The Influence Mechanism Of Robinia Pseudoacacia Plantation Conversion Model On Its Community Structure And Soil Properties

At a global scale,biological invasion can cause serious environmental degradation,thus changing the composition of local species and ecosystem functions,which is one of the main reasons for the loss of biodiversity in terrestrial ecosystems.Changes in land-use patterns will increase the incidence of biological invasions.Forest conversion is a common way of land utilization and an activity whereby dwarf forest and the composite forest were replaced by high forest based on reserving the original tree species.It will produce different forest structures along with changes in the origin of the trees,the stand structure and the silvicultural system.With the development of the economy,large-scale commercial investment has led to an increase in the frequency of forest conversion,especially in developing countries,where forest conversion is most common.As a developing country,most of its natural forests have been converted into artificial ecosystems（e.g.,farmland,orchard,tea plantation,and plantation,etc.）,producing a diversity of forest ecosystem conversion model.This conversion can alter the composition of ecosystem vegetation community,affect the return of litter nutrients,and cause changes in soil nutrient supply capacity,which in turn changes important ecosystem functions and services,and further affects the stability of forest ecosystems.Therefore,it is very important to study the impact mechanism of forest conversion on ecosystem,which is of great significance to improve the theory of forest conversion.As a widely-growing nitrogen-fixing pioneer species,Robinia pseudoacacia（RP）plays an active role in improving the soil quality and preventing soil erosion in barren hills.It has strong adaptability,fecundity and spreading ability,which is mainly vegetative propagation such as suckering regeneration and sprouting.However,after 2-3 rotations,the coppice RP plantation will be formed,and which had a lower ecological and economic value.This study aimed at the harm of RP coppice plantation in Mount Tai.Based on the leading function of“Biodiversity conservation and water conservation”in Mount Tai,the two conversion models of first generation seedling plantation conversion into second generation coppice plantation and third generation coppice plantation and conversion from RP plantation to Pinus densiflora（PD）plantation were studied.The growth characteristics of RP stand were obtained through local investigation and indoor analysis of the first generation RP seedling plantations,second generation coppice RP plantation and third generation coppice RP plantation.The nutrient allocation characteristics of different RP plantations were obtained by combining stand biomass.Based on ordination analysis and random forest model,the mechanism of conversion from seedling plantation to coppice plantation on soil properties and community structure was revealed.The effects of PD plantation on community structure and soil properties were studied.Based on the two transformation models,the effects of RP plantations on soil properties and community structure were discussed.The main results obtained are as follows:（1）The productivity of RP coppice plantation decreased.The growth rate of RP coppice plantation had a higher growth rate in the early stage.However,in fifteenth years,the growth of current annual height and DBH in first generation seedling plantation was more than that of the second generation coppice plantation and third generation coppice plantation.In the twentieth years,the growth of timber volume and DBH in first generation seedling plantation exceeded that of the second generation coppice plantation,but later,the growth of height,DBH and timber volume of coppice plantation was less than that of the first generation seedling plantation.The RP coppice plantations had lower stand productivity and biomass,and the biomass allocation of leaves and roots in the coppice plantation was 8.35%and35.02%respectively,which were more than that of first generation seedling plantation.（2）The herb diversity of the RP coppice plantation increased.The conversion from first generation seedling plantation to multigenerational coppice plantation reduced the number of shrub species,but increased the number of herb species,the Shannon index,Simpson index and Pielou index of the herb layer,but had no significant effect on the Margleaf index.The shrubs were dominated by vitex negundo in first generation seedling plantation,but there were no shrub layer in third generation coppice plantations.For the herb layer,the first generation seedling plantation was dominated by Oplismenus undulatifolius and Digitaria sanguinalis,and the sencond generation coppice plantation was dominated by Antenoron filiforme,Paspalum thunbergii and Oplismenus undulatifolius,and the third generation coppice plantation was dominated by Digitaria sanguinalis,Paspalum thunbergii and Oplismenus undulatifolius.