Interaction Relationship And Mechanism Of Forest-Soil-Microbes Of Robinia Pseudoacacia In The Loess Plateau

Artificial forest ecosystem accounted for 7%of the global total forest area?Global Forest resources Assessment 2015?,and play an important role in carbon sink,biodiversity conservation,climate change and resource supply in the terrestrial ecosystem.Interaction and mechanism of Forest-Soil-Microbes is the key to understand the vegetation restoration process of plantation,evaluate the stability and function of plantation ecosystem and predict the evolution direction of ecosystem,and restore the ecological integrity of disturbed ecosystems.Therefore,this research focused on the changes and relationships of stand factors,understory vegetation community,soil physical-chemical properties and soil microbial community following Robinia Pseudoacacia afforestataion on the south of Loess Plateau.The objectives of this work were to explore?i?the co-evolution mechanism of"plant-soil-microbes"and“root-soil-microbes”system with Robinia plantation growth;???the effects of forest health status on vegetation community,bulk and rhizosphere soil nutrients and microbial communities;and???the response of“forest-soil-microbes”system to afforestation models with pure and mixed tree species.The main results are as followed:?1?Understory vegetation community compositions,soil properties and microbial community compositions in the Robinia forests were significant different among different stand ages.The highest plant diversity and richness was observed in the young Robinia forest,which was colonized by opportunistic species;the lowest values was observed in the overmature forest due to the interaction of understory light condition,soil high nitrogen status and interspecific competition,indicating that Humulus scandens completely occupied the herbaceous layer at the 35-Y sites.Although soil nutrients continually accumulate,low concentrations of available phosphorus and ammonium were observed in Robinia forest soil.Fungal community composition shifted from cropland to forestland with plant colonization,although soil bacterial community variation lagged behind the changes in the vegetation community.Soil bacterial and fungal community composition differed between early and later period of restoration.Plant richness and phosphorus were principal factors in shaping microbial community composition.?2?The establishment and development of Robinia plantation forests altered fine root nutrients,rhizosphere soil properties and microbial community composition.Fine root nutrients and C:N:P stoichiometry were affected by rhizosphere soil nutrients with linear correlation.Concentrations of total phosphorus in fine root and available phosphorus in soil gradually decreased.Robinia restoration reduced the diversity and abundance index of bacterial and fungal communities in rhizosphere soil,and its effects on bacterial community was larger than on fungal community.Rhizosphere soil bacterial communities converted from Proteobacteria-and Acidobacteria-dominant communities in early period?5 and 15 year?to Proteobacteria-dominant communities in the later period of Robinia afforestation.Fine roots and soil nutrients jointly affect the structure and composition of rhizosphere soil microbial community,and nitrogen and phosphorus in fine root and available phosphorus in soil were principal factors in shaping microbial community composition.?3?Comparing with healthy Robinia plantation,unhealthy forest significantly altered litters input,understory microenvironment,fine root nutrients,soil properties and soil microbial community.Bulk soil microbial community compositions were dominanted by Proteobacteria,Acidobacteria and Ascomycota in healthy forest,and by Proteobacteria,Actinobacteri,Ascomycota and Zygomycota in the unhealthy forest.The difference of rhizosphere soil bacterial and fungal communities between healthy and unhealthy Robinia forests were drived by Gemmatimonadetes and unclassified fungi,respectively.Bulk and rhizosphere soil nutrients could partly explain the variation in the microbial community compositions,and soil C:N and nitrate nitrogen are the key factors for shaping soil microbial community in the bulk and rhizosphere soils,respectively.?4?Understroy vegetation community,soil properties and soil microbial community were significantly affected by afforestation models with pure and mixed tree species,and mainly regulated by tree species traits and composition.The effect of afforestation models on the soil bacterial community diversity was larger than on richnenss,and the changes in bacterial community composition among forest types were dominated by Actinobacteria,Bacteroidetes and Nitrospirae.Soil fungal community dominant microorganisum,diversity,richness varied among forest types.Soil nutrients could partly explain the total variation of microbial community,and soil nitrogen?NH₄⁺and NO₃^-?and available phosphorus are principal factors in shaping microbial community composition.