Soil Microbial Community Dynamics Following Afforestation And Its Responses To Thinning Practice Of Pinus Tabulaeformis On The Loess Plateau

The“Plant-Soil-Microorganism”system,is the key to understanding the self-sustaining capacity of fragile ecosystem in the process of forest vegetation restoration.The artificial forest builted on abandoned farmland is of great significance to improve the benefits of soil and water conservation and microclimate environment in gully area of Loess Plateau.Thinning is the main management measure to improve the stability of plantation ecosystem and promote natural regeneration of forest.During the process of plantation development and thinning process,exploring the dynamics of soil microorganism and its response to the aboveground plant community and soil physicochemical characteristics will be helpful to furtherly understand the relationship between soil microorganism and forest ecosystem function and provide scientific basis for scientific management of artificial forests.This paper studied pinus tabulaeformis plantation with different forest age and different thinning intensity,and the natural secondary forest of pinus tabulaeformis was taken as the reference.In this study,we investigated the changed characteristics of understory plant diversity,biomass,soil physicochemical characteristics,enzyme activity and microbial biomass based on the method of typical sample plots.Furthermore,we analysed the diversity and composition of soil bacterial and fungal community by the means of high throughput sequencing.Finally,through Pearson correlation and RDA redundancy analysis,the synergistic relationship among plant-soil-microorganism was analyzed to explore the characteristics and mechanism of pinus tabulaeformis plantation and thinning on soil microorganism.The main results are as follows:1.The diversity of shrub layer plants increased with the increase of stand age following afforestation with Pinus tabulaeformis.While the diversity of herb layer plants appeared to be the highest in the young stand stage?12-Y?,and then decreased.Soil enzyme activity,microbial biomass and soil carbon and nitrogen content were significantly increased after afforestation and were highest in mature forest.Compared with the abandoned land,the diversity index of soil bacterial and fungal communities increased after afforestation,reached the maximum value in the young stand stage,and decreased significantly and remained stable after the middle stand stage forest.The Proteabacteria belonging to copiotrophic bacteria increased with the increase of stand age,while the Actinbacteria belonging to oligotrophic bacteria decreased with the increase of stand age.The dominant phylum Ascomycota of the soil fungi community occupied the dominant position in the abandoned land and young forest soil?12-Y?,while Basidiomycota significantly increased and occupied the dominant position in the stage of medium and mature forest?29-Y,53-Y?.The results of cluster analysis and PCoA analysis showed that the community structure of soil bacteria and fungi presented an aggregation effect in abandoned lands and young forests.Soil organic carbon?SOC?,total nitrogen?TN?,available phosphorus?AP?,carbon/nitrogen?C:N?and nitrate nitrogen?NO₃^--N?correlated closely with the dominant groups of soil microbial community.2.There was no significant difference in soil enzyme activity between mature plantation and natural secondary forest,while differences in soil enzyme appeared among different forest sites.Soil microbial biomass carbon and nitrogen were significantly higher in natural secondary forest than in artificial forest.Stand origin affectd soil bacterial community diversity index?Shannon?significantly,and geographical distribution affectd soil bacterial community abundance index?Richness?significantly.However,no significant effect was found on the diversity index of the fungal community in both stand origin and geographical distribution,which had an impact on soil fungal community abundance index.Cluster analysis and PCA analysis showed that the bacterial community structure in soil presented separation effect in Huanglong forest region and Zhengning-Fuxian forest region,while aggregation effect was found in the artificial forest and natural secondary forest in the same district.Soil fungal communities showed no significant aggregation effect in different forest areas.Soil nutrients such as TN,NO₃^--N,SOC and TP were significantly correlated with the dominant groups of soil bacterial communities.Different from the bacterial community,only the abundance index Chao in the fungal community was significantly correlated with TN and NO₃^—N,while the diversity index and dominant fungi were not closely related to soil nutrient.3.Plant community diversity,soil carbon and nitrogen content increased with the increase of thinning intensity after 11 years thinning in the Pinus tabulaeformis plantation.Thinning intensity had no significant effect on the diversity index of soil bacteria and fungi.However,thinning had significant effect on the abundance of soil microbial dominant groups.For example,Proteobacteria in severe thinning?HIT?is higher than other thinning treatments,Acidobacteria in unthinning?CK?and light thinning?LIT?sites is higher than medium?MIT?and severe thinning?HIT?.The content of organic carbon and total nitrogen in soil was positively correlated with Proteobacteria and negatively with Acidobacteria,which furtherly verified the indicator effect of the two types of soil bacteria on soil nutrient status.For the soil fungi community,the relative abundance of Basidiomycota was lower in the HIT,while the relative abundance of Ascomycota was higher in the HIT.Soil SOC,TN,TP and NO₃^--N had significant effects on the dominant communities of soil microbe.Our results showed that MIT was beneficial to the seedling regeneration density,and HIT was beneficial to the accumulation of soil organic carbon and the copiotrophic bacteria.4.The response mechanism of soil microbes to understory vegetation and soil factors is different among the development of Pinus tabulaeformis plantation,the process after thinning,and the distribution pattern of mature forest in different counties.During the process of plantation development,both plant factors and soil factors have significant effects on soil bacterial and fungal communities.After thinning,the response of soil fungal community to understory plants and soil physical and chemical properties was stronger than that of bacterial community.In the regional distribution pattern of mature plantation and secondary forest,soil nutrients have significant influence on soil bacterial community,and TN is the most important factor affecting soil bacterial communities.However,soil nutrients have weak influence on soil fungal community.