| Global changes and human activities are profoundly changing the distribution pattern of shrub species across the globe,especially in alpine ecosystems,where shrubs are expanding rapidly to higher altitudes,which may affect local ecosystem processes and functions,to a certain extent.Plant litter decomposition is the basic process of nutrient cycling and organic matter turnover in ecosystems,and the process was comprehensively regulated by various biotic and abiotic factors.Heretofore,the most studies have focused on single shrub litter decomposition in situ under shrub removal(combined with global change drivers such as warming,rainfall,and Nitrogen deposition),or single species litter reciprocal transplant decomposition under the distinct vegetation types(forest,shrubland,and grassland etc),yet how the biodiversity effects induced by shrub expansion dynamics drive litter mixtures decomposition is still unclear,this needs to be addressed urgently.In order to understand the impact of shrub expansions on litter decomposition of alpine meadow in western Sichuan,the study used the litterbag method and chose the foliar litters of evergreen shrub(Rhododendron lapponicum),deciduous shrub(Sorbus rufopilosa),and graminoids(Saussurea woodiana,Festuca wallichanica),to determine the effects of the altered shrub richness(The richness of shrub litters are 25%,50%,75%,and 100%)of evergreen shrub expansions(ESE),deciduous shrub expansions(DSE),and evergreen-deciduous shrub expansions together(EDSE)on the mass loss dynamics,elements losses dynamics,and carbon fractions losses dynamics of the mixed foliar litters decomposition of alpine meadow.Meanwhile,we measured the environmental variables,the substrate quality(substrate traits of the mixed foliar litters),the functional dissimilarity(functional dissimilarity of substrate traits of the mixed foliar litters),the chemical changes(changes in substrate traits during the decomposition process),the colonized soil fauna,and the enzyme activities as the fundamental drivers of litter decomposition,to determine how shrub expansions dynamics affect the mixed litters decomposition.After 710 days of field decomposition,the main results are as follows:1)The increased shrub richness decreased the subsutate quality of the mixed litters,increased the functional dissimilarity of the mixed litters.The increased shrub richness significantly increased the non-polar extractables content of the mixed litters,decreased the contents of nitrogen,phosphoric,and acid-hydrolyzable carbohydrates,during the decomposition process.The increased shrub richness decreased the individual density of soil fauna during initial decomposition(342 days),yet the effect on the group desnity and diversity indexs of soil fauna were not significant.The ESE and EDSE significantly decreased the leucine aminopeptidase,β-N-acetylglucosaminidase,polyphenol oxidase,and peroxidase activities.2)The increased shrub richness(25%,50%,75%,and 100%)significantly decreased the mass loss rates(55.92%,50.83%,46.53%,and 41.02%)and carbon loss rates(56.86%,53.13%,48.46%,and 39.51%)of the mixed litters during the decomposition process.Among the different shrub types,the mass loss rates(56.52%,47.67%,and 41.53%)and carbon loss rates(58.09%,47.45%,and 42.93%)caused by the DSE,EDSE,and ESE orderly decreased significantly.Under the shrub expansions,the environmental variables,substrate quality,chemical changes,and enzyme activities(in order of dominance)significantly drove the loss rates of mass and carbon of the mixed litters during the decomposition process.The relative mixing effects of litter mixtures significantly accelerated carbon loss rates during the initial decomposition(342 days).3)Under the shrub expansions,the nitrogen loss rates of the shrub richness of 25%,50% and 75%(53.7%,48.57%,and 48.97%)were significantly higher than that of 100%(32.34%),yet different shrub types effect on the nitrogen loss rates were not significant.When soil fauna was allowed,the phosphorus loss rates of the shrub richness of 25%,50%,and 75%(46.72%,47.24%,and 41.36%)were significantly higher than that of100%(26.38%);when soil fauna were excluded,the phosphorus loss rate of the shrub richness of 50%(29.05%)was significantly higher than that of 25%,75% and 100%(5.94%,8.07%,and 5.02%).The DSE(53.99%/25.98%)caused significantly higher phosphorus loss rates than the ESE(27.51%/9.12%)and EDSE(39.77%/0.56%)among the different shrub types,regardless of soil faunal was allowed or excluded.Under the shrub expansions,the environmental variables,substrate quality,chemical changes,and functional differences(decreasing in importance)significantly drive the loss rates of nitrogen and phosphorus of the mixed litters during the decomposition process.The relative mixing effect of litter mixtures significantly accelerated the loss rates of nitrogen and phosphorus during the later stages of decomposition(550 to 710 days).4)The increased shrub richness(25%,50%,75%,and 100%)significantly decreased the loss rates of non-polar extractables(79.53%,73.43%,68.23%,and 60.03%)and water-soluble extractables(75.84%,73.52%,70.23%,and 66.78%).Among the different shrub types,the loss rates of non-polar extractables(75.75% and 74.68%)and water-soluble extractables(79.58% and 74.55%)caused by the DSE and EDSE were significantly higher than that of the ESE(66.48% and 69.75%).Under the shrub expansions,the chemical changes,environmental variables,substrate quality,and functional dissimilarity(in order of dominance)significantly drove the loss rates of non-polar extractables and water-soluble extractables of the mixed litters during the decomposition process.The relative mixing effects of litter mixtures decreased and increased the loss rates of non-polar extractables and water-soluble extractables,respectively.5)The increased shrub richness(25%,50%,75%,and 100%)significantly decreased acid-hydrolyzable carbohydrates loss rates(70.57%,70.06%,61.61%,and54.06%),but no significant effect on the loss rates of acid-unhydrolyzable residue(8.84%,0.25%,4.15%,and-1.19%).The loss rates of acid-hydrolyzable carbohydrates(76.3%,61.3%,and 54.63%)and acid-unhydrolyzable residue(9.66%,3.18%,and-3.79%)caused by the DSE,EDSE and ESE were significantly decreased among the different shrub types.Under the shrub expansions,chemical changes,environmental variables,functional identiy and functional dissimilarity(in order of dominance)significantly drove the acid-hydrolyzable carbohydrates loss rate of the mixed litters during the decomposition process.When soil fauna was allowed,the relative mixing effects of litter mixtures significantly accelerated the acid-hydrolyzable carbohydrates loss rates.Chemical changes,environmental variables,and substrate quality(in order of dominance)significantly drove acid-unhydrolyzable residue loss rates.In conclusion,this study found that shrub expansions dynamics significantly decreased the the mixed foliar litters decomposition of the alpine meadows in western Sichuan.On the one hand,the loss rates of nitrogen and phosphorus of the mixed foliar litters decreased with increasing shrub richness,suggestings that shrub expansions,especially in evergreen shrubs,which may decrease the nutrient elements loss of litter decomposition,potentially exacerbating the nutrient limits.On the other hand,although the increasing shrub richness had no significant effect on the loss rate of acid-insoluble components of the mixed foliar litters,yet the loss rates of non-polar extractables,water-soluble extractables and acid-hydrolyzable carbohydrates decreased with increasing shrub richness,suggesting that the decomposition of aboveground plants foliar litters under shrub expansions may reduce soil carbon input.More important,shrub expansions decreased the substrate quality,increased the functional dissimilarity,and influenced the chemical changes,which further led to the loss rates dynamics of mass,nutrients and carbon fractions of the mixed foliar litter during the decomposition process.Overall,these results showed that shrub expansions under the background of global change will affect the surface litter decomposition process in alpine meadow,further provide the basic data for in-depth understanding of the multiple functions,material cycling,and energy flow of mountain ecosystems. |
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