no obviously trend and the minimum percentage was 10.6% and the maximum added up to 31.7%. 3.1.2. Soil bulk density, porosity, compaction and reflectivity Topsoil bulk density increased, and soil porosity and compaction decreased overtime in the grazing plots, while, they showed reversed trends in the control plots (Fig. 5). Soil compaction was maintained at consistent levels during the early desertification period and significantly decreased at the end of the experimental period, declined by 60.1%. As for the

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Coarse sand (1-0.5 mm)

6 4 2

30 20 Soil particle size distribution(%)

Fine silt Coarse silt (0.01-0.001 mm) (0.05-0.01 mm)

Fine sand Median sand (0.25-0.05 mm) (0.5-0.25 mm)

0

10 0 60 40 20 0 12 9 6 3 0 25 20 15 10 5 0 1986 1990 1994 1998 2002 1986 1990 1994 1998 20021986 1990 1994 1998 2002 Year Grazing plot Control plot

Fig. 4. Dynamics of soil particle size distribution (%)in 0–5 (a), 5–10 (b) and 10–30 cm (c) soil layers during grassland desertification process between 1986 and 2002 (mean+S.E., n ¼ 4).

soil surface spectral reflectivity, it increased steadily from 19.5% to 40.9% in the grazing plots, which was significantly higher than that of the control plots throughout the desertification process. 3.1.3. Soil nutrient dynamics Soil nutrition decreased significantly overtime in the grazing plots (Fig. 6). SOM decreased from 43.3 to 2.7, 32.0 to 3.4, 17.8 to 5.4 g kg1, respectively, in 0–5, 5–10 and 10–30 cm soil layer. At the end of the experiment, the SOM in the 0–30 cm soil layer was 89.3% lower compared to the control plots. Most of the decrease in SOM occurred in the 0–10 cm soil layer. In the 10–30 cm soil layer, the organic matter concentration was slightly greater than that of the control plots in the early desertification period. Total soil N followed the same trend as SOM. In the top 0–5 cm soil layer, total N concentration decreased by 99% compared to the control plots in the later severe

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Soil porosity (%)

Soil bulk density (g.cm-3)

2

1

0

40 20 0

Soil surface spectral reflectivity (%)

4 Soil compaction (kg.cm-2)

60

3 2 1 0 1986

1990

1994 Year

Grazing plot

1998

2002

40

20

0 1986

1990

1994

1998

2002

Year Control plot

Grazing plot

Control plot

Fig. 5. Dynamics of soil bulk density, soil porosity, soil compaction in the 0–10 cm soil layer, and soil surface spectral reflectivity changes during grassland desertification process between 1986 and 2002 (mean+S.E., n ¼ 4).

desertification stage. In the 5–10 cm soil layer, total N concentration was nearly equal between the desertified and control plots in early desertification period and significantly decreased in the later experiment period. Total soil P decreased gradually and the decline was not as great as for SOM. Total soil P at the end of the experiment declined 69.9%, 65.5% and 55.4% relative to the controls in the 0–5, 5–10 and 10–30 cm soil layers, respectively. Total soil P in the 10–30 cm layer was maintained relative consistent levels early in the desertification process and decreased sharply at the end of the experiment period. The concentration of soil K changed in a similar manner as P.

3.2. Vegetation characteristics 3.2.1. Plant species composition Species composition in the control plots was relatively stable during the experiment period in terms of dominant species (Table 1), Aneurolepidium chinensis and Stipa krylovii were the two companied dominant species in the early experiment period, however, during the later experiment period, Stipa krylovii gradually fade out and Aneurolepidium chinensis became the predominant species. In the grazing plots, original dominants were replaced by more drought-resistant and sand and wind tolerant species (e.g., Agriophllum squrrosom) with more semi-shrub species (e.g., Artemisica ordosica) appearing at the end of the experiment period. Plant communities during succession can be divided into three distinguishable stages: bunchgrass community, rhizomous grass community, and sand and annual plant community. Species replacement during succession was also reflected in changes of importance values (Table 2). The importance values of early dominant species decreased.

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Soil organic matter (g.kg-1)

60

Total soil N (g.kg-1)

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4

129

40 20 0

2

Total soil P (g.kg-1)

0 2 1

Total soil K (g.kg-1)

0 40 30 20 10 0 1986 1990 1994 1998 2002 1986 1990 1994 1998 2002 1986 1990 1994 1998 2002 Year Grazing plot

Control plot

Fig. 6. Dynamics of soil organic matter, total soil N, P and K concentration in the 0–5 (a), 5–10 (b) and 10–30 cm (c) soil layers during grassland desertification process between 1986 and 2002 (mean+S.E., n ¼ 4).

