Ecological Stoichiometric Characteristics Of Different Functional Groups Of Plant-soil In Alpine Meadow In Northwestern Sichuan

The high altitude and low temperature in the northwest of Sichuan make the alpine meadow ecosystem extraordinary fragile.Under the influence of global climate change and overgrazing,the alpine meadow in Northwest Sichuan has been degraded to different degrees.Studying the content and stoichiometric characteristics of carbon(C),nitrogen(N),and phosphorus(P)in plants and soils under different degradation gradients will help to understand the carbon,nitrogen and phosphorus cycle processes of alpine meadow ecosystems.Among the four degraded alpine meadow ecosystems:undegraded,lightly degraded,moderately degraded and severely degraded,This research analyzed the plant coverage,species diversity,temporal stability,primary productivity,soil water content,bulk density and pH;additionally,the contents and stoichiometric characteristics of C,N and P in different functional groups of plants(gramineaes,sedges,legumes,forbs),plant communities and soils were studied.Finally,we combined altitude,latitude and longitude,annual average temperature,and annual precipitation,from the perspective of ecological stoichiometry,to explore the changes and coupling relationships of environmental factors-plants-soil in the process of grassland degradation,aiming to provide a theoretical basis for the protection and management of alpine meadows in northwestern Sichuan.The results show that:1.Grassland degradation decreased soil water content,C,N and P contents,and increased soil bulk density and pH in 0-10 cm,10-20 cm and 20-30 cm soil layers.In the 0-10 cm soil layer,C:N showed a "V" shape change with the aggravation of grassland degradation,the intact grassland was significantly higher than the slightly and moderately degraded grassland;In the 20-30 cm soil layer,the C:N increased with the aggravation of degradation,C:N was significantly higher in severely degraded grassland than that of the other grasslands.N:P changed significantly only in the 0-10 cm soil layer,that is,the lowest N:P in the intact grassland was significantly lower than that in the slightly and moderately degraded grassland(P<0.05).2.Shannon-Weiner index was the highest in slightly and moderately degraded grassland,which was significantly higher than that in intact and severely degraded grassland.Pielou evenness index had no significant difference among intact,slightly and moderately degraded grassland,but significantly decreased among severely degraded grassland.Simpson diversity index in severely degraded grassland was significantly higher than that in intact,slightly degraded and moderately degraded grassland(P<0.05).3.The coverage and primary productivity of plant communities,gramineaes and sedges decrease with the increase of grassland degradation,while forbs and legumes were the highest in moderately degraded grassland.The temporal stability of the plant community decreased with the aggravation of grassland degradation,and the intact grassland was significantly higher than that of the severely degraded grassland(P<0.05).4.The carbon content of slightly and moderately degraded grassland communities was significantly higher than that of intact and severely degraded grassland communities.Grassland degradation increased N content,C:P and N:P of different functional groups and communities in alpine meadow,but decreased P content and C:N,plant growth was restricted by phosphorus.5.Pearson correlation analysis showed that soil moisture,C and N content were significantly positively correlated with the primary productivity of sedges and legumes,while soil C:N and pH were significantly negatively correlated with the primary productivity of forbs(P<0.01).Structural equation model(SEM)further analysis showed that the decrease of soil water content and the increase of soil bulk density and pH value caused by grassland degradation decrease the contents of soil C,N,and P,thereby indirectly changed the soil C:N,C:P,and N:P.The increase in pH reduced plant community species diversity,primary productivity and C:N,C:P and N:P of legumes,but increased the C,N and P contents of different functional groups.In addition,altitude,longitude and latitude had extremely significant negative effects on the contents of C,N and P of gramineaes,sedges,forbs and communities,and had extremely significant positive effects on the contents of C:N,C:P and N:P of different functional groups and communities(P<0.01).In summary,in the process of alpine meadow degradation,plant and soil degradation interact and feedback each other.The increase of plant species diversity in the slightly and moderately degraded alpine meadow increased the temporal stability,productivity and C storage of the community,and decreased the soil C:N;In severely degraded alpine meadow,species diversity was lost,community temporal stability,productivity and the proportion of high-quality forage grass decreased,while the proportion of poisonous weeds increased.The decrease of the proportion of high quality forage grasses and the increase of the proportion of poisonous weeds which were difficult to decompose resulted in the decrease of soil organic carbon input,and the decrease of soil C,N and C:N.The decrease of the proportion of high quality forage and the increase of the proportion of poisonous weeds that are difficult to decompose reduced the source and quality of soil organic carbon,resulting in the decrease of soil C,N and C:N.At the same time,the decrease of vegetation coverage increases the risk of soil wind and water erosion,and the loss of soil nutrients such as C,N and P accelerates.Conversely,as the degree of grassland degradation intensifies,soil moisture decreased and bulk density increased,leading to an increase in soil pH,further reducing the dissolution and flow of nutrients in the soil,and limiting the diversity and primary productivity of ground plants.Therefore,the restoration of degraded alpine meadow should pay attention to the synergistic restoration of vegetation and soil.