（3）At the species level,the leaves of the first generation seedling plantation had a higher carbon and nitrogen content and lower phosphorus content,while the leaves of coppice plantation had higher carbon and phosphorus content and lower nitrogen content.The higher leaves C/P and C/N ratios were higher in first generation seedling plantation and the roots C/P and C/N ratios were higher in the coppice plantation,indicating that the organs of carbon sequestration were different in seedling plantation and coppice plantation.The hypothesized leaf N/P breakpoint was N-limitation（N/P<14）and P-limitation（N/P>16）,our results showed that the leaf N/P in coppice plantation was less than 14 and that in seedling plantation was more than 16,which indicated coppice plantation was N-limitation and seedling plantation was P-limitation,the main reason was that the lower nitrogen content in coppice plantation.At the community level,there were more N,C,and P reserves in the aboveground organs of the seedling plantation,and more N content and lower C and P contents in the aboveground organs of the second generation coppice plantation,and more C and P reserves in the upper organs of the third generation coppice plantation.The biomass of arbor,shrub and grass in the sample plot was obtained by harvesting method,and the C,N and P reserves of arbor,shrub and grass as a whole were obtained.The coppice plantations were distributed to the underground with higher nitrogen and phosphorus content and lower carbon content,which led to a decrease in carbon sequestration capacity.According to the Element Plasticity Hypothesis and Nitrogen Distribution Hypothesis,the underground root system has high plasticity,which significantly affects the transport of soil nutrients.（4）The soil quality of the RP coppice plantation decreased.The conversion from RP seedling plantation to coppice plantations significantly altered soil characteristics,reduced soil quality in rhizosphere and bulk soil,decreased soil carbon and nitrogen isotope ratio and soil carbon and nitrogen turnover rate,and resulted in reducing rhizosphere soil enrichment.In fact,this conversion reduced the contents of total carbon,total nitrogen,total phosphorus,organic matter,available phosphorus and nitrate nitrogen in rhizosphere and bulk soil,but increased the content of soil ammonium,and decrease the soil Ca²⁺,Mg²⁺,K⁺,soil urease,acid phosphatase,sucrase and catalase activities,but had no significant effect on cation content of bulk soil.This conversion had an inconsistency in the influence on C/N/P stoichiometry,such as increased the C/N/P stoichiometry in bulk soil but reduced C/N/P stoichiometry in rhizosphere soil which was principally because soil phosphorus content.Correlation and NMDS analysis results showed that the dominance of soil characteristics for different RP soils was not consistent,and soil enzymes had a good correlation with soil nutrients.Overall,there was a good correlation between soil enzymes and soil nutrient,soil C/N/P and cation could explain the bulk soil,but had a weak on the rhizosphere soil.（5）The bacterial community of the RP coppice plantation was homogenized.The conversion from seedling into coppice plantations was also related to significant differences in soil bacterial community structure.It also reduced soil bacterialα-diversity andβ-diversity,resulting in spatial and temporal homogenization of the bacterial community.Furthermore,the results based on principal component analysis（PCA）showed that in second generation coppice plantations,bulk and rhizosphere soils maintained similar composition.Specially,this conversions increased the relative abundances of Proteobacteria and Rhizobium,but reduced that of Actinobacteria,which may result in a decline of soil nutrient availability.Mantel tests found that C,N,Soil organic matter（SOM）,nitrate nitrogen（NO₃^--N）and available phosphorus（A.P）correlated with bacterial community composition.The effects of soil factors on the phylum and genus of the Top3 bacterial based on Random Forest were performed.The results showed that NO₃^--N and sucrase had a strong effect on Actinobacteria,Proteobacteria and Actinobacteria.The%lncMSE value was 7.01-10.58;while the soil N/P was 7.202 for Bacillus,and the sucrase values for the Bradyrhizobium and the Acidothermus were 10.059 and 10.393,respectively.