Effects Of Plant Functional Type Loss On Microbial Community Composition And Activity Of Litter In An Alpine Timberline

Alpine areas are one of the most sensitive areas in response to global climate change.On the one hand,climate change can directly change the environment of litter decomposition;on the other hand,it can also change the plant community structure and species composition,causing species loss.There has been no consistent conclusion on the effects of litter diversity on the process of litter decomposition.Microbial community structure and function are important factors that regulate litter decomposition and nutrient cycling in ecosystems.However,there is still a lack of research on the influence of plant functional type loss on the composition and activity of microbial community,which is not conducive to understand the mechanism of biodiversity loss on ecosystem process.Therefore,we conducted a litterbag experiment,choosing 6 typical plant functional types(9 species),to explore different plant functional type loss on the process of litter decomposition in an alpine timberline ecotone.And we also explored driving factors of decomposition rate and microbial community composition and activity during litter decomposition.The results showed that:1.There were significant differences in the environment factors under different vegetation types within a short vertical distance in the alpine timberline ecotone.During the winter periods,the snow thickness,the number of freeze-thaw cycles,average temperature and positive accumulated temperature increased from coniferous forest to alpine shrubland,while the moisture content of litter decreased from coniferous forest to alpine shrubland.The mixture with different functional types may be balanced by positive and negative interactions among functional types,significantly reducing the initial quality differences among different mixed litters.2.During the two-year decomposition process,the loss of single plant functional type had no significant impact on the k value and mass loss rate.Only the loss of deciduous shrubs in coniferous forest and the loss of ferns in alpine shrubland had significantly higher k values than those of total mixture,respectively.This result indicated that,under climate change scenario,single plant functional type loss did not cause significant change in the decomposition rate of local litters during the early stage of litter decomposition.The k value of mixed litter with different functional types mainly displayed negative effects,while the litter mixture interactions of mass loss rates were additive as a whole.From the decomposition of two years,the mass loss rates were mainly affected by environmental factors(positive accumulated temperature,moisture content,negative accumulated temperature and snow thickness can explain 51.8% of mass loss variation),indicating that the direct influence of climate change(environmental factor change)on mass loss rate of litters was greater than the indirect influence(single plant functional type loss)during the early stage of litter decomposition.3.The loss of plant functional types had no significant effect on the contents of total PLFAs,fungal PLFAs,bacterial PLFAs,gram-positive bacteria PLFAs and negative bacteria PLFAs.Different from the results of microbial community composition,the loss of trees and deciduous shrubs increased the fungi/bacteria PLFAs ratio in coniferous forest,and the loss of evergreen shrubs increased the gram-positive bacteria/gram-negative bacteria PLFAs ratio in alpine shrubland in the first growing season and the second year.In general,microbial community composition and structure can be predicted by single plant functional types(additive effects).Although the ecological stoichiometry of soluble carbon,nitrogen and phosphorus affected the composition and structure of microbial community to some extent,the composition and structure of microbial community were mainly affected by environmental factors(moisture content and temperature).This result indicated that the direct effect(environmental factor change)of climate change had greater influence on the composition and structure of microbial community than the indirect effect(single plant functional type loss)during the early stage of litter decomposition.4.Loss of plant functional types had no significant effect on microbial biomass carbon,nitrogen and phosphorus.In the alpine shrubland,only loss of evergreen shrubs increased microbial biomass carbon.The single plant functional type could not always predict the microbial biomass carbon,nitrogen and phosphorus in mixed litters.From the decomposition of two years,the microbial biomass carbon and nitrogen were regulated by both environmental factors(moisture content and temperature)and the initial quality of litter.The microbial biomass phosphorus and the ecological stoichiometry of microbial carbon,nitrogen and phosphorus were mainly affected by environmental factors,and the initial quality of litter did not enter the model.5.In general,the loss of plant functional types had no significant effect on extracellular enzyme activities.Only the loss of evergreen shrubs and graminoids had significant effects on C-and N-acquiring enzyme activities,(?-glucosidase,polyphenol oxidase and ?-N-acetylglucosaminidase),while the loss of ferns had a significant effect on the C-acquiring enzyme activity(?-glucosidase).Except phosphatase and oxidase activities,the activities of the other enzymes decreased over time.On the whole,the enzyme activities of litters mixed with different functional types displayed additive effects.From the two-year decomposition,the activities of C-and N-acquiring enzymes were affected by environmental factors(moisture content,snow cover thickness)and the initial quality of litters(the ecological stoichiometry of soluble carbon,nitrogen and phosphorus).Phosphatase and oxidase were only affected by environmental factors(moisture content,positive accumulated temperature and freeze-thaw cycle times).6.The loss of trees and evergreen shrubs increased the stoichiometric ratios of C-to P-acquiring enzyme activities in the second year.This result suggested that the loss of trees and evergreen shrubs increased the energy(carbon)requirements of soil microorganisms.The mixture effects of the ecological stoichiometry of C-,N-and P-acquiring enzyme activities were non-additive in some cases during the first winter,and were additive in the rest of the two years.This result indicated that the prediction of enzyme stoichiometric ratios and microbial nutrient limitation of mixed litters by single plant functional type was more suitable after six months of decomposition.From the perspective of two-year decomposition,the enzyme stoichiometric ratios and microbial nutrient limitation were mainly affected by environmental factors,especially moisture content,snow cover thickness and freeze-thaw cycle times.During the decomposition of mixed litters with different plant functional types,the microorganisms were mainly restricted by relative carbon and phosphorus,and the restriction degree of phosphorus was higher in coniferous forest.In conclusion,the loss of different single plant functional types had no significant influence on the decomposition rate,and microbial community structure and activity of litters.The decomposition rate k values of mixed litters with different functional types displayed negative effects.In some cases,mixture effects of microbial biomass carbon,nitrogen,phosphorus and their stoichiometric ratios were non-additive.However,the composition and structure of microbial community,extracellular enzyme activities and their stoichiometric ratios of mixed litters with different functional types displayed additive effects in most cases.The result indicated that the microbial community composition and structure,extracellular enzyme activity and their stoichiometric ratios of mixed litters can be predicted by single plant functional type litters.The mass loss rate of litters and the composition,structure and activity of microbial communities were mainly regulated by environmental factors(moisture and temperature),suggesting that the direct impact(environmental factor change)of climate change on litters decomposition was greater than the indirect impact of climate change(single plant functional type loss)in the alpine timberline ecotone.This study provides basic data for the research about the effects of plant functional type losses and environmental changes on ecosystem material cycling and energy flow under climate change scenarios.