PART III Effects of Environmental Changes on the Structure of Agroecosystems 920103_CRC20_0904_CH15 1/13/01 11:14 AM Page 313 920103_CRC20_0904_CH15 1/13/01 11:14 AM Page 314 CHAPTER 15 Impact of Grazing on Soil Properties in Steppe Ecosystems Zuozhong Chen and Xiaoyong Cui CONTENTS Introduction to Steppe Ecosystems in China . . . . . . . . . . . . . . . . . . . . . . . . . 316 Brief Account of Steppe Ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . 316 Main Types of Steppe Ecosystems and Their Features . . . . . . . . . . . 316 Temperate Meadow Steppe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 Temperate Typical Steppe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 Temperate Desert Steppe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318 Effect of Grazing on Soil Properties in Steppe Ecosystems . . . . . . . . . . . . . 318 Effect of Grazing on Soil Physical Properties . . . . . . . . . . . . . . . . . . . 319 Effect on Soil Bulk Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 Effect on Capillary Water Content. . . . . . . . . . . . . . . . . . . . . . . . 319 Effect on Soil Hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 Effect on Soil Mechanical Composition . . . . . . . . . . . . . . . . . . . 319 Effect on Soil Microgranule and Structure . . . . . . . . . . . . . . . . . 322 Effect on Clay Minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323 Effect on Submicroscopic Characters . . . . . . . . . . . . . . . . . . . . . 324 Effect of Grazing on Soil Nutrient Content . . . . . . . . . . . . . . . . . . . . 325 Effect on Organic Matter (OM), Total Nitrogen, (TN), and Total Phosphorous (TP) Contents . . . . . . . . . . . . . . . . . . . . . . 325 Effect on Soil Available Nutrient Contents. . . . . . . . . . . . . . . . . 327 Effect on Soil Humus Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 Composition of Soil Humus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 Elemental Composition of Humus . . . . . . . . . . . . . . . . . . . . . . . 329 Oxygen-contained Function Groups (OFGs) . . . . . . . . . . . . . . . 329 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 315 0-8493-0904-2/01/$0.00+$.50 © 2001 by CRC Press LLC 920103_CRC20_0904_CH15 1/13/01 11:14 AM Page 315 INTRODUCTION TO STEPPE ECOSYSTEMS IN CHINA Brief Account of Steppe Ecosystems Grassland, which is considered to be a soil type in China, is defined as “a multi-functional natural complex composed of herbaceous and wooden for- age plants together with the soil supporting them.” Grasslands are classified into several types, including natural grasslands with total coverage over 5%, campos cerrados whose crown density is under 0.3 and primarily used for grazing, shrub herbosas with crown density under 0.4, secondary grasslands after fallowed over 5 years, artificial ranges, and various greenbelts domi- nated by perennial herbaceous plants. In general, grasslands are divided into two categories—natural grasslands and artificial grasslands. The former, which is also called steppe (Li, 1979), referred to plant communities domi- nantly constituent of microthermic and xerophilous perennial herbage, on some occasions of small xeric half-shrubs. Steppe is the principal component of grassland in China. This chapter’s discussion is also confined to steppes. In Eurasia, the steppe stretches about 110 degrees of longitude from lower reaches of Danube eastward to the northeast of China, crossing Romania, Russia, and Mongolia. It is the largest in the world, and unique, and generally called the Eurasian Steppe Belt. It extends from west to east between 45° and 55°N, then turns southwestward in northeast China, ending at about 28°N. Such a distributive pattern closely correlates with the config- uration of ocean and continent, as well as effects of atmospheric circulation of this region. Because the effect of monsoons from the seas in the southeast of China weakens, and the climate effect from Siberia and Mongolia strength- ens gradually along a southeast-northwest direction, the farther away from the southeast coast, the less the precipitation and the drier the climate is. Therefore, vegetation types along the direction show distinct zonal charac- teristics. In China, the steppe extends about 4500 km from the Northeast Plain, across the Great Hinggan Mountains, vast Mongolia Plateau, Erdus Plateau, and Loess Plateau, to the south edge of the Tibetan Plateau. It occurs between 51 