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放牧和围封对阿拉善荒漠草地土壤和植被的影响

Effects of Grazing and Exclosure on Soil and Vegetation in Alxa Desert Steppe

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【Author in Chinese】 裴世芳

【Supervisor】 任继周傅华

【Author's Information】 兰州大学, 草业科学, 2007, 博士

【Abstract in Chinese】 以阿拉善干旱荒漠草地为研究区域,综合运用草业科学、植物生态学、恢复生态学和土壤学等多学科理论,通过连续3年的野外调查和室内试验及数据统计分析,研究了放牧和围封过程中植物群落特征、土壤理化性状的演变,以及在此过程中灌木引致的植被与土壤资源(养分、水分、种子库)空间异质性的变化和植物群落演替的关系,主要结论如下。1.放牧和围封对草地地上植物群落结构有显著影响。围封6年至8年后,物种丰富度由15.7种增加为20.7种,增加了32%。围封样地一年生和多年生禾草的物种数显著高于放牧样地,而一年生杂类草显著低于放牧样地。放牧和围封对植被总密度没有显著影响,但在放牧样地,一年生和多年生禾草显著减少,而杂类草显著增加,表明围封改善了植物群落组成。随着围封年限的增加,植物的盖度、高度和生物量都呈显著增加趋势。3年平均的总生物量结果显示,98围封样地比放牧样地增加了86%,其中一年生禾草增加了180%,多年生禾草增加高达286%。说明过度放牧不利于牧草的生长。植物的总盖度为98围封样地比放牧样地提高104%,一年生禾草提高244%,多年生禾草高达313%,灌木提高98%。说明放牧和围封显著的影响荒漠草地植物群落的基本特征,过度放牧使草地初级生产力急剧减少,围封有利于禾草的定居和生长,促进草地初级生产力快速恢复。物种多样性随恢复演替时间的延长而增加,围封6年至8年后,多样性指数和均匀度指数分别上升了33%和21%。降雨量与植被的高度、盖度和生物量呈显著相关关系,降雨量减少导致一年生物种减少,降雨量是植物群落物种丰富度和初级生产力的主要影响因素。2.围封显著改善了草地的土壤理化性质。围封6年与自由放牧样地相比,0~20cm土层土壤有机碳和全氮含量增加22%和14%,粗沙(>0.25mm)含量减少了26%,pH值减少0.28个单位,土壤容重减小5.3%。土壤理化性状与植被特征之间呈显著相关关系,说明土壤理化性状往往制约着植物群落的变化,反过来植被状况又影响土壤养分的积蓄和盈亏。