Effect Of Grazing Disturbance On Plant Community And Soil Index Of Alpine Meadow Dominated By Potentilla Froticosa Shrub

This study was carried out in collective-owned summer grassland in Ganchaitan, which is located at the south foot of eastern Lenglongling Mountain (a branch of north Qilian Mountain), with yak and Tibetan sheep miscellaneously grazed on it. The grassland type could be classified as alpine meadow, dominated by potentilla froticosa shrub (PFS). This study, under grads grazing stress, focused on the community structure, composition, species diversity, above-ground biomass of PFS, as well as soil moisture, soil nutrition, soil microbial biomass (SMB), soil microbial carbon (SMC) and soil microbial nitrogen (SMN) of PFS and grassland amongst PFS, by using pane net sampling based on spatial distribution in stead of temporal succession. Results showed that the current grazing stress of 5.73 /hm2 has already severely exceeded the carrying capacity of the grassland surveyed. Grazing stress changed the competitive pattern, results in the decreased hierarchy of community structure, ebbed community of high-quality forage grasses, and increased community of toxic grasses. Grazing exert significant effect on vegetation, soil and soil micro-organisms.1. Grazing stress negatively related to the distance between grazing area and resident area. The height, density, coverage, above-ground biomass of PFS, and the above-ground biomass of vegetation amongst PFS were increased with the grazing distance. Total above-ground biomass in resident area decreased 84.77%, compared to that in remote area.There are relationship of logarithmic function between the height of PFS and the grazing distance(P<0.01).There are exponential function relationship of grazing distance to the density, coverage and above-ground biomass of vegetation of PFS(P<0.01)2. The species composition and structure of the lower community of PFS were also affected by grazing stress. With the increasing of grazing stress, the hierarchy of community structure and the above-ground biomass of grasses, sedges and forbs decreased, while the biomass percentage of forbs and sedges increased (from 56% to 79.69% for forbs biomass, and a little bit increase for sedges biomass). Grasses biomass took 31.2% of the total above-ground biomass at low grazing stress, and it decreased to 9.6% of the total above-ground biomass at high grazing stress. The abundance of community species decreased with elevated grazing stress. But moderate grazing would increase species diversity.3. Soil bulk density from 0 to 10 cm depth decreased significantly (P﹤0.05) with the grazing stress, while that from 10 to 20 cm depth was not affected with grazing stress. Grazing stress exerted greater impact on soil bulk density of grassland amongst PFS than on soil bulk density of PFS grassland. For PFS grassland, soil moisture from 0 to 10 cm was lower than that from 10 to 25 cm. However, this trend reversed for the grassland amongst PFS. Soil moisture of grassland amongst PFS and of PFS grassland linearly decreased with grazing stress. The amount of capillary porosity from 0 to 25 cm depth of soil of PFS grassland was greater than that of grassland amongst PFS. The capillary porosity from 0 to 10 cm depth of soil significantly decreased (P < 0.05) with grazing stress, and that from 10 to 25 cm depth of soil was not affected (P > 0.05) by grazing stress.4. As grazing pressure increased, soil organic matter (in 0-25cm layer) decreased from 108.73 mg/g to 80.30 mg/g. there was a significant linear relationship between the declining amount and the gradients of grazing pressure. Total nitrogen and total phosphorus decreased significantly (P <0.05) in grassland amongst PFS and PFS grassland (in 0-10cm soil layer), the average of total nitrogen and total phosphorus decreased by 17.07% and 29.14% respectively, while they did not varied significantly (P> 0.05) in soil 10-25 cm layer. There was a linear correlation between both the decline amount of total nitrogen and total phosphorus and the intensity of grazing pressure in grassland amongst PFS and PFS grassland (in soil 0-10cm layer); While in general, soil available phosphorus did not changed significantly. Soil available potassium remarkable increased by 41.9% and 30.18% for the PFS grassland and the grassland amongst PFS respectively. By grazing, soil pH increased significantly, and pH value was more varied in PFS grassland than that in grassland amongst PFS.5. Under different gradients of grazing pressure, bacteria was predominate biology in soil microbial biomass, fungi and actinomycetes were less and had an obviously vertical distribution among PFS and PFS grassland. As the gradients of grazing pressure increased, the amount of bacteria, actinomycetes, fungi, microbial carbon and microbial nitrogen had a trend of decline in grassland amongst PFS and PFS grassland(in soil 10-25 cm layer), and there was a linear positive correlation between the reduced degree and the intensity of grazing pressure. The amount of bacteria, actinomycetes and fungi were decreased by 49.64%, 37.76% and 38.01% respectively, compared with the non-grazing in grassland amongst PFS (in soil 0-25cm layer). The ratio of microbial biomass carbon in grassland amongst PFS to soil organic carbon was changed between 0.85%～0.43%, The ratio of microbial biomass nitrogen to soil total nitrogen was 0.90%～1.11 %, microbial biomass had a little contribution to soil nutrition base. There was a significant linear positive correlation between soil microbial biomass and soil organic matter as between the amount of soil bacteria and soil water content.