and 28°N, about 23 degrees of latitude. Grasslands of various types cover approximately 4.00 ϫ 10 8 hm 2 , about 40% of the total land area of China. The steppe in north China is the main part. It occupies some 3.13 ϫ 10 8 hm 2 , accounting for 78% of the total grass- land area. Main Types of Steppe Ecosystems and Their Features The immense territory of China covers 31 degrees of latitude and quite different climate zones. It spans five thermal climate zones: tropical, sub- tropical, warm-temperate, temperate, and cold temperate zones. Annual pre- cipitation varies dramatically. It is over 2000 mm at the southeast coast and 316 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT 920103_CRC20_0904_CH15 1/13/01 11:14 AM Page 316 less than 50 mm in the northwest interior region. Altitude changes from below 100 m to above 8000 m. Furthermore, there are very many types of soils in China. Hence, it is not surprising to see diverse grassland forms in China. There are five dominant types: temperate meadow steppe, temperate typical steppe, temperate desert steppe, alpine grassland, and tropical/subtropical grassland. Temperate Meadow Steppe Temperate meadow steppe is zonal vegetation in temperate semi-humid regions. It mainly consists of perennial meso-xerophilous cespitose and rhi- zomial grasses as well as meso-xerophilous and mesophilous herbs with more or less meso-xerophilous small shrubs. This type is developed under the most humid steppe climate so that it concentrates on the transitional zone between forest and steppe. In China, the temperate meadow steppe mostly distributes at the east end of the steppe belt, such as the hill regions under the foot of the Great Hinggan Mountains, and the upper parts of some alpine grassland zones. The famous Hulunbuir Grassland, Xilingol Grassland, and Horqin Grassland in Inner Mongolia, and Altay Grassland and Yining Grassland in Xinjiang, have extensive meadow steppes with a total area of 1.45 ϫ 10 7 hm 2 , 3.7% of the total grassland area in China. Temperate meadow steppes are developed under a temperate semi- humid climate. The annual precipitation varies between 350 and 550 mm. Cumulative temperature above 10°C is 1800–2200°C. The main soil types are chernozem, dark chestnut soil, and meadow soil. Those soils are fertile, with organic matter content normally over 3%. Meadow steppes are rich in plant species. There are 15–25 species in 1m 2 . Owing to the favorable natural conditions, plants grow well and are high in productivity. The average height of the plant community is as high as 50 cm, coverage is 70–90%, and forage output is 1500 kg.hm Ϫ2 . The meadow steppes have long been primary pastoral regions and tra- ditional stock raising bases in China because of the favorable natural condi- tions, high productivity, and fine forage quality. They are also good places for developing cattle, fine and half-fine wool sheep, and wool-and-meat sheep production. Temperate Typical Steppe This grassland type is developed in the interior continent under a tem- perate semi-arid climate. The flora is mainly euxerophilous and euryxe- rophilous perennial cespitose grasses or together with shrubs and small half-shrubs under some conditions. The typical steppe distributes most extensively and is the most representative one in China. The typical steppe spreads mainly in the west of Hulunbiur Plateau, most areas of Xilingol Plateau, hills of the north foot of Yinshan Mountains, IMPACT OF GRAZING ON SOIL PROPERTIES IN STEPPE ECOSYSTEMS 317 920103_CRC20_0904_CH15 1/13/01 11:14 AM Page 317 the south part of the Great Hinggan Mountains, and Xiliaohe Plain. The total area is about 4.11 ϫ 10 7 hm 2 , around 10.5% of the grasslands in China. In typical steppe areas, winter and spring are cold and dry due to the effect of the Mongolia high-pressure mass, while summer is temperate and humid under the influence of monsoons from the southeast. Therefore, the plant growing season is short but abundant with climate resources. Annual average temperature is 1.5–6°C, annual rainfall is 350–400 mm, and accumu- lated temperature above 10°C is 2100–3200°C. Soils in these regions are chiefly chestnut soil of moderate fertility with organic matter around 2–4% (Chen and Huang, 1985). There are 10–20 plant species in 1m 2 area in a typical steppe. Plant growth here is second only to that of meadow steppes. The average height of the community is about 25 cm, total coverage 50%, and