3.2.2. Species diversity Species richness in the control plots increased from 16 to 22/m2 during the 17-year period, while that in the grazing plots decreased from 16 to 7/m2, a 56% reduction (Table 3). Most of the species in the grazing plots at the end of the experimental period were annuals, such as Agriophllum squrrosom, Artemisica ordosica and Salasola collina community. Density of plants changes in the control plots showed an increasing trend, however, that in the grazing plots decreased significantly from 628 to 71/m2. Species evenness in both treatments was maintained consistent levels in the early experimental period and then decreased, but the decrease was much higher in the grazing plots than the control. Both Simpson index and Shannon–Wiener index declined significantly in the grazing plots, while that in the control plots showed a more gradual decrease. 3.2.3. Soil seed bank The species richness in the soil seed bank of grazing plots showed a similar declining trend as in the aboveground plant community with time (Table 4). Species number significantly decreased throughout the desertification process. Species and seed number in

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Table 1 Species replacement in plant communities in the grazing plots and control plots between 1986 and 2003 Study year

1986 1989

1991

1993 1995

1998

2003

Grazing plots

Control plots

Life form

Dominant plant species

Living form

Dominate plant species

Perennial Perennial Perennial Perennial Perennial Perennial Semi-scrub Semi-scrub Semi-scrub Perennial Semi-scrub Perennial Annual Perennial Annual Semi-scrub Annual Semi-scrub

Aneurolepidium chinensis Stipa krylovii Stipa krylovii Cleistogenes aquarrosa Aneurolepidium chinensis Cleistogenes aquarrosa Koeleria cristata A. frigida A. frigida Thymus mongolicus ronn A. frigida Potentilla acaulis Corispermum didutum Potentilla acaulis Agriophyllum squrrosom Artemisia ordosica Agriophyllum squrrosom Artemisia ordosica

Perennial Perennial Perennial Perennial

Aneurolepidium chinensis Stipa krylovii Aneurolepidium chinensis Stipa krylovii

Perennial Perennial

Aneurolepidium chinensis Stipa krylovii

Perennial Perennial Perennial

Aneurolepidium chinensis Stipa krylovii Aneurolepidium chinensis

Perennial

Aneurolepidium chinensis

Perennial

Aneurolepidium chinensis

Table 2 Dynamics of species importance value (%) (mean, n ¼ 4) throughout the grassland desertification process for the grazing plots Species

Stipa krylovii Aneurolepidium chinensis Cleitogenes squarrosa Koeleria cristata A. frigida Thymus mongolicus ronn Potentilla acaulis Corispermum didutum Agriophyllum squrrosom Artemisia ordosica Eragrostis Setaria vridis Stellera chamaejasme Lespedeza davurica A. argyi Scutenllaria scordiflorum

L

P P P S S P P A A S A A P P P P

Importance value (%) 1986

1988

1990

1992

1994

1996

1998

2000

26.8 19.7 14.3 2.3 4.9 3.0

26.2 22.0 15.6 9.8 5.4 4.1

24.6 15.9 23.3 10.3 6.6 8.9

13.3 14.2 20.6 18.7 16.4 11.6 2.3 1.6

9.6 11.8 7.7 4.6 18.1 20.7 5.7 10.8

3.8 24.7 6.9 3.9 19.7 6.4 16.9 16.6

3.3 4.3

4.7

5.5

3.9

2.3

4.4

2.2

2.1

4.6 3.2

3.6 6.4

4.3

5.5

3.1

L—Life form; P—perennial; S—semi-scrub; A—Annual.

1.6 6.2

14.2 3.4 16.3 8.7 5.8

22.1 21.4 4.6 15.7

9.7

6.0

2002

25.7 34.7 5.1 14.3

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Table 3 Changes of species richness, plant density, Simpson index, Shannon–Wiener index and Evenness in the grazing plots (G) and control plots (C) during grassland desertification process Study year

Species richness (m2)

Plant density (m2)

