Regulation Mechanisms Of Alpine Grassland Ecosystem Multifunctionality By Herbivore Assemblages

As a complex integrated system,grassland ecosystems combine ecological,production and living functions and are responsible for ecosystem security,grassland cultural heritage and sustainable development of pastoral economies.In recent years,due to excessive human interference and climate change,the degradation of grassland ecosystems in China has become increasingly severe,and the functions of grassland ecosystems have declined significantly.Therefore,an urgent issue to be addressed is how to safeguard the function of grassland ecosystems and improve their use efficiency in the context of global change and human interference.Grazing is one of the most common uses of grassland ecosystems,but overgrazing is also one of the main causes of degradation of grassland ecosystems.As a typical ’ecologically fragile area’,alpine grasslands are particularly sensitive to overgrazing.They are susceptible to degradation and difficult to recover from excessive external disturbance.Recognising this,the government has introduced a series of policies to protect grasslands,such as the Grassland Contract Responsibility System.With the implementation of the grassland protection policies and the management choices made by herders,three typical grazing practices have gradually emerged in the alpine grasslands of the Tibetan Plateau,namely yak-only grazing,Tibetan sheep-only grazing and a mixture of the two.By selectively feeding,trampling and returning excreta,grazing livestock can alter the characteristics of vegetation,soil and microorganisms,regulating changes in ecosystem multifunctionality.Therefore,an important theoretical basis for ensuring the stability of ecosystem functions and achieving their sustainable use is a thorough understanding of the ways and mechanisms by which the multifunctionality of alpine grassland ecosystems is regulated by the activities of different types of grazing livestock.Furthermore,the key importance of grazing experiments lies in the possibility of using patterns found at the scale of grazing experiments(small scale)as generalisable patterns that can be extended to the scale of ecosystem management(large scale)to ensure sustainable ecosystem development.At present,however,most studies have focused on the effects of individual grazing livestock species on grasslands at local spatial scales.In grazing systems,livestock trampling and dung return pathways alter the spatial heterogeneity of soil resources at smaller scales.Their selective foraging alters the spatial heterogeneity of vegetation communities at larger scales and indirectly alters the spatial heterogeneity of soil resources.Finally,these heterogeneous effects are coupled and redistribute soil resources at the landscape scale.Therefore,the effects of different types of grazing livestock activities on vegetation,soils and micro-organisms at different scales in grassland ecosystems need to be taken into account.Soil resources are not only the basis for vegetation growth but also the vehicle for microbial activity as one of the main components of standing conditions.How these three elements and their interactions are influenced by different grazing practices and spatial scales,and how these factors and processes further regulate changes in the multifunctionality of grassland ecosystems,is still not systematically understood and appreciated.This limits further improvement of grassland management.In this study,the alpine grasslands of Xihai Town,Haiyan County,Qinghai Province,were subjected to different grazing methods,and the key indicators regulating the multifunctionality of alpine pastures,namely the structural characteristics of the vegetation community network,the biomass compensation,the biomass temporal stability,and the spatial heterogeneity of key soil microbial communities and soil resources,were systematically investigated at a multilevel and multiscale system level by grazing on natural alpine pastures under long-term moderate grazing intensity.The following results and conclusions were obtained.The following results and conclusions have been obtained:(1)Herbivore assemblages had an effect on the keystone species of the alpine grassland plant community,but had no effect on plant community diversity.The keystone species for yaks grazing and mixed grazing in a 1:2 ratio was Kobresia humilis.The keystone species for Tibetan sheep grazing was Stipa tianschanica,the keystone species for mixed grazing in a 1:4 ratio was Potentilla acaulis,and the keystone species for mixed grazing in a 1:6 ratio and no grazing was Leymus secalinus.The key soil physicochemical factors driving changes in different herbivore assemblages are soil available nitrogen,water moisture and bulk density,respectively.From the point of view of the structural stability of the plant community,mixed grazing performed better than grazing alone,and the level of organisation and stability of the plant community was better when yak and Tibetan sheep were mixed in a ratio of 1:2 and 1:6.