Effects Of Different Vegetation Restoration Types On Soil Quality And Its Mechanism In Rocky Desertification Area In Northwest Guangxi

Rocky desertification is an extreme form of land degradation in karst areas,and it is a major ecological,economic and environmental problem worldwide,which is characterized by soil erosion,exposed bedrock and sharp decline in productivity.As the country with the largest and most widely distributed karst landforms,China has achieved remarkable results in the control of rocky desertification through the implementation of projects such as "comprehensive control of rocky desertification in karst areas".Restraining soil degradation and improving soil quality are the primary tasks of rocky desertification control.Vegetation ecological restoration is the best way to control rocky desertification,and the key to vegetation restoration lies in the selection and rational allocation of tree species.Therefore,in the process of vegetation restoration and management of rocky desertification,how different vegetation types affect soil quality and what are the main driving factors are worthy of further study.In this study,the typical karst peaks and depressions of the Huanjiang Karst Ecosystem Observation and Research Station of the Chinese Academy of Sciences in northwest Guangxi were used as the experimental area,the artificial disturbance degraded forest(ADF),and the deciduous broad-leaved tree forest(DF),the deciduous evergreen broad-leaved mixed forest(EDF),the evergreen broad-leaved tree forest(EF),which have been artificially promoted for 13 years,were selected as the research objects.The natural enclosed forest(NF)was used as the control.Carry out research on the change law of vegetation and litter from the aboveground part to the root system,soil microorganisms,soil enzyme content,soil physical and chemical properties of the underground part,to analyze the effect of different vegetation restoration types on soil quality in the karst rocky desertification area.Through the screening of main influencing factors,on the basis of constructing the structural equation model of "vegetation-root system-litter-soil microorganism-soil quality",to reveal the driving factors and influencing mechanisms of soil quality of different vegetation restoration types.(1)The vegetation types(DF,EDF and EF)of artificially enhanced forests recovered faster than natural enclosed forests(NF),and arbor layers had been formed.The arbor diameter class structure of the three artificially promoted forests all showed an inverted "J" type,and the height class structure showed a positive skewed distribution.Compared with EDF and EF,DF has 4 diameter classes and 1 higher tree class,which effectively improves the average stand density,tree height and DBH.NF and ADF are the largest species in shrub and grass layer of 5 vegetation types,respectively.The composition of dominant species in each type of shrub layer is similar,all of which are Vitex negundo L.or Jasminum nervosu,while the sub-optimal species are different.,NF is mainly based on a variety of positive drought-tolerant pioneer species such as Mallotus barbatus(Wall.)Muell.Arg.and Rhus chinensis Mill.,DF is mainly based on a variety of shade-tolerant and humid species such as Leucaena leucocephala(Lam.)de Wit and Alangium chinense(Lour.)Harms.Almost all types of herbaceous layers are perennial shade-tolerant herbs,and the dominant species were Wetia serrata.DF has the most rare species,with high community heterogeneity and great diversity.The four diversity indices of the tree layer showed that DF and EDF were significantly greater than EF;each index of the shrub layer showed that ADF was significantly the smallest;the richness index,Shannon-wiener index and Simpson index of the herb layer all showed that ADF and NF were larger,but the uniformity index of the two types were small.ADF has the least similarity with other types,The order of community stability of each vegetation type is DF>EDF>NF(EF)>ADF.(2)The litter volume of different vegetation types is significantly different,which is EF>EDF>DF>NF;the natural moisture content is ranked as DF>NF>EF>EDF.The water holding capacity,water holding rate,water loss and water loss rate of litter of each vegetation types increased logarithmically with the increase of time,and the water absorption rate and water loss rate decreased with the increase of soaking time as a power function.At the same time,the maximum water holding capacity,maximum interception capacity,effective interception capacity and accumulation of litter of different vegetation types are all proportional to the volume.The order of maximum water holdup,maximum interception rate and effective interception rate is NF>DF>EDF>EF,while the order of maximum water loss rate is NF>EDF>EF>DF.Four main indicators affecting the water retention capacity of litter were screened out: litter volume,maximum water retention rate,average natural water content and maximum water loss rate.The comprehensive score of water conservation capacity of each vegetation type is EF(0.65)>EDF(0.34)> DF(-0.12)>NF(-0.87).Vegetation restoration type,soil depth and their interactions significantly affected FRB,RLD,RLD,RSD,SRL,SRA,AD and SRF.The FRB,RLD,RSD and AD of artificially promoted arbor forest(EDF,EF,DF)were significantly lower than those of natural enclosure(NF),and the SRA and SRT were significantly higher than those of NF.FRB,RLD and RSD of DF were significantly larger than those of EDF and EF in the 0-20 cm soil layer,and SRL,SRA,SRT,SRF of DF were significantly the largest in the 20-40 cm soil layer.Four main response indicators of fine roots were screened out: biomass,root surface area density,specific surface area and ratio of bifurcation density.