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