| The area of plantations is increasing worldwide,and the main afforestation goal is to establish plantation with ecological protection.At present,there are some problems with plantations,such as a single understory structure and low biodiversity.Therefore,how to conversion from plantations to the near-natural forest through artificial tending measures,so that their ecological functions can be fully and sustainably developed,has become the hotspot and difficult point in current restoration ecology.At present,due to the lack of systematic comparison between planted and natural forest communities,it is difficult to scientifically evaluate the community characteristics of plantations formed by different tending measures.In this research,the Pinus tabulaeformis forest on the Loess Plateau was used as the research object,and the differences of vegetation community structure,soil physiochemical properties and soil microbial characteristics under the two vegetation restoration types（artificial afforestation,natural restoration）and different stand densities(low(plantations:725-875 stems·hm-2),medium(plantations:1375-1500 stems·hm-2;natural forest:1250-1400 stems·hm-2),high(plantations:2150-2200 stems·hm-2;natural forest:2000-2700 stems·hm-2)were compared systematically.The results are as follows:（1）The vegetation restoration types significantly affected the diameter distribution of Pinus tabulaeformis and their vertical structure.Compared with natural forests,the diameter class in plantations was displaying an open downward“unimodal”trend,and the curve moves to the left with the increase of stand density.In natural forests,the proportion of small tree size Pinus tabulaeformis increased,which indicated that the regeneration capacity of Pinus tabulaeformis in natural forests was stronger than that in plantations.The Shannon index of structural diversity in plantations（0.98-1.48）were lower than that in natural forests（1.41-1.47）.Moreover,as the stand density increased,the Shannon index of structural diversity significantly decreased.However,compared with the natural forests,the stand volume of Pinus tabulaeformis in plantations was higher than in natural forest,and had the maximum value(328 m3·hm-2)in the medium-density.Also,vegetation restoration types and stand density significantly affected species composition and diversity.The species richness index and diversity index in plantations were higher than those in natural forest,and had the maximum value in the low-density plantations（1.24 and 1.3）.In plantations,the species richness index decreased with the stand density increased,while it significantly increased in natural forests.However,the species diversity index significantly decreased both in plantations and natural forests with the increase of stand density.RDA results showed that soil water content and slope degree were important factors for the change of community species composition.（2）The vegetation restoration types and stand density significantly affected the soil physicochemical properties.Compared with natural forest,soil water content（30%-34%）and soil pH in plantations（8.39-8.46）higher than that in natural forests（23%-24%;8.30-8.31）,but soil bulk density(0.91-0.95 g·cm-3)lower than that in natural forests(0.97-1.01 g·cm-3).In terms of soil nutrients,compared with natural forests,the surface soil organic carbon(13.04-17.73 g·kg-1),total nitrogen(1.25-1.30 g·kg-1),mineral nitrogen(27.54-34.55 mg·kg-1),and available phosphorus(2.54-3.27 mg·kg-1)content in plantations was significantly lower than that in natural forests(27.41-30.61g·kg-1;1.94-1.96 g·kg-1;35.37-39.10 mg·kg-1;6.91-8.15 mg·kg-1),while soil total phosphorus content(0.53-0.59 g·kg-1)was significantly higher than that in natural forests(0.43-0.52 g·kg-1).With the increase of stand density,the content of soil total phosphorus significantly decreased,while that of soil total nitrogen(1.30 g·kg-1)and mineral nitrogen(34.55 mg·kg-1)content in the middle-density plantations were significantly higher than other densities.Correlation analysis results showed that soil organic carbon,total nitrogen,and available phosphorus were positively correlated with litter depth and litter biomass,but negatively correlated with Margalef richness index,Simpson diversity index and Shannon diversity index.Soil water content and total phosphorus were positively correlated with Margalef richness index,Simpson diversity index,and Shannon diversity index,but negatively correlated with litter depth and litter biomass.There was also a significant positive correlation between soil total phosphorus and Pielou evenness index.Soil pH was negatively correlated with litter depth and litter biomass.In addition,compared with natural forest,the soil carbon storage of different soil layers（0-20cm,20-40cm and 40-60cm）in plantations decreased significantly by51%,53%,and 54%,respectively.Compared with natural forests,the different activated carbon content of different granular soil aggregates in plantations was significantly lower than that in natural forests,in which the content of high,medium and low activated carbon decreased by 43%,49%,and 48%respectively.However,there was no significant difference in the content of globulin between plantations and natural forests.