The Effects And Regulation Mechanisms Of Interaction Between Grazing Livestock And Plant Diversity On Grassland Litter Decomposition

Litter decomposition is a key process in terrestrial ecosystems,controlling carbon and nutrient cycling.Litter decomposition rates are influenced by a range of biological and environmental factors,including litter quality,decomposer organisms and soil micro-environmental variables.Recently,the plant diversity of grassland has been gradually reduced,due to the aggravation of global environmental change and the interference of human activities,and its impact on ecosystem processes such as litter decomposition has attracted more and more attention.Livestock grazing,an important management and utilization way in grassland,can constitute a feedback loop with plant diversity to affect litter decomposition.In this paper,we tested the effects of plant diversity,herbivore grazing and their interaction on the above-ground and blow-ground litter decomposition in grassland,and systematically analyzed the regulation mechanism from the factors of litter quality to soil microenvironment.This study have important implications theoretically and practically for understanding the effects and mechanisms of the plant diversity loss and grassland grazing management on ecosystem process,but also protecting biodiversity,effectively managing the grassland and maintaining the function of grazing grassland ecosystems.This study using a set of strictly controlled experiments,including the litter-mixing decomposition experiment,artificially simulated grazing experiment and natural grassland grazing experiment,we obtained the main findings as follows.1)The litter decomposition rates and patterns were different between above-ground(leaf)and below-ground(root)parts of dominant species-Leymus chinensis.The fine roots decomposed more quickly than leaf litter,which might be explained by the higher litter quality and belowground environment that is more beneficial to litter decomposition.Furthermore,there was net P mineralization and N mineralization after a temporary N immobilization for leaves,and net P immobilization and N mineralization for fine roots.N is likely to be an important rate-controlling factor in L.chinensis leaf litter decomposition,however P is likely to be an important rate-controlling factor in L.chinensis root litter decomposition.2)Plant diversity can significantly increase L.chinensis leaf and root litter decomposition rate.Plant diversity enhanced L.chinensis litter decomposition and nutrient release rate through producing synergistic non-additive effects and improving decomposition microenvironment conditions,thereby accelerating nutrient cycling in grassland.The direction and magnitude of non-additive effects were mainly driven by species composition in litter mixture.Litter synergistic non-additive effects were stronger when more species with higher physical and chemical traits were mixed.In addition,the presence of functional dissimilar neighbor plant species can increase soil microbial biomass,thereby facilitating the litter decomposition of focal plant leaf and fine root.3)Livestock grazing significantly affected the above-ground and below-ground litter decomposition of L.chinensis,and the effects strongly depended on pre-grazing plant diversity.At the low plant diversity levels,sheep grazing can increase litter decomposition rate of L.chinensis leaf,stem and root.However at the high plant diversity level,sheep grazing can decrease root litter decomposition rate,and had no significant effect on leaf and stem litter decomposition rate.The relationship between the decomposition of aboveground and belowground litter is decoupled by sheep grazing,which showed that the responses and mechanisms of plant aboveground and belowground litter decomposition to livestock grazing and plant diversity were different.In the study,sheep grazing affected plant aboveground(leaf and stem)litter decomposition predominantly through positive changes in soil microbial biomass,followed by the photodegradation;root litter decomposition through positive changes in soil microbial biomass,followed by the soil water content.Plant diversity affected plant aboveground(leaf and stem)litter decomposition mainly through positive changes in soil microbial biomass,followed by the photodegradation,and negatibe changes in photodegradation;root litter decomposition through changing soil microbial biomass,followed by the soil water content.4)Plant diversity loss and grazing disturbance significantly affected annual nutrient input of aboveground and belowground litter of dominant species-L.chinensis in grassland,and had obvious interaction.Compared to normal grassland(high plant diversity grassland without grazing),livestock grazing significantly decreased annual nutrient inputs of L.chinensis leaf,root and total litter,and plant diversity loss significantly decreased annual nutrient input of L.chinensis leaf,stem,root and total litter.When plant diversity decreases,livestock grazing significantly increased annual nutrient inputs of L.chinensis leaf,stem and total litter.Therefore,moderate livestock grazing can relieve this negative effect of plant diversity loss on the annual nutrient input of litter in grassland.5)The interaction between sheep grazing and plant diversity have important spatial negative feedback loop to modify litter decomposition of dominant species in this grassland ecosystem.In the local species-poor communities where litter decomposition rate was low,herbivore defoliation could stimulate root exudation from grass species,which increased soil microbial biomass,and decreased plant cover,elevated photodegradation,thereby promoting L.chinensis litter decomposition.By contrast,in the local species-rich communities where litter decomposition rate was high,sheep foraging can decrease the quality of plant litter inputs and soil water content,thereby lowering soil microbial biomass,and consequently reducing the litter decomposition rate.Therefore,sheep grazing could homogenize grassland nutrient cycling in space by balancing litter decomposition rate among local communities,which therefore could increase the homogenization of metacommunity structure.In summary,livestock grazing utilization of grassland and plant diversity characteristic have important influence on the decomposition rate and annual nutrient input of aboveground and belowground litter.Reduction of plant diversity has obviouse effects on litter decomposition rate of aboveground plan-leaf and stem,but also belowground plant-root,thereby likely reducing the nutrient cycling and ecosystem function in grassland.Our study further demonstrated the important significance of the protection of plant diversity,from the perspective of the key pathway of nutrient cycling-litter decomposition.In addition,livestock grazing as an important utilization way and disturbing factor in grassland,can interact with plant diversity to produce negative feedback regulation mechanism,which affects the input and decomposition rate of litter.Therefore,under the condition of lower grassland plant diversity,moderate grazing may be more beneficial to the nutrition cycle of grassland.The findings further showed that both the decomposition rate and pattern of the aboveground(leaf)and belowground(root)litter and the response to livestock grazing are obviously different.Therefore,in the study of litter decomposition in the future,the litter of the aboveground(leaf)and belowground(root)parts of the plant should be considered simultaneously.