forage output 1200 kg . hm Ϫ2 . Typical steppes are principally pastures and traditional livestock hus- bandry bases in China. They are also good sites for developing fine wool sheep and wool-and-meat sheep industries. Temperate Desert Steppe The temperate desert steppe is typical vegetation under temperate arid climate. The dominant plants are small perennial xerophilous cespitose grasses accompanied by some xerophilous and strong xerophilous small half-shrubs and shrubs. This form occupies a narrow belt to the west of the typical steppe. In China, it is within 75–114E and 37–47N, including the mid- dle and western part of Inner Mongolia, the north of Ningxia, the middle of Gansu, and Xinjiang Provinces. These areas are strongly influenced by the Mongolia high pressure mass but at the end of the efficient extent by the mon- soon from the southeast oceans. Hence, the climate has strong continental characteristics. The annual precipitation is 150–250 mm, annual mean tem- perature is 2–5°C, accumulated temperature above 10°C is 2200–3000°C. The main soil type is calcic brown soil. It is poor in fertility, and organic matter content is usually less than 2% (Chen et al., 1991). The composition of the plant community is quite simple here. There are 10–15 plant species in 1m 2 . Plants grow poorly and productivity is low. Average height of the grass layer is 10–15 cm and total coverage is 15–30%. Generally the hay yield is 500 kg . hm Ϫ2 , but it can be as high as 1000 kg . hm Ϫ2 under certain conditions. EFFECT OF GRAZING ON SOIL PROPERTIES IN STEPPE ECOSYSTEMS There are mainly two uses for natural grasslands in Inner Mongolia, namely, grazing and mowing. Grazing has existed for thousands of years while mowing only for several decades. During the long history of grassland animal husbandry, utilization of grasslands by grazing was a synonym of 318 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT 920103_CRC20_0904_CH15 1/13/01 11:14 AM Page 318 grassland animal husbandry. With nomadism, moving for water and grass was the dominant pattern then. Such a lifestyle was gradually replaced by settlement with the development of the economy. However, grazing remained the primary way of using natural grasslands. Therefore, it is impor- tant to study the effects of grazing on the grassland ecosystem including those on the soil system. Effect of Grazing on Soil Physical Properties Effect on Soil Bulk Density Bulk density reflects soil tightness to some extent. Therefore, it is closely correlated with soil porosity, aeration, and water-holding capacity. Soil vol- ume weight increased steadily as grazing pressure elevated. Volume weight under heavy grazing was 1.15 times that without grazing (Table 15.1; Jia et al., 1997). The result suggested that soil was tightened and its volume weight increased by long-term trampling of animals under overgrazing conditions. Effect on Capillary Water Content Capillary water content is the amount of water maintained by capillary attraction. It decreased about 4% in a heavily grazed site compared with a lightly grazed site (Table 15.1). The experiment showed capillary water con- tent of soil in the heavily grazed site was only 78–85% of ungrazed site. Effect on Soil Hardness Like bulk density, soil hardness significantly increased in surface as well as upper layers (0–20 cm) after heavy grazing (Table 15.2). Surface hard- ness was 3.16, 6.453, 9.146, and 11.107 kg · cm Ϫ2 , respectively, in ungrazed, lightly grazed, moderately grazed, and heavily grazed sites in chestnut soil. Soil surface hardness in the heavily grazed site was 3.5 times that in the ungrazed site. The result implied overgrazing by long-term high stocking rate deterio- rated soil physical attributes. Effect on Soil Mechanical Composition Overgrazing had great influence on soil mechanical composition because of long-term trampling by animals. The most significant effect was on the surface layer in which sand content obviously increased while clay content greatly decreased (Gu and Li, 1997; Kang et al., 1997). These studies indicated that change of soil particle composition was the main reason for soil sandifi- cation and erosion in degraded grasslands. In dark chestnut soil under IMPACT OF GRAZING ON SOIL PROPERTIES IN STEPPE ECOSYSTEMS 319 920103_CRC20_0904_CH15 1/13/01 11:14 AM Page 319 320 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT Table 15.1 Changes of Soil Physical Features and Capillary