Effect Of Enclosure On Community Structure And Productivity Of Subalpine Grassland In The Qilian Mountains

Climate change and human activities are the main causes of environmental damage and grassland degradation.Enclosure,one of important management strategy in grassland restoration,has been widerly implemented in China.The grassland in the Qilian Mountains are representative in northwest China.It has a great significance to restore the degraded grassland and maintain the rational utilization of grassland resources for the stabilization of Qilian Mountains ecological environment and the development of animal husbandry economy.Understanding the mechanism of enclosure on the community structure and productivity of subalpine grassland in the Qilian Mountains can provide basic theoretical guidance for the restoration of grassland(prairie)resources in northwest China.This study was conducted in a subalpine grassland in Qilian Mountains,and analysized the response of grassland community structure and productivity to different enclosed duration(CK,enclosed 2years,enclosed 4 years and enclosed 6 years)and its underling mechanisms.The main research results are as follows:(1)Enclosures changed the species composition in the community:improved the dominant position of Gramineous species,and weakened the dominance of miscellaneous grasses.Enclosures reduced the root-shoot ratio of grassland.The biodiversity decreased with the increasing of enclosed duration,and the biodiversity decreased significantly after 6 years enclosure,but not significantly after 2 years;The primary productivity was increased after enclosure,and reached the maximum value(5.4 kg/(m~2·a))after 2 years,and then decreased with further enclouses.therefore,the effect of short-term enclosed on species diversity and productivity is more significant,but not with long-term enclosure.(2)Enclosure changed plant non-structural carbohydrates(non-structural carbohydrates,NSC)concentrarions(bothsoluble sugar and starch).From July to September,the change of soluble sugar content in the aboveground of five plants in the same sample plot was“V”font,and the content was lowest in August.For starch and NSC content,except for Poa annua L,the overall change in the content of other plants from July to September is also“V”font.The content of NSC in the aboveground parts of Elymus nutans,Poa annua L and Carum carvi L was relatively higher after 4 years of enclosed,and the change of the content was relatively stable.overall,changes in the aboveground NSC content of various species indicated that the four-year enclosure management have positive effects on plant adaptability.According to the correlation analysis,except for the control plot,the content of soluble sugar and starch of Elymus nutans had a certain correlation with biomass in the enclosed plot,while other species had a certain correlation only in the individual enclosed plot.(3)Enclosure also induced changes in soil physical and chemical properties.The soil bulk density and soil electrical conductivity of grazing plots(except for the10-30 cm soil layer enclosed for 6 years)were higher than that in the enclosed plots.In the 0-10 cm and 10-30 cm soil layers of the plot enclosed 4 year,the soil bulk density had the minimum value,which were 0.79 and 0.87 g/cm~3,respectively.The soil porosity values of the enclosed plots were higher than in grazing plots,and have the largest value after 4 years enclosure.In the 10-30 cm soil layer,the soil moisture content was increased after 4 and 6 years enclousers.In the 0-10 cm,the soil conductivity of the enclosed sample plot was significantly lower than that of the control plot;while in the 10-30 cm soil layer,the soil conductivity of the enclosed plot was the lowest.Overall,the contents of soil organic carbon,nitrate nitrogen,available phosphorus and available potassium increased slowly with the extension of the enclosure period,and began to increase after 4 years;at the same time,the value of C:P after enclosed 2 and 4 years were the highest.In short,the plants enclosed in the plots for 2 and 4 years absorbed relatively more soil total nitrogen,total phosphorus and ammonium nitrogen in the 0-30 cm soil layer.we found a positive correlation between aboveground biomass,ammonium and nitrate nitrogen content,and species richness and ammonium nitrogen content.the underground biomass significantly correlated with the soil total nitrogen and phosphorus content.The species richness and Shannon index were negatively correlated with soil total phosphorus content.Enclosure management measures have certain effect on the restoration of degraded subalpine grassland.our results show that although enclosure can promote the restoration of grassland via vegetation and soil characteristics,enclosure management period should not be too long.For example,the restoration effects of vegetation and soil after 6 years was lower than after 2 and 4 years.Proper enclosure management of the grassland will result in a stable grassland ecosystem.The stable natural grassland ecosystem can provide a good living and production environment for the local people and the surrounding areas,and promote the sustainable development of the ecosystem and the agriculture and animal husbandry economy.