| Phosphorus is the second nutrient after nitrogen in plants,which plays a vital role in plant growth and development.Phosphorus is easily fixed by organic matter and ions like iron,aluminum and calcium in the soil and thus becomes a limiting factor during plant growth and development.Forest age has an important influence on the forms,migration and transformation of soil phosphorus.Soil microorganisms are important regulators in the process of soil phosphorus transformation.Therefore,this study selected the Robinia pseudoacacia plantations with different ages?8Y,18Y,30Y?in the hilly area of the Loess Plateau as the research object.The research contents were as follows:?1?phosphorus forms in different soil layers?0-10 cm,10-20 cm,20-30 cm?using an improved Bowman-Cole fractionation method;?2?the microbial community structure involved in phosphorus transformation in 0-20 cm soil by quantitative PCR and High-throughput sequencing;?3?In addition,pot experiments were used to explore the effects of phosphorus addition on soil phosphorus forms and phosphorus cycling microbial community structure of Robinia pseudoacacia seedlings.so as to provide a theoretical basis for deeply understanding the dynamics of phosphorus and the mechanism of microbes in phosphorus cycling of Robinia pseudoacacia L.forest.The main results are as followed:?1?The growth and development process of plantation had a significant impact on soil nutrient content.The total organic carbon content,total nitrogen content,total phosphorus content,alkaline phosphatase activity and microbial biomass phosphorus in each soil layer gradually increased with the increase of forest age.The maximum appeared in the 30Y Robinia pseudoacacia plantation.The contents were 14.91 g·kg-1,1.71 g·kg-1,0.72 g·kg-1,1.76 mg·g-1·d-1 and 19.66 mg·kg-1,respectively.The soil p H showed the opposite trend.In the same age,the content of total organic carbon,nitrogen,and phosphorus decreased with soil depth.Soil p H increased with soil depth.The content of total phosphorus in each soil layer was not significantly different.?2?The moderately labile organic phosphorus was the main form of organic phosphorus in Robinia pseudoacacia plantation.In plantation forests at different development stages,its content was 19%-42%,75%-122%and 291%-525%higher than that of the moderately stable organic phosphorus,the highly stable organic phosphorus and the labile organic phosphorus,respectively.With the growth of the forest age,the organic phosphorus content of the above four forms in each soil layer decreased to different degrees.Among them,the reduction of labile organic phosphorus was the largest.In 30Y Robinia pseudoacacia forest land,the content of the labile organic phosphorus was reduced by 15%to 25%and 12%to 21%respectively compared with 8Y and 18Y forest land.Among inorganic phosphorus components,Na OH-Pi had the highest content,followed by HCl-Pi and Labile-Pi.With the growth of forest age,only HCl-Pi content in each soil layer gradually increased,and the content of the other two components decreased to different degrees.Among them,the decrease amplitude of Labile-Pi was greater than that of HCl-Pi.?3?The phoD gene abundance of Robinia pseudoacacia plantation increased first and then decreased with the growth of the forest age.The abundance of phoD gene in 18Y forest land increased by 103%compared with 8Y forest land.The 30Y forest land decreased by 52%compared with 18Y forest land.There was no significant difference in species richness and diversity during the developmental of Robinia pseudoacacia plantation?P>0.05?.The phoD-harboring microorganisms were mainly composed of Proteobacteria,Acidobacteria,Actinobacteria,and Cyanobacteria.Based on the genus level,the dominant bacteria were Bradyrhizobium,Variovorax,and Kribbella.The relative abundance of Bradyrhizobium and Kribbella showed a trend of first decreasing and then increased with the increase of forest age,and the maximum value appeared in 30Y forest land.The relative abundance of Variovorax gradually decreases.Forest age had no effect on the species richness and diversity of the microbial community phoD encoders.Soil p H,available phosphorus,total phosphorus,alkaline phosphatase,soluble organic carbon,and total nitrogen were the main factors affecting the microbial community that phoD encoders.?4?Phosphorus application had a significant effect on the soil phosphorus forms and the microorganisms phoD encoders in pot experiment.Except for the moderately stable organic phosphorus,the phosphorus content of each component increased with phosphorus fertilizer application varying degrees,and the content of the labile organic phosphorus and Labile-Pi increased the most.The phoD gene abundance showed the opposite trend.Under the three phosphorus fertilizer levels,the phoD-harboring microorganisms were mainly composed of Proteobacteria,Acidobacteria,Cyanobacteria,and Actinobacteria.The main dominant bacteria were Mesorhizobium,Bradyrhizobium,Beijerinckia,Streptomyces.Under low phosphorus fertilizer levels,the abundance of microbial communities was significantly higher than that of medium and high phosphorus fertilizer levels.The phosphorus fertilizer levels had no effect on the diversity of soil microbial community.In addition,phosphorus application significantly increased the content of available phosphorus in the soil,while declined soil phosphatase activity. |
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