| Grazing is the most intensive land use activity on grassland,affecting grassland biodiversity,ecosystem function and stability.Overgrazing has caused increasingly serious grassland degradation and declining grassland function in China.Determining the appropriate grazing intensity can provide theoretical and technical support for grassland management.In this paper,the alpine meadow in the southern foot of Qilian Mountains was taken as the research object,and four different grazing gradients were set up:light grazing(LG,4.5 sheep·hm-2),moderate grazing(MG,7.5 sheep·hm-2),heavy grazing(HG,15 sheep·hm-2)and extremely heavy grazing(SG,30 sheep·hm-2).Through the combination of field monitoring and laboratory experiments,the effects of grazing intensity on the characteristics of grass layer,plant community characteristics and soil moisture characteristics were analyzed,and the relationship and variation between grass layer and its influencing factors in alpine meadow ecosystem were analyzed.The main results are as follows:（1）The thickness of the felt layer and the ratio of root to soil increased with the increase of grazing intensity,reaching a maximum of 6.78 cm and 2.55%in extremely heavy grazing,and the spatial heterogeneity was very obvious.The shear strength increased first and then stabilized,reaching a maximum of 2 kg·cm-2in moderate grazing.With the increase of grazing intensity,the bulk density of the felt layer of the grass was first stable and then increased,which was significantly reduced to 0.59 g·cm-3in heavy grazing,and the bulk density of the felt layer of the extremely heavy grazing was significantly increased by 47.46%compared with heavy grazing.The total porosity and capillary porosity increased first and then decreased with the increase of grazing intensity,and reached the maximum value of 75.22%and 66.80%in heavy grazing,and decreased by 10.08%and 7.24%respectively in extremely heavy grazing.The maximum water holding capacity,capillary water holding capacity,field water holding capacity and saturated hydraulic conductivity increased first and then decreased with the increase of grazing intensity,reaching the maximum value of 127.03%,114.29%,78.82%and 21 mm·min-1under heavy grazing,and the proportion of extremely heavy grazing decreased by 26.90%,26.07%,67.29%and 13.59%,respectively.（2）With the increase of grazing intensity,the indexes of aboveground plants showed a trend of stable first and then significant change.Compared with heavy grazing,the plant coverage of extremely heavy grazing decreased by 24.73-58.33%,and the plant height of each functional group decreased by 56.37-96.99%.The biomass of grasses decreased significantly by 83.13-99.03%,the biomass of sedges increased significantly by 0.83-1.52 times,and the biomass of weeds did not change significantly.The dominant species in the plot changed from Poao rinosa to Potentilla anserina,and the plant diversity index decreased significantly by 20.4-68.5%.The regression fitting of grazing intensity and community species diversity index showed an obvious parabolic trend.The species diversity reached the maximum at moderate grazing,and then gradually decreased with the increase of grazing intensity.Aboveground and belowground biomass were significantly positively correlated with species diversity.It is worth noting that compared with heavy grazing,the underground biomass of 10-20cm soil layer under extremely heavy grazing decreased significantly by 13.90%,33.20%,68.13%and 41.53%from June to September,respectively.（3）With the increase of grazing intensity,the indexes of soil physical properties in0-10 cm soil layer showed a trend of stable first and then significant change.Compared with light grazing,the soil bulk density of 0-10 cm soil layer under extremely heavy grazing increased by 12.6%,the soil saturated water holding capacity decreased by7.90%,the soil saturated hydraulic conductivity decreased by 74.76%,and the soil water storage decreased by 24.9%.Plant diversity index was positively correlated with soil water holding capacity and soil water storage（P<0.05）,and negatively correlated with total porosity and soil saturated water holding capacity.（4）The results of the redundancy analysis showed that the grass felt layer capacity and shear strength were the most significant factors affecting the moisture characteristics of the grass felt layer.The correlation analysis showed that the felt layer thickness and root-to-soil ratio were significantly and positively correlated with the biomass of sedge plants（P<0.05,R=0.632,0.824）,and the shear strength of the felt layer was significantly and negatively correlated with the biomass of sedge plants（P<0.05,R=-0.251）;the felt layer weight was significantly and negatively correlated with the biomass and height of grasses and weeds（P<0.05,R=-0.726,-0.679）.Therefore,maintaining a continuous layer of grass carpet can not only ensure the balance of water-holding function to a certain extent,but also buffer the damage to plant community and soil structure caused by external factors,which is the key to the reasonable utilization and conservation of alpine meadows.In order to maintain high meadow utilization and sustainable development,medium grazing intensity can be used as the reference grazing intensity. |
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