| Soil microbial communities affect ecosystem processes such as nutrient cycling and energy flowing,both of which can be affected by soil environments.The improvement of soil quality by vegetation restoration not only exerts ecological impacts on topsoils,but also on the deep soils where soil microbial diversity and community structure change accordingly with environments.Moreover,nitrogen and phosphorus additions were also expected to affect soil microbial diversity and community structure in varying degrees.Here,I used high-throughput sequencing to investigate bacterial and fungal diversity and their community structure in relation to various environments in coniferous Pinus tabulaeformis(Carr.)planted forests in the hilly Loess Plateau regions of China.This study was conducted in southern Yan'an,Shaanxi Province.The local environment has been greatly improved for several decades with the "Grain for Green" project.Soil microbial developed with vegetation restoration,the poor-nutrient and deep loess soils provide a new insight into soil microbes for us to better understand the nature of microorganisms that change with vegetation restoration and soil depth.The purpose of this study was to investigate the environmental responses of microbial community structure and diversity in P.tabulaeformis plantated forests in the following four aspects:changes in deep soils down to a depth of 20 m,soil microorganism dynamics during vegetation restoration,and the effects of nitrogen and phosphorus additions on soil microorganisms.First,I investigated the changes in soil microbial diversity and community structure across a soil profile gradient ranging from 0 to 20 m.I obtained 7 incontinuity soil strata(i.e.,0-0.2,0.8-1,1.8-2,3.6-4,7.6-8,13.5-14 and 19.5-20 m)and found that,with increasing soil depth in planted forests,the bacterial richness declined from the topsoil to a depth of 2 m,and then increased slowly with soil depth from 2 to 14 m.Shannon diversity declined from the topsoil to a depth of 8 m,and then increased to a soil depth of 14 m.Below the depth of 14 m,bacterial diversity declined again with increasing soil depth,and the lowest values of chromium,nickel and manganese contents in 14 m depth indicated that some microelements could suppress bacterial diversity.Proteobacteria,Actinobacteria and Acidobacteria dominated the soil,while Basidiomycota,Ascomycota,and Zygomycota prevailed over major fungal components.I also found that the relative abundance of a-Proteobacteria was higher in subsoils than in topsoils,and it was highest at a 20 m depth in the forest,indicating that a-Proteobacteria can survive dry,alkaline,and low-nutrient environments.The relative abundance of Acidobacteria was higher in topsoils than in subsoils,indicating that Acidobacteria prefer to nutrient-and oxygen-rich environments.These results suggest that soil bacterial communities are affected by nutrient availability that are associated with soil depth;and some microelements were the second important influencing factor to soil microbial community structureSecond,I investigated the changes in rhizosphere microbial diversity and community of coniferous P.tabulaeformis planted forests in degraded lands across a chronosequence that spanned over 60 years(10,25,40 and 60 years since restoration,i.e.,four forest stands).I found significant differences in soil bacterial and fungal communities among forest stands with differing ages.Proteobacteria,Actinobacteria,and Acidobacteria dominated the rhizosphere,while Basidiomycota,Ascomycota,and Zygomycota prevailed over fungal components.With stand development,bacterial Shannon diversity decreased,but fungal Shannon and Simpson diversity and richness increased.Non-metric multidimensional scaling analysis separated the clusters of bacterial community well by forest stands.Fungal community clusters of 25-and 60-year-old stands overlapped.The dominant bacteria Acidobacteria showed the highest relative abundance at the 40-year-old stands.Relative abundance differed by 65.6%between Basidiomycota and Ascomycota in 25-year-old stands,and only 27.2%difference in the 60-year-oldstands.Soil microbial communities were significantly correlated with macronutrients(soil organic carbon,total nitrogen and total phosphorous).Additionally,the relative abundance of ?-proteobacteria was positively correlated with soil organic carbon and total nitrogen.Thus,restoring vegetation in degraded temperate forests enhanced some macronutrient contents and influenced microbial communities.These results revealed that restoring vegetation in degraded lands could decrease the diversity of bacterial communities over time.In contrast,soil fungal diversity increased after restoration and fungal communities in the 25-and 60-year-old forest stands overlapped on degraded soilsThird,I investigated the age-related effects of nitrogen addition(20 g N·m-2·yr-1)on the bacterial and fungal community compositions along a 40-year chronosequence of coniferous P.tabuliformis plantated forests.The observed results indicated that,the Shannon diversity of both bacterna