Biological Characteristics In Litter-soil Continuum After Reestablishment Of Sand-fixation Vegetation

The feedback and mutual regulation between microorganisms and aboveground vegetation play a key role in regulating long-term evolution and stability of forest ecosystem.Understanding the changes of microbial community structure during the long-term vegetation restoration has an important implication in maintaining the ecological services and functions of sand-fixation forests.Therefore,we selected the chronosequences from two typical sand-fixation forests afforested 23～54 years in the Mu us Sandy Land,Shaanxi province,China,as research object,and sampled the litter-soil continuum along the gradient of decomposition（fresh litter layer,humus layer,soil organic layer and mineral layer）of soil organic matter（SOM）,to analyze the evolutionary characteristics of microbial community structure in the long-term transformation and accumulation process of SOM,and explored the long-term evolution mechanism of two types of sand-fixation forests from the perspective of microbial community evolution.Our main result as follows:（1）Revegetation on semi-fixed sand resulted in a significant inputs of carbon（C）and nitrogen（N）into the soil in the form of litter.After 54 years of afforestation,shrub and arbor had imported 16.98 and 36.56 times higher C and 10.94 and 36.90 times higher N into the soil in the form of litter than semi-fixed sand,respectively,which resulted in that shrub and arbor lands accumulated 7.1 and 14.6 times higher total organic carbon（TOC）and soil total nitrogen（TN）increased by 4.4 and 6.6 times,respectively than semi-fixed sand.The continuous input of litter also caused an increase in microbial biomass carbon（MBC）,by73.9 and 165.32 mg kg^-1 in shrub and arbor land,respectively,while microbial biomass nitrogen（MBN）increased by 16.8 and19.6 mg kg^-1,respectively.Compared to the arbor lands,the dissolved organic matter（DOM）in shrubland showed a higher microbial source and a relatively simple molecular structure.In addition,the C/N showed that the transformation of organic matter in both forestlands followed a same pattern（increased and then decreased）,which indicated that the transformation of organic matter in the litter layer depended mainly on that the degradation of easily degradable fractions,and the accumulation of complex fractions;while in the soil layer,the transformation mainly along the direction that more easier utilization by microorganisms,and showing an increase in microbial metabolites.（2）In general,the change of bacterial community was dominated by the increase in the relative abundance of Actinobacteria（increased by 12.7%on average）after shrub afforestation,while dominated by the increase of Proteobacteria（increased by 14.1% on average）in arbor land.Shrub afforestation resulted in the bacterial community with the higher homogeneity,dominance,and diversity indices compared to arbor afforestation,however,only in the litter layer,the fungal community in shrub land was observed the higher values than in the arbor lands,which showed the lower values in the soil layer.Along the litter to soil layer,the microbial community in the shrub woodland changed from with the predominance of Ascomycota in the litter layer,to the predominance of Actinobacteria in the organic layer,and finally to Acidobacteria in the mineral soil.In arbor forests,however,there was a shift from the dominance of Amoebacteria in the litter layer to Basidiomycota,and finally to Mortierellomycota in the soil layer.The network structure of the microbial community showed that the structure of the microbial community became more complex in the litter layer after afforestation of both sand fixation forests,however,which tended to be simpler in the deep soil layer.In addition,the microbial communities showed stronger competitive intensity after shrubland afforestation,compared to revegetation of arbor,as well as the competitive intensity between microbial communities tended to increase from the litter layer to the soil layer.This indicates that the ecological succession of microbial communities in the process of desertification restoration is a resource-regulated and is a"bottom-up"evolutionary process.（3）Compared to shrub rehabilitation,arbor restoration showed a lower oxidase activity and a higher nitrogen hydrolase activity,while a lower stoichiometric ratios of C hydrolase to N hydrolase in the litter-soil continuum.This suggests that organic matter is more easily mineralized by microorganisms in shrubland,while arbor exhibit a stronger N limitation.Therefore,compared to arbor revegetation,the shrub restoration showed a slower increase in soil organic matter and a lower improvement in physicochemical properties.The vegetation diversity index showed a decreasing trend after 30 years of shrub restoration.In contrast,in the arbor land,although there was some new species invasion in the early stage,it eventually evolved into a pure forest of Pinus sylvestris with the restoration extension.Our results showed that the establishment of both shrub and arbor on semi-fixed sand land could greatly increase the SOM content,and improve the soil physicochemical properties,as well as the structure of the microbial community.Shrub litter was easier to degrade by microorganism,which is beneficial for microbial growth and nutrient release,but unfavorable for SOM accumulation.However,under arbor afforestation,the litter biomass is large and is difficult to degrade by microorganisms,which is beneficial to organic matter fixation but inhibit the nutrient release as well as the microbial and vegetation diversity.Therefore,future research is required to investigate the regulation mechanism between microorganisms and vegetation,and optimize the vegetation allocation,to alleviate the contradiction between nutrient release and SOM accumulation.