在风蚀较严重的阿拉善荒漠草地,过度放牧对草地植被、土壤养分及其周围环境有极严重的负面影响,采取封育措施会增加草地植被多样性、盖度和生物量,促进土壤肥力的增加。3.放牧和围封显著的影响着霸王灌丛根部和灌丛下的土壤肥力,土壤有机C、全N、全P均为围封6年>围封2年>自由放牧,pH值为围封6年<围封2年<自由放牧;放牧和围封也显著的影响着灌丛根部和灌丛下的土壤养分富集率,土壤有机C、全N、全P的富集率值均为围封6年>围封2年>自由放牧。霸王灌丛生物量与灌丛根部、灌丛下土壤有机C、全N和全P的含量呈显著的正相关,与pH呈显著的负相关,而与灌丛间地的土壤有机C、全N、全P和pH没有明显的相关关系。4.灌木显著的影响草本植物土壤种子库植物密度,灌丛下与灌丛间地相比,一年生禾草增加72%,多年生草增加150%,而一年生杂类草减少107.9%(围封样地数据)。灌丛对一年生禾草和杂类草的影响不同,说明灌丛下微环境并不是对所有草本植物有利。灌木显著的影响草本植物物种组成,灌丛下与灌丛间地相比,一年生禾草增加43%,多年生草增加50%,一年生杂类草减少14%(围封样地数据)。围封样地灌丛下和空地土壤种子库密度(灌丛下:850比485株/m~2;灌丛间地:846.7比535.0株/m~2)和物种数以及植被的密度、物种数、一年生禾草和多年生草的高度、生物量和盖度均高于放牧样地。灌丛下,围封样地一年生禾草(32比8g/m~2)、多年生草(9.3比4.6g/m~2)和一年生杂类草(16.0比12.9g/m~2)生物量显著高于放牧样地。围封样地一年生禾草(25%比8.1%)、多年生草(4.7%比0.8%)和一年生杂类草(18%比13.1%)盖度显著高于放牧样地。围封促进了灌丛周围一年生禾草和多年生草的定植生长,而对一年生杂类草影响较小。尽管灌木入侵草地通常被认为是草地退化过程,但研究显示在极易受损的干旱生态环境中灌木有利于植物群落的恢复演替,灌木对防止干旱荒漠草地进一步的荒漠化极为重要。5.阿拉善荒漠草地广泛分布的几种灌木和半灌木对草地土壤肥力有显著的影响。0~10cm上层中,灌丛下与灌丛间地相比土壤有机碳和全氮分别高38%和40%,且差异显著,土壤pH值降低0.14个单位,土壤全P含量增加7%,但都无显著的差异。灌丛根部与灌丛间地相比,灌丛根部土壤有机碳、全氮和全磷分别高118%,103%和20%,土壤pH值降低0.34个单位,且土壤各性状均显示差异显著(P<0.051。灌丛根部土壤有机碳最大值达5.4 g/kg,比空地最大值高160%,根部土壤全氮的最大值达0.523 g/kg,比空地最大值高128%。灌木降低风速的作用及对凋落物,风蚀物等的截获,使更多的有机物质和养分积累在灌丛下,形成灌丛肥岛效应,从而使灌丛周围土壤肥力和植被得以改善。在严重退化的荒漠草原中,灌木是保护生态系统,降低风蚀的最后屏障。

