| Vegetation restoration is one of the effective measures for restoring ecological environment.The Loess Plateau is a typical region with fragile ecological environment in China.In order to restore the ecological environment,the government has carried out a series of soil and water conservation measures in this region since the 1950s.Especially,the implementation of“Grain for Green”program since 1999 has dramatically improved the ecological environment.The planting area of deep-rooted plants increased with vegetation restoration,which inevitably affected the properties of soil profile,such as soil water,organic carbon（SOC）turnover,and activities,distributions,and interactions of microbial communities.These alterations of soil profile might in turn affect the ecosystem services and functions.However,owing to the complexity of the above processes,the dynamics of soil moisture,organic carbon and microorganisms during vegetation restoration and their driving mechanisms remain unclear.The Wangdonggou watershed located in the gully region of Chinese Loess Plateau was selected to carry out this research.Firstly,the spatial distribution pattern of SOC content in surface（0–20 and 20–40 cm）soils at the watershed scale was analyzed using the geostatistical tools and its driving mechanisms were explored.The variations in fine root distribution,SMC,and SOC content and density in 0–500 cm soil profiles under two deep-rooted plants（black locust,Robinia pseudoacacia;and apple tree,Malus pumila）with different stand ages were investigated and their soil moisture deficit（SMD）and SOC sequestration effect were evaluated.The soil extracellular enzyme activities were determined by the microplate fluorimetric assay method and microbial communities were measured by high-throughput sequencing method,which can help to evaluate the effects of vegetation restoration on soil extracellular enzyme activities,enzymatic stoichiometry,and microbial communities at different depths of deep soil profiles.Based on the results of high-throughput sequencing,the effects of vegetation restoration on the interactions of microbial communities at different depths of deep soil profiles were clarified by using the molecular ecological network method.The main results are as follows:（1）The spatial pattern of SOC content throughout the watershed was generally patchy,and both the topographic and land use type exert significant influence on the spatial distribution of SOC.In the 0–20 cm soil layer,the average SOC content of different topographic types decreased in the following order:gully(9.88 g·kg–1)>sloping land(8.28 g·kg–1)>tableland(7.51 g·kg–1).Therefore,the relatively higher SOC content in this soil layer was mainly distributed along the low-lying gully within the watershed.However,in the 20–40 cm layer,the average SOC content decreased in the following order:tableland(5.60 g·kg–1)>gully(5.14 g·kg–1)>sloping land(4.78g·kg–1).And the relatively higher SOC in the 20–40 cm layer was mainly distributed in the northern and western parts of the tableland.Land use type affected SOC content in0–20 cm and 20–40 cm soil layers,and woodland and grassland soils had great carbon sequestration potential.（2）Vegetation restoration significantly affected the soil moisture and SOC dynamics in the deep profiles（0–500 cm）under black locust plantations（8,18,22,and38a）and apple orchards（5,9,19,and 28a）of different stand ages.After the establishment of black locust plantations on cropland,the depth-averaged SMC in deep profiles generally decreased first and then increased slightly with increasing stand age.Compared with long-term cultivated cropland,different levels of moisture deficit of all soil layers within the 0–500 cm profile under all black locust plantations were confirmed in this study.After the establishment of apple orchards on cropland,the depth-averaged SMC in deep profiles increased obviously during the initial 5 years and then decreased over time.And all soil layers within deep profiles under the 19-and 28-year-old apple orchards exhibited different levels of moisture deficit.The temporal variation pattern of SOC over time in deep soils was different from that in shallow soils.The SOC sequestration in deep soils mainly occurred below depths of 300 cm,indicating that the deep soils play an important role for carbon sequestration process.However,SOC decrease was noted in deep layers under the old black locust plantation.Compared with apple orchards,SOC sequestration in black locust plantations was more easily affected by moisture condition.（3）Vegetation restoration altered the extracellular enzyme activities and enzymatic stoichiometry at different soil depths within deep profiles（0–500 cm）.Based on the vector length and angle of soil enzymatic stoichiometry,it can be concluded that the establishment of woodland and grassland after cropland abandonment,microbial carbon limitation was enhanced below depths of 40 cm.In general,the establishment of woodland enhanced microbial nitrogen limitation in the deep profiles.While the establishment of grassland alleviated the soil profile microbial nitrogen limitation,with stronger microbial phosphorus limitation below 280 cm layers.Soil extracellular enzymes were mainly affected by soil nutrients and water content conditions during vegetation restoration.（4）The establishment of woodland and grassland after cropland abandonment generally increased the phylotype richness and Shannon index of bacterial and fungal communities at different soil depths within the 0–500 cm profiles,except the 0–100 cm layers of woodland.This indicated that vegetation restoration improved the microbial diversity and richness in deep soils.The effects of vegetation restoration type on different functional soil bacteria and fungi communities were different in deep profiles.Bacteria were more affected by the variations of soil physical and chemical properties induced by vegetation restoration,relative to fungi.Soil nutrient supply（i.e.,contents of available phosphorus,total nitrogen,SOC,total phosphorus and ammonium nitrogen）,SMC,and p H were the main factors affecting soil bacterial community in deep profiles.Whereas the available nutrients（i.e.,dissolved organic carbon and available phosphorus contents）were the main factors affecting soil fungal community.（5）The soil microbial network（composed of bacterial and fungal communities）became more complex and the relationships between microbial species became closer in 0–500 cm profile after woodland establishment on cropland.While,the establishment of grassland resulted in a simpler microbial network in the 0–500 cm soil profile.Besides,relatively complex microbial networks were also noted in deep soil layers（such as 280–300 and 380–400 cm layers）,compared with other soil layers（except for the 40–60 cm layer）.During vegetation restoration,majority of the microbial interactions in deep profiles tended to be co-occurrence.Bacteria plays a more important role in microbial networks than fungi.The keystone taxa of microbial networks varied with land use types and soil depths.In conclusion,this study clarified the importance of deep soil layers for carbon sequestration.Moreover,the responses of microbial carbon and nutrients limitation,diversity,community composition,and the microbial community interactions in deep profiles to vegetation restoration were characterized.Results of this study provide the theoretical basis for evaluating the ecological effects of vegetation restoration,as well as the influences of vegetation restoration on ecosystem services,functions and sustainability. |
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