Studies On Response Mechanism Of Plant Community And Livestock To Grazing Pressure In An Alpine Meadow

Grassland ecosystem,as an important barrier to land ecological security in China,not only plays an important role in regulating climate,conserving water resources,soil and water conservation,wind protection and sand fixation,but also plays an important role in maintaining grassland biodiversity and supporting grassland animal husbandry production.It is not only the key material support for the development of grassland animal husbandry,but also the basis for herdsmen and basic means of production for their survival.Grazing,as the most basic means of grassland utilization,is also considered as one of the most important human disturbance factors,which has a universal and profound impact on grassland ecosystem services.At present,the degradation of grassland caused by overgrazing has become the main factor that destroys the ecological environment of grassland and restricts the development of animal husbandry economy in pastoral areas.It is of great theoretical and guiding significance to study and reveal the response mechanism of grassland ecosystem service function to grazing pressure for curbing grassland degradation,protecting grassland ecological environment,supporting grassland animal husbandry production and realizing sustainable development of grassland.Based on the experimental platform of fixed grazing control of Kangle grassland in Sunan County,which is a national wool sheep industry technology system,and taking the local summer grazing pasture-alpine meadow ecosystem as the research object.This research systematically analyzed and studied the effects and mechanisms of grazing intensity on grassland plant community characteristics,community diversity,grassland productivity,nutrient quality and service functions of Gansu Alpine Fine Wool Sheep under the different rainfall years[wet year（2015）,dry year（2016）,wet year（2017）and normal year（2018）]and different grazing seasons（June-September）.In addition,a visible/near infrared spectroscopy（Vis/NIR）prediction model for nutrient quality of natural forage was established.The field experiment platform was established in 2008.There were four grazing intensities with 0,3.7,5.3 and 7.6 sheep/ha.0sheep/ha was used as enclosure control,3.7 sheep/ha was used as light grazing treatment,5.3sheep/ha was used as medium grazing treatment and 7.6 sheep/ha was used as heavy grazing treatment.A randomized block experiment was conducted in 12 plots with 4 treatment levels,3repetitions and single factor（grazing intensity）.The main results are as follows:1.Responses of plant community height,coverage and aboveground biomass to grazing intensity and annual rainfall.（1）Grazing significantly reduced the height,coverage and aboveground biomass of plant community,and with the increase of grazing intensity,the height,coverage and aboveground biomass of the community decreased significantly;（2）The height,coverage and aboveground biomass of fencing and grazing plots showed a trend of first decreasing,then rising,then declining and then again rising with the inter-annual change of rainfall（2015-2018）;（3）Grazing intensity and inter-annual rainfall interaction significantly affected community height,coverage and aboveground biomass.2.Responses of plant community density,species richness index（R）,Shannon-Wiener index（H）,Simpson index（D）and Evenness index（E）to grazing intensity and annual rainfall.（1）Grazing significantly increased community density,species richness index（R）,Shannon-Wiener index（H）,Simpson index（D）and Evenness index（E）significantly increased with the increase of grazing intensity;（2）Annual variation of rainfall first decreased and then increased the grazing sample land community density,species richness index（R）,Shannon-Wiener index（H）,Simpson index（D）and Evenness index（E）,while the enclosure showed a gradual downward trend（2015-2018）;（3）Grazing intensity and inter-annual rainfall interaction significantly affected community density,species richness index（R）,Shannon-Wiener index（H）,Simpson index（D）and Evenness index（E）.3.Response of plant community nutrient quality to grazing intensity,annual rainfall and seasonal variation.（1）Grazing significantly increased the nutritional quality of mixed herbages.Grazing significantly improved the nutritional quality of mixed pasture in the wet,normal and dry years,and significantly increased with the increase of grazing intensity;（2）The inter-annual variation of rainfall reflected the inter-annual variation of the nutritional quality of mixed herbages.The nutritional quality of mixed herbages in the wet year was significantly higher than that in the normal and dry years,and the normal year was significantly higher than the dry year.For the grazing plots,the nutritional quality of the mixed herbages showed at first decreasing,then increasing and then decreasing trend with the inter-annual variation of rainfall,while that of fencing plots showed a downward trend over time（2015-2018）;（3）The nutritional quality of mixed herbages showed obvious seasonal dynamics,and the maximum value appeared during the period of seedling establishment in June,and the lowest value appeared in the withering period of herbages in September.The