（6）The N-fixed bacteria community of the RP coppice plantation was homogenized,and the net soil N mineralization（Rm）was decreased.Converting seedling plantation to coppice plantations decreased the soil nitrogen（N）and phosphorus（P）,microbial biomass N（MBN）,microbial biomass C（MBC）,soil nitrification（Rn）and net soil N mineralization（Rm）,but significantly increased the concentration of soil NH₄⁺-N.The dominant species in bulk soil and rhizosphere changed from Rhodopseudomonas（22.62%and 15.76%）,Alphaproteobacteria（22.37%and 29.28%）,Rhizobiales（15.40%and 13.31%）,Bradyrhizobium（12.00%and 11.74%）in seedling plantations to Bradyrhizobium（45.95%and 47.86%）and Rhodopseudomonas（43.56%and 41.84%）in coppice plantations,respectively.It indicated that the coppice plantation can normalize soil diazotrophic communities.Mantel test showed that Rn,MBN,Ra and Rm were the important factors in shaping the diazotrophic communities.Redundancy analysis（RDA）showed that Rn,MBN and Rm can explain 15.90%,14.90%and 13.10%of the data variability,respectively.Our results demonstrated that in order to better play the role of nitrogen fixation of RP,it is a disable to reduce the proportion of the coppice plantations.The random forest model was used to analyze the top3 genus.The results showed that the Rm,Rn and Ra had greater effect on Bradyrhizobium,and the%lncMSE values were 0.31,0.29 and 0.22,respectively.The%lncMSE values for Alphaproteobacteria were 0.37,0.15 and 0.24,respectively,while the Ra,N/P and NH₄⁺-N had the greatest effect on Rhodopseudomonas,and the%lncMSE values were 0.31,0.22 and 0.20,respectively.（7）Species diversity of undergrowth in PD plantation was increased.The conversion from RP plantation to PD plantation significantly changed the composition of understory vegetation,which increased the number of understory herb and shrub species and the Shannon index,Simpson index and Margleaf index of understory shrub and herb layer.The shrub layer of PD plantation was dominated by Rhamnus davurica,Lespedeza bicolor,Forsythia suspensa,Broussonetia papyrifera and Spiraea salicifolia,while Vitex negundo was the dominant species in RP plantation;the herb layer of PD plantation was mainly dominated by Oplismenus compositus,Oxalis corniculata,Carex rigescens and Paspalum thunbergii,and the RP plantation was dominated by the Paspalum thunbergii,Elsholtzia ciliata and Athyrium sinense.（8）Soil texture of PD plantation was declined.The conversion from RP plantation to PD plantation had significant effects on soil nutrient,which can significantly reduce soil available nitrogen,available phosphorus,total carbon and organic matter,and increased soil C/N,but had no significant effect on total nitrogen,meantime,it could reduce the soil stable carbon and nitrogen isotope ratio and then slowed the turnover rate of soil carbon and nitrogen.This conversion could change the soil particle size composition such as increased soil clay and silt content but decreased sand content,which will promote the transformation from sandy loam to loam.This conversion could increase soil fractal dimension,then reduced soil porosity.For multifractal dimension,there had no significant effect on the Dq-q and Hurst index,but have a greater influence on the singular spectrum（α-f（α））,which could reduce the non-uniformity and asymmetry（Δα）of soil particle composition and the distribution density（Δf）of soil particles in small probability subsets.The correlation analysis and RDA results showed that there was a significant correlation among soil nutrient,particle size and Dv,but a poor correlation with multifractal parameters.Soil stable carbon isotope can better shape soil singlet and multifractal parameters.In this study,we analyzed the response mechanism of soil ecosystem to the conversion system of RP plantation.The conversion from RP seedling plantation to the coppice plantation resulted in the decline of soil quality and rhizosphere enrichment effect,and the homogenized and homogenized soil bacteria and nitrogen-fixing bacteria in the rhizosphere soil and bulk soil in the coppice plantation.The negative synergistic change of soil multi-dimensional ecological factors exacerbates the imbalance of soil micro-ecology,at the same time,because of the increasing of coppice plantation density,the shrub layer of understory vegetation disappeared and the diversity of herb layer increased,leading to the niche of soil resources.The distribution of nutrients in the underground organs of the community is less,which slows down the activities of the roots.Especially the sprouting forest is a nitrogen-restricted environment,indicating that the reduction of biological nitrogen fixation and nitrogen mineralization rate is difficult to meet the normal growth requirements of the sprouting forest,which will resulted in the decline of the coppice plantation.Although the RP had a significant benefits in soil improvement,but the coppice plantation had lower applied value in preserving soil ecosystem health and species diversity.The conversion of RP plantation to PD plantation model can lead to a decline in soil quality to a certain extent,but it can increase the number and diversity of understory shrubs and herbs.Therefore,Based on the leading function of“Biodiversity conservation and water conservation”in Mount Tai,conifer species should be protected,the proportion of RP plantation should be maintained,and the proportion of coppice plantation should be reduced.