Simpson index

Shannon–Wiener index

Evenness

1986 1988 1990 1992 1994 1996 1998 2000 2002

G 16b 19a 18ab 20a 19a 15b* 11c* 9d* 7d*

G 628a 563b 550b* 547b* 438c* 440c* 239d* 106e* 71f*

G 0.78a 0.72ab 0.80a 0.67b* 0.54c* 0.46c* 0.30d* 0.33d* 0.26d*

G 2.21a 2.02b 1.87bc 1.96b* 1.75c* 1.43d* 1.17e* 1.11e* 1.03e*

G 0.69b 0.74b 0.80a 0.68b 0.44d* 0.53c 0.41e* 0.30e* 0.36e*

C 16b 16b 17b 19ab 20a 20a 21a 22a 22a

C 630c 658c 702b 733b 781a 788a 794a 817a 809a

C 0.74bc 0.76b 0.80ab 0.83a 0.81ab 0.80ab 0.77b 0.72c 0.66d

C 2.24b 2.33b 2.39b 2.60a 2.27b 2.19bc 2.04c 1.88c 1.65d

C 0.66ab 0.67a 0.72a 0.64ab 0.60bc 0.55c 0.53c 0.42d 0.40d

Means followed by the same letters within a column are not significantly different among years at pp0.05; *Significant between grazing and control plots at the same year at pp0.05 according to LSD (mean, n ¼ 4).

Table 4 Soil seed bank species richness, seed number, Shannon–Wiener index and Evenness changes in the grazing plots (G) and control plots (C) during the grassland desertification process between 1986 and 2003 Study year

Species richness (m2)

Seed number (m2)

Shannon-Wiener index

Evenness

1986 1991 1995 1998 2003

G 16a 16a 13b* 9c* 6d*

G 5071a 2179b* 1621b* 2071b* 873c*

G 2.81a 2.80a 2.45a 1.41b* 1.22b*

G 0.91a 0.89a 0.78b 0.47b* 0.34c*

C 15b 16b 18a 19a 19a

C 4865c 5310b 5415b 5839a 6074a

C 2.83a 2.79a 2.66a 2.43b 2.55b

C 0.92a 0.83a 0.89a 0.85a 0.76b

Means followed by the same letters within a column are not significantly different among years at pp0.05; * Significant between grazing and control plots at the same year at pp0.05 according to LSD (mean, n ¼ 4).

the grazing plots decreased by 63% and 40.8%, respectively, at the end of the experiment, however, that in the control plots increased by 26% and 25%. During the middle desertification process, the seed number in the grazing plots maintained a consistent level. Species diversity indices and species evenness of the soil seed bank in the grazing plots declined over time, but showed less variation than that in the aboveground plant community and not as sensitive to desertification process. The significantly desertification effect on soil seed bank occurred only during the later desertification process. 4. Discussion and conclusions Early studies indicate that livestock overgrazing and increasing aridity are the major causes for grassland desertification (Mcpherson, 1995; Wondzell and Ludwig, 1995), though there complex interactions of factors maybe responsible for desertification (Humphrey, 1958; Reynolds et al., 1999). Fire plays an important role for grass mortality and replacement. Species replacement of grass induced by fire was evident in desert

