Regulation Mechanism Of Siol Microorganisms On Soil Phosphorus Availability Of Pinus Tabuliformis Plantation Of Loess Plateau

Phosphorus（P）is one of the primary factors influencing the primary productivity and ecological functioning of forest ecosystems.Due to the low solubility and easy adsorption of P compounds in soil,phosphorus often becomes a limiting nutrient in forest ecosystems.Plant P acquisition relies on rhizosphere processes.However,the specific characteristics of the response of rhizosphere microbial community structure and microbial P metabolism constraints to soil P availability,as well as the mechanisms of interaction between plants and rhizosphere microorganisms in the P acquisition process,remain unclear.Pinus tabuliformis forests are a significant component of northern China’s forest ecosystems.Therefore,this study investigated the rhizosphere soil P fractions,enzyme activities,and microbial community along a Pinus tabuliformis chronosequence to elucidate the relationships between soil microbial communities,microbial P metabolism limitation,and P fractions.In conjunction with pot experiments involving P additions and synthetic microbial communities（Symcom）,we employed non-targeted metabolomics of root exudates,high-throughput sequencing,and synthetic microbial community methods to uncover the regulatory strategies of root exudates,rhizosphere microorganisms,and their interactions on soil P availability.The main findings are summarized as follows:（1）In the Loess Plateau,the total P content of P.tabuliformis plantation soil is relatively abundant,but the available P is at a low level.With the increase in stand age,soil P availability significantly decreased.The available P content in bulk soil decreased from 1.60 to 1.24mg·kg^-1,whereas the available P content in the rhizosphere soil remained relatively constant,ranging from 2.68 to 3.05 mg·kg^-1.Furthermore,the available P content in the rhizosphere was significantly higher than in the bulk soil.HCl-P fration was the dominant P fraction in the soil,with contents ranging from 7.82 to 10.76 mg·kg^-1,followed by Citrate-P,Ca Cl₂-P,and Enzyme-P frations.As the stand age increased,the HCl-P content in both bulk and rhizosphere soils significantly decreased,and the HCl-P content in rhizosphere soils at all stand ages was significantly lower than in the bulk soils.Both bulk and rhizosphere soil microbial metabolism was generally limited by P,with a significant increase in the degree of P limitation in rhizosphere soil with increasing stand age.Regression analysis revealed a significant negative correlation between microbial P metabolism limitation and HCl-P,and a significant positive correlation with Citrate-P and Enzyme-P.（2）Soil bacterial community is primarily composed of Proteobacteria,Acidobacteria,Actinobacteria,and Chloroflexi,while fungal community consists mainly of Ascomycota and Basidiomycota.As stand age increased,the complexity of bacterial co-occurrence networks gradually increased,while there was no significant change in the complexity of fungal co-occurrence networks.The assembly processes of bacterial and fungal communities were predominantly influenced by dispersal limitation,with fungal community being more affected by dispersal limitation.Microbial P metabolism limitation is a major driving factor for bacterial and fungal communities,especially in the rhizosphere soil.Keystone species composition of both rhizosphere bacteria and fungi were driven by soil total phosphorus TP and the C:P ratio.Bac＿Mod 3 was identified as the primary functional module regulating soil P fractions and showed a significant negative correlation with microbial P metabolism limitation.An increase in the abundance of Bac＿Mod 3 led to higher HCl-P content.Dominant genera within Bac＿Mod 3,such as Mycobacterium,Pseudoxanthomonas,Ramlibacter,Rubrobacter,and Streptomyces,exhibited strong correlations with soil P fractions.（3）P addition significantly altered soil P fractions,root morphology,root exudate composition,and rhizosphere bacterial and fungal community structures.P addition significantly increased soil total P,available P,Ca Cl₂-P,Citrate-P,HCl-P,microbial biomass P,root length,root surface area,and fine root percentage,and significantly decreased alkaline phosphatase activity.P addition significantly increased the content of oxamic acid and 1,2,4-Benzenetriol in root exudates but significantly decreased oxalic acid content.The relative abundance of ectomycorrhizal fungi significantly decreased after P addition.P-sensitive bacterial indicator species（mainly from Solirubrobacter and Streptomyces）significantly enriched in low-P environments,most of which possessed P activation or P transport capabilities.Oxalic acid showed significant negative correlations with Ca Cl₂-P,Citrate-P,and HCl-P fractions,with the strongest correlation observed with the HCl-P fraction.Additionally,oxalic acid exhibited significant positive correlations with the most of P-sensitive bacterial indicator species.The structural equation model indicates that fine roots could directly impact plant P acquisition and indirectly affect P acquisition by influencing ectomycorrhizal fungi,as well as indirectly influencing P acquisition by regulating P-sensitive bacterial indicator species through oxalic acid.（4）In the rhizosphere soil of P.tabuliformis,a total of 45 bacterial strains were isolated,primarily belonging to the genera Streptomyces,Bacillus,and Pseudomonas.Among these,19 strains exhibited the capability to dissolve organic P.Fourteen strains possessed the ability to dissolve inorganic P.Fifteen strains had the capacity to produce auxins,and 14 strains exhibited nitrogen-fixing abilities.In low-P environments,13 genera,including Streptomyces,Opitutus,Chitinophaga,and Luteimonas,were significantly enriched.Based on the significantly enriched bacterial genera in low-P environments and the isolates’plant growth-promoting traits,a Syncom with P-solubilizing capabilities was constructed.Under low-P conditions,inoculating the Syncom significantly reduced the HCl-P content in rhizosphere soil while significantly increasing available P,Ca Cl₂-P,Citrate-P,root P concentration,fine root percentage,and the activities of root system resistance-related enzymes.However,in high-P environments,the plant growth-promoting effects of the Syncom were diminished.Therefore,inoculating the Syncom can effectively enhance soil P availability,promote plant growth,but its effectiveness depends on soil P levels.In conclusion,this study found that the soil P availability in the P.tabuliformis plantation soil on the Loess Plateau is low and decreases gradually with forest age.Facing low-P environment,P.tabuliformis adapts by regulating rhizosphere processes,which include adjusting root exudates（oxalic acid）and the microbial community（Streptomyces）to dissolve HCl-P fractions,thereby enhancing soil P availability.The findings are helpful for deeply understanding P limitation mitigation mechanisms in forest ecosystems and the’plant-microbe’interaction mode,provide a valuable model and theoretical basis for the implementation of sustainable forest management practices.