（3）The vegetation restoration types and stand density significantly affected soil enzyme activities.Compared with natural forests,the activities of C-source enzyme(29.52-36.37 nmol·g-1·h-1 dry soil)and N-source enzymes(78.93-107.59 nmol·g-1·h-1dry soil)in the surface soil in plantations were significantly lower than that in natural forests(51.07-55.02 nmol·g-1·h-1 dry soil;123.92-124.98 nmol·g-1·h-1 dry soil),but P-source enzyme activity(104.20-148.46 nmol·g-1·h-1 dry soil)was significantly higher than that in natural forests(100.83-114.98 nmol·g-1·h-1 dry soil);with the increase of stand density,the activities of N and P-source enzymes in plantations increased significantly.Moreover,RDA analysis showed that soil organic carbon content,C:P of available nutrients,and richness index of vegetation were the main factors that caused the change of soil enzyme activity.In addition,the stoichiometric ratio of soil enzymes in natural forests（1:1.22:1.18）and plantations（1:1.31:1.39）deviated from the global average.Vector analysis showed that the soil microorganisms in natural forests were limited only by C and N（vector angle<45°）,not by P.Plantation construction slowed down the C limit,but aggravated N limit,and caused the P limit（vector angle>45°）.（4）The diversity index of bacterial community was significantly affected by vegetation restoration types and stand density.Compared with the natural forest,the bacterial OTU richness index（2637-2773）andαdiversity index（6.38-6.46）in plantations were significantly higher than those in natural forests（2324-2461,6.18-6.20）.Moreover,with the increase of stand density,the diversity index of bacterial community significantly decreased.Also,the vegetation restoration types significantly affected the composition of fungal and bacterial communities in the Pinus tabulaeformis forests.Compared with the natural forest,at the same stand density,the relative gene abundances of Acidobacteria（22.81-23.32%）,Plantomycetes（8.83-9.6%）and Bacteroidetes（2.21-3.16%）phylum in the bacterial community of plantations were significantly higher than that in natural forest（18.55-21.02%,7.09-7.2%,1.66-1.85%）,while the relative gene abundances of Actinobacteria（15.1-16.18%）phylum was significantly lower than that in natural forests（21.97-25.51%）.On the other hand,with the increase of stand density,the relative gene abundances of Plantomycetes and Gemmatimondetes phylum significantly increased in plantations,while the relative gene abundances of Nitrospirae significantly decreased.Moreover,the relative gene abundances of Bacteroidetes phylum were significantly lower than those of other densities.In natural forests,with the increase of stand density,the relative gene abundances of Nitrospirae significantly increased,while that of Gemmatimonadetes phylum significantly decreased.In the fungal community,compared with the natural forest,at same stand density,the relative gene abundances of Basidiomycota phylum（36.97-39.4%）in plantations were significantly higher than that in natural forests（22.54-29.06%）.On the other hand,in the plantation,the relative gene abundance of Basidiomycota（36.97%）phylum in the middle-density was significantly lower than that of other densities.While the relative gene abundance of Zygomycota（4.45%）phylum was significantly higher than that of other densities.In natural forests,with the increase of stand density the relative gene abundance of Basidiomycota phylum significantly increased.PCoA results showed that there were significant differences in soil bacterial and fungal communities between plantations and natural forests,and the soil fungal and bacterial communities in the low and middle-densities were significantly different from those under the high densities.RDA analysis results showed that the content of soil nutrients（soil available P and total N content）was the main factor causing the change of soil microbial community.In addition,vegetation restoration types and stand density significantly affected the relationship between species in soil bacterial communities.Compared with natural forests,the relationship between bacterial species in plantations was loose and the network was unstable（low connectivity）;compared with high density forests,the relationship between bacterial species in the middle density forest was closer and the network was more stable.However,there was no significant difference in the network structure among the fungal communities with different restoration types and different stand densities.In addition,the key species of fungi and bacteria are different under different treatments,which may be caused by different environmental factors.In a word,on the Loess Plateau,medium-density(1375-1500 stems·hm-2)was the best afforestation density for wood production,and the soil bacterial network was more stable under this density.In addition,low-density afforestation(725-875 stems·hm-2)was the best afforestation density to maintain plant and microbial diversity,and under this density,it eases the limitation of soil nutrients on microbial metabolism.Therefore,the afforestation of Pinus tabulaeformis on the Loess Plateau should not be higher than1500 stems·hm-2. |
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