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grasslands on Sonoita Plains in southern Arizona (Bock, 1997). Ludwig and Esteban (2000) documented that even in the absence of livestock grazing and fire, desert grassland vegetation was very responsive to precipitation change over relatively short time period. However, overgrazing was the primary cause for the grassland desertification in agropastoral transition zone in northern China (Zhao et al., 1999). In this paper, soil nutrients and particle size distribution in the control plots maintained at consistent levels during the 17-year study period and changes in species composition were not significantly, thus, we could conclude that precipitation and aridity are not the primary factors causing grassland desertification in this region. From 1986 to 2003 under continuous heavy grazing, vegetation community and soil properties changed significantly. At beginning of the experiment, predominant plants were Stipa krylovii, Stipa grandis, Aneurolepidium chinensis, and Ulmus pumila intermixed with a total canopy coverage of 95%. However, after 17 years of overgrazing, the research field has been heavily damaged, with shifting sand dunes. The dominant plants species became Agriophllum squrrosom, Artemisica ordosica, and Caragana with canopy coverage under 20%. Based on the desertification gradient classification method (Kang et al., 2005, Wang et al., 2005) and the observation of the vegetation and soil properties changes in the experiment, the desertification process can be divided into four stages. (1) Fixed sandy stage (1988–1991): light wind erosion, few visible sandy particle on the soil surface, unchanged dominant vegetation, and vegetation canopy coverage between 60% and 75%. (2) Semi-fixed sandy stage (1991–1996): distinctive wind erosion, observed sandy potholes and sandy heap phenomena, o30% erosion area, changed dominant vegetation, and vegetation canopy coverage between 40% and 60%. (3) Semi-shifting sandy desert stage (1996–1999): aggravated soil erosion, 430% bare sandy, changed plant community, and dominant plants replaced by more drought-resistant and sand-wind tolerant species, and vegetation canopy coverage between 20–40%; (4) Shifting sandy dune stage (1999–2003): few plants witho20% canopy coverage, and abundant shifting sandy dune over 50% of the plots. Soil chemical and physical properties declined under heavy grazing disturbance in the agro-pastoral transition zone of northern China as desertification progressed from the native steppe grassland to the shifting sand dune stage. The dynamics of the soil particle size distribution indicated that the desertification effects facilitated the removal of silt particles resulting in coarsening of soil texture. In the desertification process, most of the alteration of soil particle composition occurred in the fixed and semi-fixed sandy stages. Soil erosion by wind is the main cause for changes in soil particle composition (He et al., 2004), and trampling by animals promotes the loss of top soil by wind erosion. Gibbens and Beck (1988) found a net surface soil loss of 34 mm (0.8 mm yr1) from 1935 to 1980 in a desert grassland of New Mexico after grazing. After the completely elimination of grazing between 1955 and 1980, net soil deposition occurred (+23 mm). There are linear relationships between soil particle size distribution and desertification degree, and soil particles size distribution can be used as a parameter to monitor soil degradation and to estimate the degree of soil desertification (Yong et al., 2004). While, in the experiment the interaction of soil removal and sheep trampling affected soil properties changes, and the changes were not appear to be linear along with the desertification process. The increase in soil bulk density is usually accompanied by reduction in porosity during the grassland desertification process (Wang et al., 2004). However, the effects of desertification on soil bulk density and porosity was not significant during early of the

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desertification process, possibly due to the interactions between soil particle alteration and soil compaction by livestock. A previous study indicated that the topsoil surface spectral reflectivity could be used as an important indication of different grassland desertification stages (Wang, 2001). In this paper, the results also showed that the soil surface reflectivity increased significantly as desertification became more severe. The decline in plant canopy coverage is likely the cause for increased soil spectral reflectivity. The concentration of total soil N and SOM is associated with the silt fraction rather than sand (Lobe et al., 2001). Therefore, the removal of the majority of silt during the grassland desertification process directly resulted in depletion of total N and SOM. In the transformation from potential desertified land to extremely desertified land in Horqin Sandy Land (in northern China), the sand content at the 0–15 cm soil layer increased from 69% to 93%, while organic C and total N contents decreased by 65% and 69%, respectively (Yong et al., 2004). In this paper, the SOM concentration in 0–30 cm soil layer declined by 89.3% relative to the control plots during the desertification process. Several other factors may affect soil nutrient losses: (1) Over grazing reduce vegetation growth and expose soil surface to erosion, leading to direct soil nutrient losses; (2) Over grazing reduces the return of litter to the soil. Unlike SOM and total N, which sharply declined as grassland desertification happened, the concentration of total soil P decreased gradually. The animal wastes may be the primary factor that promoted the total P maintained relative higher levels. The vegetation succession pattern in grassland desertification process was of the regressive type, which was in agreement with that report in the literature (Wang, 2000). Early reports indicated that C4 species occurred in early desertification succession stages because they could use the limited water resource in the growing season and survive the dry season in the form of dormant seeds. The number of C4 species and proportion of C4/C3 increased with desertification progressed in Hunshandake Desert (Wang, 2004). Plant water status was higher on the active sand dune than off-sand dune facilitated their survive (David et al., 2005). In this paper, the results showed that along with the grassland desertification progressed, species richness and density decreased significantly, and natural steppe grassland community was replaced by annual and sand tolerance species under over grazing. With grassland desertification ongoing, the invasion of higher drought-resistant and sand and wind-tolerant species decreased the competitive capacity of the native plants, and reduced their dominance. Therefore, these native plants became subordinate components in the early community and dropped out gradually or finally disappeared. Meanwhile, the psammophytes plants were well suited to the changing living conditions and at last became the dominant community. Species such as Agrophyllum aqurrosom, and Artemisia ordosica were the main community compositions at the shifting sand dune stage. Soil seed banks of different vegetation types have been studied frequently (An and Lin, 1996; Coffin and Lautenroth, 1989; Hederson et al., 1988; Thompson, 1986), although few studies were carried out on the soil seed banks subject to grassland desertification processes (Xiong and Zhong, 1992). Lu et al. (2005) found that in Hulunbeier steppe desertification process in China, the species richness of soil seed bank decreased significantly by 75.5%, 83.4% and 84.3% at semi-fixed sand stage, semi-shifting sand stage and shifting sand dune stage compared to the natural grassland, respectively, and the proportion of plant species in soil seed bank was dominated by annuals and biennials (50%). In this study, the species richness in soil seed bank decreased significantly by 68% compared to the control plots at the end of the experimental period. However, the seed number was maintained a consistent

