Responses and feedback of litter properties and soil mesofauna to herbaceous plants expansion into the alpine tundra on Changbai Mountain,China

Global climate changes result in the expansion of lower elevation plants to higher elevations.The rapid upward expansion of herbaceous plants into the alpine tundra on Changbai Mountain resulted in changes in different levels of ecosystem organization.However,the responses and feedback of litter properties and soil mesofauna to herbaceous plants expansion have not been studied yet.To understand the mechanisms underlying those changes,we conducted a field experiment in the range of 2250-2300 m in the alpine tundra of the Changbai Mountain and collected a total of 288 samples from four degrees of herbaceous plants expansion to study the litter physiochemical properties,soil mesofauna,and soil nutrient contents,and their relationships in that tundra ecosystem suffered from various degrees of herbaceous invasion.We found that herbaceous plant expansion is responsible for a major shift in the dominant species of soil mesofauna from mites to collembolan and has significant impacts on the community structure(R2=0.54,p=0.001)and diversity of soil mesofauna(Shannon-Weiner index,p=0.01).The increasing herbaceous plant expansion resulted in a significant increase in litter biomass from 91 g·m^(-2) in the original tundra vegetation(OIT)to 118 g·m^(-2) in the moderately invaded tundra(MIT),and an increase in litter thickness from 2.37 cm(OIT)to 3.05 cm(MIT).And,the litter total nitrogen content significantly increased,but the values of the litter carbon content,the lignin content,the C/N ratio,and the lignin/N ratio decreased with increased herbaceous coverage(both p<0.05).The litter physical properties pathway(biomass and thickness)directly explained 31% of the total variance in soil mesofauna diversity and 59% of the total variance in soil mesofauna community composition.Furthermore,both the soil available nutrients(incl.AN and AP)and plant biomass(incl.the total plant biomass and herbs/shrubs biomass)significantly increased with increasing coverage of herbaceous plant(both p<0.05),and litter chemical properties pathway directly explained 50% of the soil nutrient content variance and indirectly explained 20% of soil nutrient by affecting soil mesofauna.We found that both soil available nutrients and soil mesofauna were positively correlated with the herbaceous expansion from OIT to MIT,indicating a positive feedback of herbaceous expansion,and the abundance of soil mesofauna decreased in the severely invaded tundra vegetation,suggesting a negative feedback.While,both litter N content and soil available nitrogen were consistently increased in the severely invaded tundra vegetation,indicating a positive feedback of herbaceous expansion.Therefore,this study provides new insights into the process of herbaceous plant expansion into tundra,and provides possible evidence for further expansion according to responses and feedback of in litter properties and soil mesofauna to herbaceous plants expansion.Furthermore,these positive or/and negative feedback systems in the Changbai alpine tundra ecosystem in relation to herbaceous expansion have important implications for the tundra protection,and thus,need to be deeply studied.