(3)Proteobacteria and Acidobacteria were the dominant phyla in the soil bacteria of the five vegetation types.Artificially enhanced forests(DF,EDF and EF)all increased the relative abundance of Proteobacteria,but decreased the relative abundance of Acidobacteria.Ascomycota and Basidiomycota were the dominant phyla in the soil fungi of the 5 vegetation types.DF and EDF increased the relative abundance of Ascomycota,while EF decreased.Alpha diversity analysis showed that artificially enhanced forests(DF,EDF,and EF)decreased soil fungal diversity,while DF increased soil bacterial diversity.The Beta diversity analysis showed that the species composition of soil bacteria and fungi in EDF and DF was more similar,that of ADF and NF was more similar,and the difference between EF and other vegetation types was the largest.The functional forecast shows that,compared to NF,three types of artificially promoted forests reduced the abundance of chemoheterotrophic bacteria and aerobic heterotrophic bacteria,but increased the abundance of phototrophic bacteria.DF increased the abundance of pathotrophic fungi,EDF and EF increased the abundance of symbiotic trophic fungi,and all three artificially promoted forests decreased the abundance of saprophytic fungi.Artificially promoted forests all increased the relative abundance of Basidiomycota,with EF being the most significant.Vegetation restoration can change soil MC,TN,TK,AP,MBC content and p H value by controlling the increase and decrease of microbial groups such as Dependentiae,Acidobacteria,Latescibacteria,Rhodes and the changes of microbial Shannon and Simpson indices.Bacterial diversity plays a role in improvement,fungal diversity plays a negative role,and the improvement effect of DF is the most significant.MC and p H were the key drivers that positively affected the species diversity of bacterial communities;MC and SAC were the key drivers that positively affected the species diversity of fungal communities.(4)Different vegetation types had significant effects on soil physicochemical properties,microbial biomass and enzyme content in karst areas.Deciduous broad-leavedforests(DF)have higher SOC,TN,TK,AN,AK,MBC,MBN,CAT,SAC and ALP content,It’s soil chemical properties are better.Evergreen and deciduous broad-leaved mixed forest(EDF)have higher MC,CP,TPO,TP,AP,URE,It’s soil physical properties are better.Based on the minimum data set(MDS)method,six indicators including NCP,SOC,TP,p H,MBC and URE were selected to evaluate the soil quality of different vegetation types.The order of soil quality in the five vegetation restoration modes was: DF(0.618)> EDF(0.593)> NF(0.472)> EF(0.443)> ADF(0.284).Stepwise regression analysis showed that soil organic carbon accounted for 79.9% of the variation of soil quality indicators,which was the main indicator affecting soil quality.(5)Through the construction and analysis of structural equation model,it is found that in the process of vegetation restoration of rocky desertification control,different vegetation restoration types directly affect the input of stand litter and fine roots,and then the litter and fine roots affect the composition of soil microorganisms.Finally,the soil quality was changed by the combined action of litter,fine roots and soil microorganisms,and there was an obvious interaction between soil quality and soil microorganisms.Among the effects of different vegetation restoration types on litter and fine roots,deciduous broad-leaved forest(DF)mainly affected natural water content,specific surface area and specific bifurcation density,with path coefficients of0.58,0.78 and 0.79,respectively.Evergreen broad-leaved forest(EF)mainly affects litter volume and bifurcation density,with path coefficients of 0.69 and 0.8.Among the effects of fine roots and litter on soil microorganisms,litter mainly affects bacterial community structure and fungal Shannon index through the maximum water holding capacity,with path coefficients of 0.8 and 0.58.Fine root biomass mainly affects bacterial community structure and fungal Shannon index,with path coefficients of 0.69 and 0.59.Root surface area density mainly affects bacterial Shannon index and fungal community structure,with path coefficients of 0.6 and 0.7.Among the effects of litter,fine roots and soil microorganisms on soil quality,the root surface area density and specific surface area density of fine roots have a greater impact,with path coefficients of 0.84 and 0.42;the bacterial Shannon index has a greater impact,with path coefficients of 0.83.Soil quality had a significant effect on microbial feedback,all of which were positive effects.Soil quality had the greatest impact on bacterial Shannon index and fungal community structure,with path coefficients of 0.68 and 0.54.