Effect Of Stocking Rate On Ecosystem Stability Of Stipa Breviflora Desert Steppe

Grassland grazing ecosystem is one of the most efficient consumption system of terrestrial ecosystem. There are few compable repeated studies in long-term effects of stocking rate on grazing ecosystem stability. This study was conducted to examine the effects of stocking rate on grazing ecosystem stability in vegetation, soil and livestock performance, discussed the variability in grassland productivity of grazing ecosystem and the optimal stocking rate on Desert Steppe of Inner Mongolia.A randomized complete block design was used with three replication and four stocking rate treatments Control (non-grazed exclosure (CK)), light stocking rate (LG), moderate stocking rate (MG) and heavy stocking rate (HG)) by sheep in a continuously grazed system (June to November), with the stocking rates of 0, 0.91, 1.82 and 2.71 sheep ha-1 half year-1, for the CK, LG, MG and HG treatments, respectively.The community coverage and other vegetation indicators, soil water content and bulk density were measured once in each month from June to October, 2004 to 2008.Aboveground net primary productivity (ANPP), soil chemical properties, soil respiration rate and soil compaction, soil particle size were measured at the peak of the growing season (August) each year. Living weight gain was determined as the weight difference between the beginning and end of grazing season in each year.The results showed that the stocking rate, year and their interaction has significant effects (P<0.05) on ANPP, vegetation coverage and height of both community and dominant species. ANPP and coverage of plant community and dominant species decreased significantly (P<0.05) with increasing of stocking rates. With the increasing of stocking rates, percentage of C4 plants and perennial bunchgrasses increasing, significantly, while percentage of shrubs and semi-shrubs decreased significantly (P<0.05). The different functional groups which were classified according to plant life forms and photosynthesis pathway have significant (P<0.05) negative correlation among shrubs and semi-shrubs, perennial bunchgrasses, annuals and biennials and C4 plants (P<0.05). The ecological compensary effects existed among different functional groups associated with different stocking rates. The ecosystem stability measured by the coefficient of variation in terms of ANPP at the individual species level were significant lower in CK than any stocking rate treatments (P<0.05), while there is no significant difference between any two stocking rates (P>0.05). While the lowest coefficient of variation at the plant functional group and community level existed in light stocking rate treatment. Light stocking rate has a positive effect on ecosystem stability, and the grazing ecosystem was most stable in light stocking rate. The species diversity of plant community in LG treatment was the highest, optimal stocking rate benefits to the maintenance of grassland ecosystem biodiversity.Heavy stocking rate introduced grassland soil degradation. Soil bulk density and compaction in soil surface significant (P<0.05) increased with increasing the stocking rates. The compaction effect would accelerate soil moisture lost in heavy stocking rate. However, stocking rate has no consistent significant effect (P>0.05) on soil chemical properties. 5 years continuous grazing did not introduced essential changes in soil nutrients content. Soil chemical properties, such as soil organic matter, total and available nitrogen, phosphorus and potassium have not changed significantly (P>0.05) enen in heavy stocking rate.For livestock performance, the mean daily live weight gain per head (r2 =0.59,P<0.05) and per hectare (r2 =0.50,P<0.05) quadratically response on the stocking rate increased.The mean daily live weight gain per head was significant higher in LG and MG than HG treatment (P<0.05). The optimal stocking rate can be abstracted from the comparation of the mean daily live weight gain per head and sheep production per hectare, which was 0.91-1.14 sheep ha-1 half year-1 from 5-year data, 2004 to 2008, and the average is 1.02 sheep ha-1 half year-1 for Desert Steppe of Inner Mongolia.ANPP increased significantly (P<0.05) with the increasing of annual precipitation. Species diversity and lively weight gain increased first, then decreased with the increasing of annual precipitation.Grasslands ecosystem vegetation, soil and livestock attributes at the optimum, which means its relative stable under light stocking rate (0.91-1.82 sheep ha-1 half year-1) in the Desert Steppe, with the utilization rate of 31% to 42%. The resistance and resilience can interpret its function which benefits on the maintenance of ecosystem stability.Basisd on the controlled stocking rates, comparable repeated experiment was conducted to understand the stability of grazing ecosystem in vegetation, soil and livestock, considered the year as the principal component to evaluate the effect and mechanism of annual precipitation on grazing ecosystem characteristics, using the coefficient of variation in terms of ANPP and decrease and increase of functional groups to reveal the ecosystem stability and ecological compensatory effect in grazing ecosystem. The result not only richens the study contents of ecosystem stability, but also provides a guide for adaptive rangeland management plans, which are promoted the ecological and economic sustainable development.