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level during semi-fixed sand stage and semi-shifting sand stage period. This may be due to the fact that in the desertification process, annual plants such as Agriophllum squrrosom, Setaria vridis and Kochia prostrata were the dominant vegetative species, many of these have higher seed production ability than their counterparts. The changes of Shannon–Wiener and evenness index were not significantly in the early desertification process. Our study has shown that the soil properties and vegetation succession changed significantly in the grassland desertification process under overgrazing. Grassland desertification was accompanied by severe soil erosion, soil nutrition decline and species diversity losses. This information will provide a useful baseline for better understanding the grassland desertification process in agro-pastoral transition zone in northern China. Acknowledgments Funding was provided in part by a Ministry of Agriculture ‘‘973’’ Basic Research Project of ‘‘Key Techniques for Degraded Grassland Comprehensive Melioration and Reconstruction in agro-pastoral transition region (G2000018606)’’. Authors thank Dr. Yuguang. Bai., (University of Saskatchewan, Canada) for his helpful comments in the preparation of the manuscript. References An, S., Lin, X., 1996. A preliminary study on the seed banks of the dominant vegetation forms on Bao Hua Mountain. Acta Phytoecologica Sinica 20, 41–50. Bahre, C.J., 1991. A Legacy of Change. University of Arizona Press, Tucson, AZ, 231pp. Bock, C.E., Bock, J.H., 1997. Shrub densities in relation to fire, livestock grazing, and precipitation in an Arizona desert grassland. Southwestern Naturalist 42, 188–193. Chen, Y.H., Li, X.B., 2003. Intra-annual vegetation change characteristics in the NDVI-Ts space: application to farming-pastoral zone of northern China. Acta Botanica Sinica 45, 1139–1145. Chen, G., Dong, Z., Yan, P., 1996. Desertification: international research topics and research strategies of China. Exploration of Nature 15, 1–5. Coffin, D.P., Lautenroth, W.K., 1989. Spatial and temporal variation in the seed bank of a semiarid grassland. American Journal of Botany 76, 53–58. David, M.R., Ludwig, F., Lisa, A.D., 2005. Plant responses to an edaphic gradient across an active sand dune/ desert boundary in the great basin desert. International Journal of Plant Sciences 166, 155–247. Decai, Z., 1998. Dynamical Evolution of Sand Sea in China. Gansu Culture Press, 20pp. Department of Biology, Inner Mongolia University, China, 1986. Plant Ecology Experimentation. Higher Education Press, Beijing, 75–79pp (in Chinese). Dregen, H.E., 1998. Desertification assessment. In: Lal, R., Blum, W.H., Valentine, C., Stewart, B.A. (Eds.), Method of Assessment for Soil Degradation. CRC press, New York, pp. 441–458. FAO/UNESCO/WMO, 1977. World map of desertification. Food and Agriculture Organization, Rome. Gibbens, R.P., Beck, R.F., 1988. Changes in grass area and forb densities over a 64-year period on grassland types of the Jornada Experimental Range. Journal of Range Management 41, 186–192. Havstad, K.M., Herrick, J.E., Schlesinger, W.H., 2000. Desert rangelands, degradation and nutrients. In: Arnalds, O., Archer, S. (Eds.), Rangeland Desertification. Kluwer, Dordrecht, pp. 77–87. Hederson, C.B., Petersen, K.E., Redak, R.A., 1988. Spatial and temporal patterns in the seed bank and vegetation of a desert grassland community. Journal of Ecology 76, 717–728. He, W.Q., Gao, W.S., Tuo, D.B., Zha, P.Y., 2004. Study on some factors influencing soil erosion by wind tunnel experiment in North Ectone between Agriculture and Pasture. Journal of Soil and Water Conservation 18 (3), 1–8. Hodgson, J., Illius, A.W., 1996. The Ecology and Management of Grazing Systems. CAB International, Wallingford, United Kingdom, 466pp.

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