effects of grazing on nutritional quality of mixed herbages were different between different grazing months,and the nutritional quality of mixed herbages significantly increased with the increase of grazing intensity;（4）Nutritional quality of mixed herbages was affected by grazing intensity,inter-annual rainfall and inter-seasonal variation;（5）The accuracy of the prediction equation of dry matter digestibility DMD（%）of mixed herbages was significantly affected by grazing intensity,inter-annual rainfall and inter-seasonal variation.Comprehensive analysis showed that the nutritional quality of herbages was the highest when the grazing intensity was 5.3 sheep/ha.4.Response of nutrient quality of four dominant species in plant community to grazing intensity,annual rainfall and seasonal variation.（1）Grazing significantly improved the nutritional quality of four dominant species（Kobresia humilis,Polygonum viviparum,Potentilla fruticosa and Caragana sinica）in wet,normal and dry years,and with the increase of grazing intensity,the nutritional quality of Kobresia humilis,Polygonum viviparum and Caragana sinica significantly increased,but had minor effects on the nutritional quality of Potentilla fruticosa;（2）The nutritional quality of four dominant species showed a trend of changing with the inter-annual variation of rainfall.The nutritional quality of four dominant species in wet year was significantly higher than that in normal and dry years,and that in normal year was significantly higher than that in dry year.For grazing plots,the nutritional quality of four dominant species showed a first decreased,then increased,then decreased and then increased with the inter-annual variation of rainfall,while the fencing plots showed a downward trend over time（2015-2018）;（3）The nutritional quality of four dominant species in the community showed obvious seasonal dynamics,and the maximum value appeared in the period of seedling establishment in June,and the lowest value appeared in the withering period of herbages in September.The effects of grazing on nutritional quality of dominant species were different between different grazing months,and nutritional quality of four dominant species significantly increased with the increase of grazing intensity;（4）The nutritional quality of mixed herbages was determined by dominant species.The nutritional quality of dominant species showed obvious species-specific responses to grazing pressure,such as tolerance（i.e.,strong regeneration ability,such as Kobresia humilis and Polygonum viviparum）and grazing avoidance（i.e.,lower palatability,such as Potentilla fruticosa）;（5）The relationship between nutritional indicators of dominant species and the accuracy of dry matter digestibility prediction equation was affected by grazing intensity,inter-annual variation,inter-seasonal variation,and species-specific;（6）The interaction of grazing intensity,sampling year and sampling season significantly affected the nutritional quality of the four dominant species,and the nutritional quality of the dominant species showed obvious species-specific responses to grazing intensity,inter-annual and seasonal patterns.Comprehensive analysis showed that Kobresia humilis and Polygonum viviparum had the highest nutritional quality at grazing intensity of 5.3 sheep/ha,while Potentilla fruticosa and Caragana sinica had the highest nutritional quality at grazing intensity of 7.6 sheep/ha.5.Responses of grassland productivity and sheep productivity to grazing intensity and annual rainfall.（1）Rainfall is the main factor that determined the aboveground net primary productivity（ANPP）and nutritional quality of herbages.The higher the grazing intensity,the higher the nutritive value of forage,the lower thee ANPP;（2）Grazing intensity was highest,LWG_haa was highest in short term,but it will endanger the ANPP and profitability of grassland grazing ecosystem in the long run.Moderate grazing intensity（5.3 sheep/ha）can provide 78%of the gain of LWG_ha,which meets the requirements of sustainable grazing management.6.Establishment and validation of visible/near infrared spectroscopy（Vis/NIR）model for Forage Nutritional Quality Prediction in alpine meadow.（1）The contents of dry matter,crude protein,crude fat,acid detergent fiber,neutral detergent fiber,dry matter digestibility and metabolizable energy of mixed forage and four dominant forage species from different rainfall years（2015-2018）,grazing months（6-9）and grazing intensity（0,3.6,5.3,7.6 sheep/ha）varied greatly;（2）Apart from crude fat,other nutritional indicators such as dry matter,crude protein,acid detergent fiber,neutral detergent fiber,dry matter digestibility and metabolizable energy content were predicted by Vis/NIR prediction model with RSQ ranging from 0.992 to 0.999 and RPD ranging from 3.82 to 5.97.The best calibration results were obtained,and the calibration equation showed good predictive ability,and can be successfully applied to daily analysis;（3）Compared with other nutritional indicators,RSQ of crude fat was 0.837,RPD of external verification was 2.30,and the calibration effect was not ideal.Although the calibration model cannot be used for accurate quantitative analysis,it can still be applied to rough determination of crude fat of forage.