Water Content Under Different Grazing Pressure Treatments Lightly Moderately Heavily Number of Ungrazed grazed grazed grazed samples Soil types Items Mean Variance Mean Variance Mean Variance Mean Variance Dark Bulk density 1.162 0.0814 1.260 0.0497 1.302 0.0841 1.379 0.0586 30 chestnut (g · cm Ϫ3 ) Surface hardness 3.186 1.659 6.453 1.169 9.146 2.381 11.107 2.489 50 (kg · cm Ϫ2 ) Capillary water 39.24 2.814 34.04 1.065 33.29 0.550 30.55 1.042 9 content (%) Bulk density 1.162 0.0282 1.312 0.0261 1.314 0.0285 1.329 0.0319 (g · cm Ϫ3 ) Typical Surface hardness 2.903 2.431 7.122 2.235 4.726 3.351 7.169 2.451 chestnut (kg · cm Ϫ2 ) Capillary water 34.17 0.733 33.77 1.282 31.60 2.097 29.20 0.923 content (%) 920103_CRC20_0904_CH15 1/13/01 11:14 AM Page 320 IMPACT OF GRAZING ON SOIL PROPERTIES IN STEPPE ECOSYSTEMS 321 Table 15.2 Mechanical Composition under Different Grazing Pressures Contents of the soil particles in different classes (g . kg ؊1 ) Very Coarse Medium Fine Very fine coarse sand sand sand sand Silt Clay Depth Stones sand 0.5–1 0.25–0.5 0.1–0.25 0.05–0.1 0.05 Ͻ 0.002 Microgranule Soil type (cm) Ͼ 2mm 1–2mm mm mm mm mm mm mm (g . kg ؊1 ) Dark chestnut 0–5 / 2 24 42 159 355 250 168 9 5–20 / 2 25 56 240 366 179 132 11 20–40 / 2 23 46 239 391 189 109 40–90 / 3 15 35 236 447 164 101 Ͼ 90 / / 5 25 405 437 48 80 Dark chestnut 0–5 / / 34 59 185 351 212 159 5 (overgrazing) 5–20 / 3 28 73 252 388 137 119 8 Typical 0–12 / 2 60 44 123 410 247 114 12 chestnut 12–26 / 3 52 47 116 412 216 154 10 26–32 / 3 40 38 104 417 251 147 32–61 / 2 34 34 102 441 216 171 61–96 / 4 25 14 65 442 277 173 Typical 0–12 / 10 90 64 176 349 175 136 7 chestnut 12–37 / 7 86 65 143 381 178 140 10 (overgrazing) 37–72 / 10 52 39 117 354 274 154 Note: / denotes nil. 920103_CRC20_0904_CH15 1/13/01 11:14 AM Page 321 heavy grazing, clay content in 0–5 cm soil layer was 159 g.kg Ϫ1 , about 95% of that in the ungrazed site where the content was 168 g.kg Ϫ1 (Table 15.2). On the contrary, particles larger than 0.1 mm were 278 g.kg Ϫ1 and 227 g.kg Ϫ1 in the heavily grazed and ungrazed site, respectively. The former contained 1.2 times the clay content of the latter. Effect on Soil Microgranule and Structure Animal trampling under overgrazing had a profound effect on soil struc- ture and microgranule. Microgranule was the typical structure in grassland soil. It largely determined water, nutrition, and other soil fertility character- istics. In overgrazed sites, soil microgranules were enormously reduced. For example, the content was 20 g . kg Ϫ1 in dark chestnut soil under natural con- ditions. However, it was only 13 g . kg Ϫ1 in the overgrazed site, approxi- mately 65% of the former. It was the same in typical chestnut soil. The structures referred to the relatively stable aggregates that were larger than 0.25 mm and composed of soil particles in different sizes binding together by diverse materials. Soaking the original-state soil for a period of time, then screening it in the water, the structures would remain on the mesh. The content of structures reflected the soil structure status to some degree. Table 15.3 shows the content of soil structures in variously degraded soils caused by different grazing pressures, and Figure 15.1 characterizes the mor- phology of aggregates. There were a great number of structures in dark chestnut soil under nat- ural conditions. Structures (Ͼ0.25 mm) accounted for 92.1% of total soil weight in 0–5 cm soil layer and 87.7% in 0–20 cm. However, they were only 38.2 and 36.2% in the same soil layers in seriously degraded dark chestnut soil, decreasing 51 and 54%, respectively. Not only the number declined, but also its morphology changed enormously. Most structures were elliptic, 322 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT Table 15.3 Contents of Soil Structures in Degraded Dark Chestnut Soils Soil Structures ( Ͼ 0.25mm) Degree of soil Depth weight Percent of degradation (cm) (g) Weight (g) soil weight (%) Undegraded 0–5 20 18.42 92.1 5–20 20 17.56 87.8 Slightly degraded 0–5 20 16.84 84.2 5–20 20 16.36 81.8 Moderately degraded 0–5 20 10.17 50.9 10–15 20 8.46 42.3 Seriously degraded 0–5 20 7.64 38.2 10–18 20 7.26 36.2 920103_CRC20_0904_CH15 1/13/01 11:14 AM Page 322 [...]... organic binding matter Quartz content was increased and binding force between minerals was decreased so that the complex was poorly structured and unstable (Figure 15. 