and fungi increased significantly in the 25-year-old stand and also the same was true for the abundance-based coverage estimator of bacteria in the 40-year-old stand.Proteobacteria,Actinobacterna and Acidobacterna dominated the soil,while Basidiomycota,Ascomycota,and Zygomycota prevailed over fungal components.Nitrogen addition enhanced the relative abundance of Bacteroidetes in the 40-year-old stand and the relative abundance ofAcidobacteria/Zygomycota in the 25-year-old stand.In contrast,the relative abundance of Thaumarchaeota was reduced by nitrogen addition in the 40-year old stands.Using non-metric multidimensional scaling analysis,I found that the effect of nitrogen addition was stronger on bacterial than on fungal community structure.Moreover,the effect of nitrogen addition was stronger in the 25-year-old stands than in the 40-year-old stands.These results indicated that the effect of nitrogen addition on microbial community structure diminished over time in temperate forests on Loess Plateau,China.The stronger effect of nitrogen addition was found to be on the community structure of bacterna than on that of fungi,and on the microbial community structure in the 25-year-old stand than in the 40-year-old stand.My findings suggested that planted forest ecosystems at a certain stage(?40-year old)were less sensitive to disturbance such as nitrogen addition than young forests.Fourth,I investigated the effects of phosphorus addition(addition rates included six levels,i.e.,0,1,2,4,8,16 g P·m-2·yr-1)on bacternal and fungal diversity and communities compositions in 25-year-old P.tabulaeformis planted forests for three consecutive years.The observed results showed that soil available phosphorus content significantly increased,while soil pH values and soluble calcium contents declined after phosphorus addition.The increased bacternal and fungal diversity and richness after phosphorus addition indicated that the phosphorus addition at a rate of 16 g P m-2 yr-1 was not too high and fell within a reasonable range.Proteobacterna,Actinobacterna,and Acidobacteria dominated the soil,while Basidiomycota,Ascomycota,and Zygomycota prevailed over fungal components.The addition of phosphorus decreased the relative abundance of Proteobacterna but increased the relative abundance of Actinobacteria.Except for the plots with a rate of 8 g P m-2 yr-1,the bacterial community structure was significantly enhanced by phosphorus addition,indicating that the effects were dependent upon P concentration.The results of NMDS analysis indicated that bacternal communities were more sensitive to the addition of phosphorus than fungal communities.The positive effect of phosphorus addition on bacterial richness,which was also positively related to available phosphorus,and further suggesting the high sensitivity of bacterial communities to soil phosphorus availability.My findings revealed that soil organic carbon decreased with increasing depth but increased over time as restoration proceeded.My findings suggest that soil microbial communities in the young forest,a system that is still at a developing stage,are the components of belowground subsoils and respond with large fluctuation to the disturbances of external soil nutrient sources before the forest ecosystem reaches a steady stage and forms enough flow paths for material and energy circulation.Bacterial communities were more sensitive to environmental changes than fungal communities during the disturbance period after nitrogen and phosphorus additions,likely attributing to the fact that bacteria was directly affected while fungi was indirectly affected by the addition of nitrogen and phosphorus.In summary,soil organic carbon,which comes from the litter and root secretion,is the main soil nutrnents affecting soil microbial communities with vegetation restoration.Along with the soil depth increasement,the effects of litter on soil microbial community diminished.In the areas where plant roots cannot reach,the effects of root secretion also disappear.With vegetation restoration,the organic carbon in topsoils increased and then decreased over time.The importance of soil nitroge/phosphorus effects on soil microbial communities followed next to the soil organic carbon.For the soil nutrients aspects,vegetation restoration is a process that soil organic carbon accumulates in plants and disturbances of external soil nutrient.Then soil microbial communities could respond to soil quality in different ways at various development stages and also at different environments where they inhabited.My experiments of nitrogen and phosphorus addition revealed that both nitrogen and phosphorus could significantly impact soil nutrient availability and thus microbial diversity and structure,indicating the potential influences of anthropogenic disturbances on soil microorganisms and also for the environment changes.The present analyses and findings will be useful to policy-makers and planners for implementation of sustainable forest management at both local and regional scales. |
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