(2)The biomass allocation pattern of alpine grassland plant communities follows the optimal allocation theory,but shows a different response between enclosure and grazed conditions.In the absence of grazing disturbance,the biomass allocation of the plant community tends to favour the belowground component,whereas in the presence of grazing disturbance,the biomass allocation of the plant community tends to favour the aboveground component.The trade-offs in the biomass allocation patterns of the plant community are mainly regulated by soil bulk density and available nutrients.The different herbivore assemblages all contribute to the aggregation of the roots of the alpine grassland plant community in the topsoil.At the functional group level,the proportion of Gramineae plant biomass increased significantly when yaks and Tibetan sheep were mixed grazed at a ratio of 1:6,while the proportion of Cyperaceae plant biomass decreased significantly.(3)Herbivore assemblages changed the soil multifunctionality of alpine grasslands.Yak grazing reduced soil multifunctionality in alpine grasslands,while the other treatments increased soil multifunctionality in alpine grasslands,reaching a maximum when Tibetan sheep were grazed and yak and Tibetan sheep were mixed in a ratio of1:6.Herbivore assemblages also altered the richness indices of soil bacterial and fungal communities,and their richness indices were positively correlated with the soil multifunctionality.The main physicochemical factors driving changes in soil multifunctionality in alpine grasslands under different herbivore assemblages were soil total nitrogen,available nitrogen,available phosphorus,bulk density and water content;the main physicochemical factors driving changes in bacterial communities were soil bulk density,available phosphorus,available nitrogen and water content;the main physicochemical factors driving changes in fungal communities were soil total organic carbon,available nitrogen,bulk density and available phosphorus.Soil multifunctionality was negatively correlated with the relative abundance of keystone species in the bacterial community and positively correlated with the relative abundance of keystone species in the fungal community.(4)Changes in alpine grassland plant and microbial communities are jointly driven by soil available nitrogen,moisture and bulk density.The spatial heterogeneity of soil resources was dependent on herbivore assemblages,with Tibetan sheep grazing and yak and Tibetan sheep mixed in a ratio of 1:6 increasing the spatial heterogeneity of soil available nitrogen,moisture and bulk density.Although the coupling effect of mixed grazing reduced frequency and extent of yak and Tibetan sheep activity,the inhibitory effect decreased with increasing proportion of Tibetan sheep.Tibetan sheep grazing and mixed grazing of yak and Tibetan sheep in a ratio of 1:6 contributed most to the spatial heterogeneity of key soil resources.(5)Species asynchrony mediates the increase in functional stability of alpine grassland ecosystems under different herbivore assemblages.Herbivore assemblages suppress the ’insurance effect’ provided by the biodiversity of the alpine grassland plant community,but enhance the overall functional stability of alpine grassland ecosystems by changing the relative importance of species asynchrony and spatial asynchrony.The’insurance effect’ provided by species asynchrony is much higher than that provided by spatial asynchrony.Furthermore,different grazing practices reduced the α,β and γdiversity and β and γ richness of alpine grassland plant communities,as well as species stability and γ stability,but increased species asynchrony and total asynchrony.The contribution of species asynchrony to the ’insurance effect’ became more pronounced as the proportion of Tibetan sheep in the mixed grazing community increased,indicating that the grazing livestock had significantly higher foraging preferences for the species than for the different palnt patches.(6)Herbivore assemblages changed the multifunctionality of the alpine grassland ecosystem.The pattern of change was almost the same as that of soil multifunctionality.All grazing treatments increased the multifunctionality of the alpine grassland ecosystem,except for yak grazing,which decreased the multifunctionality of the alpine grassland ecosystem.The highest level was reached when yak and Tibetan sheep were mixed in a ratio of 1:6.In addition,vegetation,soil and microbial key factors explained22.37%,26.25% and 51.37% of the variation in alpine grassland ecosystem multifunctionality under different herbivore assemblages,respectively,with the microbial contribution being the largest.Herbivore assemblages mainly improved ecosystem multifunctionality by changing key indicators of plant,soil and microbial,while key microbial communities played the most important role in regulating changes in alpine grassland ecosystem multifunctionality.In conclusion,this study suggests that a reasonable management strategy for the use of alpine grasslands should be based on moderate grazing intensity,and the regulation of different herbivore assemblages should be an important means to ensure the structural and functional stability of the alpine grassland ecosystem,taking into account the key soil resources and their spatial heterogeneity.The combination of yak and Tibetan sheep grazing in a ratio of 1:6 may be a sustainable way to use alpine grasslands on the QTP.This study is of great theoretical value for understanding the main factors regulating the multifunctionality of alpine grassland ecosystems and for the adaptive management of alpine grassland-livestock ecosystems.