3) 920103_CRC20_0904_CH15 324 1/13/01 11:14 AM Page 324 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT Figure 15. 2 Organic matter covering on the clays Figure 15. 3 Membranous inorganic and organic matter between aggregates... with and without Grazing 920103_CRC20_0904_CH15 Page 328 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT 920103_CRC20_0904_CH15 1/13/01 11:14 AM Page 329 IMPACT OF GRAZING ON SOIL PROPERTIES IN STEPPE ECOSYSTEMS 329 2 Enormous decreases of extractable humus and ration of HA/FA Extractable humus content was 1.60 and 1.47% in the top layer, free of and with grazing Proportions of HA and. .. Table 15. 4 Soil Nutrient Content in Soils under Different Grazing Pressures 920103_CRC20_0904_CH15 Page 326 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT 920103_CRC20_0904_CH15 1/13/01 11:14 AM Page 327 IMPACT OF GRAZING ON SOIL PROPERTIES IN STEPPE ECOSYSTEMS 327 Changes of TN and TP were similar with OM They decreased with an increase of the stocking rate Grasses grew poorly and vegetation... prismatic and large, and there were fewer organic binding materials between soil particles when overgrazed Consequently, the structures were unstable and prone to collapse upon soaking and shaking in water Effect on Clay Minerals Clay minerals were dominated by cloudy smectite and lamellate hydromica in the 0–5cm layer in dark chestnut soil There was only a small proportion of kaolinite and incomplete... effect of grazing on humus in top layer of different sites was similar to that along soil profile in the grazed site As the influence of grazing declined from top layer down, C/H and C/N increased FA responded differently Its C and H contents were lower while N and O content were higher with grazing Thus, C/H and H/O were obviously higher while C/N and C/O lower after grazing Oxygen-contained Function Groups...920103_CRC20_0904_CH15 1/13/01 11:14 AM Page 323 IMPACT OF GRAZING ON SOIL PROPERTIES IN STEPPE ECOSYSTEMS 323 Figure 15. 1 Morphology of the aggregates inside (B1) and outside (B2) the fence inlaid with pores in different sizes under natural ungrazed conditions Soil particles were bound tightly with plant debris and organic matter The structures were stable even upon soaking and shaking in water In contrast, structures... were 0.82 and 0.78% in the former and 0.73 and 0.74% in the latter condition Elemental Composition of Humus Carbon (C) and oxygen (O) composed most of HA and FA HA had 50–60% of carbon and 30 –35% of oxygen The hydrogen (H), nitrogen (N), and sulphur contents were 4 –6%, 2–3%, and 0 –2%, respectively Grazing influenced elemental ratio of humus as shown in Table 15. 6 Proportions of C and N in HA were... minerals were coated or interlaced by some membranous organic matter secreted by roots (Figure 15. 2) to form an inorganic-organic-complex Clay mineral types in overgrazed dark chestnut were the same as those in natural soils free of grazing Yet the morphology of minerals was different Using an electron microscope it was found that the hydromica was smooth with distinct edges and little organic binding... COOH and Ph OH 0.92 1.45 1.49 0.95 1.19 1.16 T Ratio of QC ‫ ؍‬O and KC ‫ ؍‬O 0.46 0.39 0.31 0.51 0.38 0.32 Ratio of Ph OH and Al OH 11:14 AM 3.68 3.86 3.99 Alcoholic OH (Al OH) 1/13/01 1.88 1.48 1.26 Phenolic OH (Ph OH) 332 Table 15. 8 Effect of Grazing on the Content of OFGs in FA 920103_CRC20_0904_CH15 Page 332 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT 920103_CRC20_0904_CH15 1/13/01... carbonyl groups significantly lower In FA besides quinoic group and total carbonyl, alcoholic hydroxy and total carbonyl were also higher under grazing conditions Content of carboxyl and total hydroxies trended to increase after grazing There were many factors contributing to the changes of function groups in HA and FA in which vegetation variance induced by grazing was important Grazed Ungrazed Grazed . parts of some alpine grassland zones. The famous Hulunbuir Grassland, Xilingol Grassland, and Horqin Grassland in Inner Mongolia, and Altay Grassland and Yining Grassland in Xinjiang, have extensive. (Figure 15. 5). 324 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT Figure 15. 3 Membranous inorganic and organic matter between aggregates. Figure 15. 2 Organic matter covering on. 327 920103_CRC20_0904_CH15 1/13/01 11:14 AM Page 327 328 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT Table 15. 5 Change in Humus Composition in Chestnut Soil with and without Grazing Humus