| After long-term high intensity utilization and soil erosion,black soil degradation is serious in the northeast region of China.The decrease of soil organic matter content leads to the loss of soil phosphorus（P）and potassium（K）.Available phosphorus（AP）contents are generally deficient and available potassium（AK）contents of partial areas are insufficient in the crop growing season.The utilization rate of phosphate and potassium fertilizer are low,and excessive application leads to deterioration of soil physical and chemical properties,non-point source pollution and increase agricultural costs.A large amount of fixed phosphorus and potassium exist in the plough layer of black soil.Phosphorus and potassium-solubilizing microorganisms can effectively promote P and K transformation.Biochar can serve as a carrier for immobilizing plant growth-promoting bacteria,promotes the proliferation of microorganisms,supplies P and K for crops.However,soil microecosystem is very complex.After inoculation of microbial agent with biochar,both biotic and abiotic factors will affect the activity and function of exogenous microorganisms.Few r eports on the mechanism about exogenous microbial community,soil microbial community and soil environmental factors jointly affecting P and K transformation in terms of interaction mode,interaction intensity,influence value and driving force,which is s ignificant to reveal the microecological mechanism of P and K transformation in black soil environment.In this study,several phosphorus and potassium-solubilizing strains were screened from black plough soil of different crops.Pot experiment and green fluorescent protein（GFP）labeling technology were used to study the growth-promoting effect and colonization ability of functional bacterium in maize rhizosphere soil and plant.The changes of soil AP and AK content and maize growth after inoculation were analyzed.The effects of biochar on the growth and capacity of functional bacterium were studied.Orthogonal test was used to determine the proportion of strains and prepare the compound microbial agent.Response surface methodology（RSM）was used to optimize the culture conditions.The effects of adding exogenous compound microbial agent,biochar and P and K substrate on soil characteristics and maize growth were analyzed.High-throughput sequencing technology was used to explore the effects of above treatments on soil bacterial community composition.Multivariate statistical analysis was used to explore the interaction between exogenous microorganisms and soil bacterial community.The correlation between soil environmental factors and bacterial community s tructure was analyzed.The relationships between P,K and other main environmental parameters and bacterial community diversity were quantified.The key bacterium and environmental factors that affecting P and K transformation were identified,which revealed the mechanism of P and K transformation on the perspective of microecology.It provided a basis for the synergistic effect and relative contribution of exogenous microorganisms,soil microorganism and soil environmental factors to regulate the soil P and K transformation,and provided technical support for black soil protection.The results were as follows:（1）Phosphate and potassium-solubilizing strains of Beijerinckia sp.WN-2 and Azotobacter sp.JWJ-5,and phosphate-solubilizing strains of Enterobacter sp.JYJ-2 and Rhizobium sp.JYJ-3 were isolated and screened.After inoculation,it was found that the above strains could be successfully colonized in rhizosphere soil and maize plants,and the colonization quantity of maize roots was more than that of stem and leaf tissues.The content of AP and AK increased significantly.The promoting effects on maize growth were significant.The characteristics of IAA,organic acid and extracellular polysaccharide secreted by strains were significantly correlated with maize growth.The addition of maize stalk biochar could significantly improve the growth,adsorption and phosphate and potassium-solubilizing capacity of functional bacterium.（2）Strains of WN-2,JYJ-2,JYJ-3 and JWJ-5 were prepared according to the ratio of 3:2:1:3.After optimized culture conditions,the content of solubilizing phosphorus and potassium increased by1.05 and 1.07 times,respectively.Compared with the control group,in the treatment group of compound microbial agent+biochar+calcium phosphate+potash feldspar showed that soil p H,OM and CEC increased by 12.24%,13.41%and 11.25%,respectively.The contents of AN,AP and AK contents increased by 11.75%,54.28%and 37.87%,respectively.The activities of soil PP,UA and CL increased by 12.01%,9.92%and 11.46%,respectively.The maize plant height,stem diameter,leaf area,fresh weight of aboveground and underground increased by 43.11%,35.79%,33.20%,39.43%and 38.82%,respectively.The contents of TN,TP and TK in maize leaves increased by 19.69%,34.62%and 26.83%,respectively.（3）Theαandβdiversity analysis of soil bacterial community showed that the diversity and abundance of soil microorganisms increased significantly after inoculated with compound microbial agent（p<0.05）.The distance threshold of the sample group added microbial agent was similar,which had a significant difference with the sample group without microbial agent,indicating that compound microbial agent was the key factor that affecting soil bacterial community structure.The bacterial community on phylum level:Actinobacteria and Proteobacteria belonged to the dominant phylum,and the exogenous microorganisms screened belong ed to Proteobacteria.In the samples added microbial agent and biochar,the relative abundance of Proteobacteria increased by 3.23%,4.08%and 9.82%,respectively,which affected the relative abundance of other bacterium.The bacterial community on genus level:Beijerinckia and Enterobacter were detected in samples added compound microbial agent with high abundance,which showed extremely significant differences among multiple groups of samples（p<0.01）.The addition of exogenous microorganisms and biochar activated the beneficial bacteria community such as Arthrobacter,Gaiella,norank＿f＿＿Roseiflexaceae,which jointly affected the changes of soil bacterial community.（4）Spearman analysis showed that soil OM,p H,AP,AK and PP were significantly affected bacterial community and significantly correlated with most OTU.OTU49,OTU116 and OTU4072were positively correlated with AP.OTU49 and OTU217 were positively correlated with AK.The results of network double-factor analysis showed that species contained by Actinobacteria and Proteobacteria were positively correlated with AP and AK.Path analysis showed that AP,AK,p H and OM were positively correlated with soil microbialαdiversity.The direct influence order was AP(1.8655**)>OM（1.0223*）>p H(1.0042**)>AK（0.6734*）.AP was the best predictor and main contributing factor for soil microbial diversity changes.AP and AK synergistically affected soil microbial diversity changes.（5）Path analysis about synergistic effect of exogenous microorganisms and soil microorganisms on P and K transformation showed that the exogenous bacterium including Beijerinckia,Enterobacter,Rhizobium and soil microorganisms including Arthrobacter,unclassified＿f＿＿Intrasporangiaceadrove soil P transformation in a synergistic manner.The exogenous bacterium including Beijerinckia,Azotobacter and soil microorganisms including Gaiella,Arthrobacter,g＿＿RB41 drove soil K transformation in a synergistic manner.The indirect effects of bacterium on soil P and K transformation also indicated the interaction between bacterium.The results further verified the unique exogenous bacterium including Beijerinckia and Enterobacter,exogenous and soil microorganisms shared bacterium including Rhizobium and Azotobacter played a synergistic role in driving P and K transformation in black soil.（6）Path analysis about synergistic effect of exogenous microorganisms and environmental factors on soil P and K transformation showed that the exogenous bacterium including Beijerinckia,Rhizobium,Enterobacter and key environmental factors including p H and OM drove soil P transformation in a synergistic manner.The exogenous bacterium including Beijerinckia,Azotobacter and key environmental factors including OM,p H and AP drove soil K transformation in a synergistic manner.AP had a direct effect on AK（0.5347*）,which indicated that soil P transformation had direct driving force on K transformation.The microbial community and other environmental factors indirectly affected K transformation through driving P transformation.In conclusion,path analysis could better clarify the key microbial communities and environmental factors that regulated black soil P and K transformation and their direct and indirect influence coefficients.The results indicated that the exogenous microorganisms,soil microorganisms and soil environmental factors drove black soil P and K transformation through microecological mechanism of synergistic interaction. |
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