| Grazing has great influence on typical steppe ecosystem services. Currently. grassland degradation caused by overgrazing has become the main limiting factor of economic development and ecological environmental protection of pastoral areas. Deep research based on response of grassland ecosystem services to grazing and its mechanisms has great theoretical and practical significance in restraining grassland degradation, recovering ecological balance of grassland and realizing the sustainable development of grassland utilization. This study is aimed to examine the effects of grazing intensity, topography (flat vs. slope) and fluctuations in precipitation (wet vs. normal years) on biodiversity, above-ground productivity, carbon and nitrogen reserve, and soil and water conservation in a typical steppe of the Inner Mongolia grassland, based on the long-term grazing experiment set up by the Inner Mongolia Grassland Ecosystem Research Station, CAS. The experiment site conducted in2005. The experimental treatment was composed of7levels of grazing intensities (i.e..0.1.5.3.0.4.5.6.0.7.5. and9.0sheep hm-2). SRO was as control, SR1.5and SR3.0as light grazing treatments, SR4.5and SR6.0as moderate grazing treatments. SR7.5and SR9.0as heavy grazing treatments. The mixed system was managed including14experiment plots. The main results are as follows:1. Grazing had significant effect on community constructon and biodiversity of typical steppe ecosystem. The biodiversity (α-, β-and γ-diversity) declined with increasing grazing intensity. Over-grazing caused convergence of the plant species composition. The responses of a-diversity of dominant species, common species, and rare species to grazing differed substantially, with the highest negative response of rare species, intermediate negative response of common species, and least response of dominant species. The local-scale biodiversity declined which is triggered by grazing-induced loss of common species and rare species. Our findings provide a better understanding of that precipitation is key factor governing the relationships between grazing intensity and biodiversity in semiarid grasslands.2. Both grazing intensity and interannual climate changes had significant influence on the annual net primary production (annual net primary production, ANPP). With the increase of grazing intensity. ANPP was reduced after the first rising trend, the light grazing treatment SR3.0gained the highest mean ANPP of two-year experiment. Sheep’s weight gain increased with the increase of grazing intensity. However, the average weight gain per sheep per hectare was reduced after the first rising trend. the treatment SR3.0was the highest. Comprehensive analysis on the change trend of results in primary and secondary productivity, the grazing intensity with3.0sheep hm-2reached maximum productivity of grassland.3. The plant carbon and nitrogen reserve of grassland showed significant differences with different grazing intensity, and various response patterns were found among different topography (flat vs. slope) and fluctuations in precipitation (wet vs. normal years). In flat system, plant carbon and nitrogen reserve declined with increasing grazing intensity. However, in slope system, plant carbon and nitrogen reserve was reduced after the first rise trend. The heavy grazing treatment had the lowest plant carbon and nitrogen reserve. The carbon and nitrogen content of soil and root were significantly affected by grazing in0-10cm surface soil layer. less effect on the deep soil.there was no significant effect was found in deeper soil layers. Significant difference of total grassland carbon reserve was found among different grazing intensity, with heavy grazing treatment minimum. No significant difference was found between total grassland nitrogen reserves.4. Rainfall intensity is one of the key factors of grassland soil and water loss. The loss amount of runoff water. runoff soil, carbon and nitrogen increased with increasing rainfall intensity. Heavy grazing aggravated soil erosion and water runoff with the heavy rainfall intensity. With the increase of grazing intensity, vegetation coverage and litter biomass decreased. In flat system, the above-ground standing plant biomass declined with increasing grazing intensity, however, it was reduced after the first rising trend in slope system, with heavy grazing treatment minimum. The loss amount of runoff water, runoff soil, carbon and nitrogen increased with increasing grazing intensity. The results also showed that size of the rainfall intensity determined whether the runoff to appear. Under the same rainfall condition, the grazing intensity, terrain slope and the vegetation coverage determined the loss amount of runoff water, runoff soil, carbon and nitrogen. In order to avoid serious soil and water loss and to keep proper vegetationon coverage, the grazing intensity should be limited no more than4.5sheep hm-2.In a word. Grazing has significant influence on typical steppe ecosystem services, such as biodiversity. productivity, carbon and nitrogen reserves and soil and water conservation.Under the conditions of this two-year experiment. with the grazing intensity of4.5sheep hm-2. the flat grazing system showed results with annual average species richness of13. ANPP of239.25g m-2. total carbon and nitrogen reserves in plants-soil system of1863.00g m-2and164.75g m-2respectively, the annual runoff water amount of1.2kg m-2, the average weight gain per sheep per hectare of8.23kg sheep-1hm-2. The species richness. ANPP. total carbon and nitrogen reserves and the average weight gain per sheep per hectare declined significantly while the annual loss amount of runoff water, runoff soil, carbon and nitrogen increased significantly with grazing intensity more than4.5sheep hm-2. |