【Abstract】 Livestock grazing is recognized as one of the main causes of vegetation and soil degradation and desertification in arid and semiarid northern China. Based on a series of field investigation and laboratory analysis, effects of grazing and exclosure on soil properties, plant characteristics, soil seed bank and the role and effects of shrubs in a typical degraded area in desert steppe of Alxa were studied. Some conclusions can be drawn as follows:1. Plant community structure was greatly influenced by grazing and exclosure. Exclosure for 6 and 8 years, species number increasd 32%, and species number of plant favourite to exclosure were annual gramineous grass and perennial grass in exclosure site, whereas favourite to grazing areas was annual forb grass. With increasing exclosure years, the species number of plant was adding. Plant density of annual gramineous grass and perennial grass in exclosure sites were higher than that in grazing sites, but total plant density in exclosure and grazing were not significant difference, this showed that although exclosure was no influence on total plant density, it could alter plant community structure and was beneficial to plant community succession. Plant total biomass increased 86%, annual gramineous grass biomass increased 180%, perennial grass biomass increased 286% in 98EX. Plant total coverincreased 104%, annual gramineous grass cover increased 244%, perennial grass cover increased 313%, and shrub cover increased 98% in 98EX. Plant diversities index increased 33% after exclosurefor 6 and 8 years, and homogenize index increased 21%. The cover, high and biomass of plants were all bigger in exclosure area than in frazing area, There were remarkable correlations between rainfall and plant high, biomass, plant cover respectively, rainfall decrease resulted in speicse number and plant dry weight decrease.2. Frazing and Exclosure significantly influence properties of soil physics and chemistry. Soil organic carbon and total N in the 0-20 cm soil increased significantly with exclosure period, with 22% higher SOC, and 14% higher N, soil coarse sand fraction (>0.25 mm) decreased 26%, pH value decreased 0.28, and soil bulk density decreased 5.3% in 6EX as compared with FG There were significantly correction between soil properties and plant characteristics. This suggests that soil fertility controls the plant communities, in reverse, plant community conditions influence soil fertility profit and loss. The results suggest that while continuous overgrazing in the erosion-prone desert steppe was detrimental to soil and vegetation, this can be reversed and significant increases in soil fertility, vegetation diversity, cover and biomass can be achieved with the implementation of protecting practices.3. Chemical properties of soils nearby shrub root and under the canopy were affected significantly by exclosure and grazing. The trend of the concentrations of C, N, and P at the same position of shrubs were 6EX>2EX>FG The enrichment ratios of most soil properties nearby root in exclosure areas significantly higher compared with those of soils in free-grazing areas, especially in 0-10 cm soils. Soils nearby root had significantly higher concentrations of C, N and P as well as lower pH value compared to soils in open land, these effects occurred in 0-10 cm and 10-20 cm soils. Concentrations of C and N in the soils under the canopy increased significantly compared with those of soils in open land, the pH showed only slight but significant differences, but there was no significant difference in P between the soils under shrub canopies and in open land, these effects occurred mainly in surface soil (0-10 cm). This work shows that shrubs play a vital important role in the accumulation of nutrients and maintenance of soil fertility following exclosure in arid desert steppe of Alxa.4. Soil seed bank and plant communities were greatly influenced by shrubs. Mean number of germination density and species favourite to underneath the canopy microsite were annual gramineous grass (72% heighter) and perennial grass (150% heighter), whereas germination density and species favourite to open areas were annual forb grass (exclosure site data). The trends of plant dry weight and ground cover in August were similarly with the mean number of germination density. The height of grass was all showed taller under the shrub canopy than in the open areas. Height, crown diameter, shrub dry weight, mound height around shrub root, mean number of species, mean number of germination density in April and plant density in August were all significant higher in EX sites than in FG sites. The mean number of species, mean number of germination density in April, plant density in August, plant height, plant dry weight and ground cover of Annual gramineous grass and perennial grass were all influenced by grazing, but there was little influence on annual forb grass by grazing. Under shrub biomass of annual gramineous grass (32 vs 8 g/m~2), perennial grass (9.3 vs 4.6 g/m~2), and annual forb grass (16.0 vs 12.9 g/m~2) in 6 EX were significantly increased than that in FG Under shrub cover of annual gramineous grass (25% vs 8.1%), perennial grass (4.7% vs 0.8%), and annual forb grass (18% vs 13.1%) in 6 EX were significantly increased than that in FG This suggests that in our study the shrub facilitation process may be important for community succession on destructible environmental conditions. Although shrub invasion into the grassland is generally considered as a desertification process, preservation of shrubs from overgrazing is of vital importance in preventing the rangeland from further desertification.5. Several shrubs and subshrubs distributed widely in Alxa desert steppe are of vital importance for accumulation and maintenance of soil fertility in desert steppe ecosystem. In 0-10 cm soil, concentrations of SOC and total N under shrub canopies were significantly higher 38% and 40% than open land, while pH value decreased 0.14, and total P increased 7%, but there were no significant. The soil fertility near root compare with open land, soil SOC significant increased 118%, total N significant increased 103%, total P significant increased 20%, pH value decreasd 0.34. Shrubs can reduce wind velocity as natural barriers, leading to deposition of wind-blown soil material, dust and litters in the vicinity of the shrub crowns, hence more organic matter and fertility accumulate under shrubs, which improve soil fertility around shrub. In severely degraded rangelands, shrubs are one of the dominant life forms and play an important role in protecting the area from desertification.

【Keywords in Chinese】 放牧围封荒漠草地植被特征土壤性状灌木
【Key words】 GrazingExclosureDesert steppePlant characteristicsSoil propertiesShrub
  • 【Contributor】 兰州大学
  • 【Year of Internet Publish】200704
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