Effects Of Phosphorus Addition On Soil Microbial Community Structure And Function In Chinese Fir Plantation

As an important part of the forest ecosystem,soil microorganisms participate in all biogeochemical cycles of the ecosystem and play an important role in regulating organic matter decomposition,nutrient cycling,plant diversity,and productivity.At the same time,subtropical forest soil is highly weathered,and most phosphorus is fixed by iron and aluminum oxides.Phosphorus is considered to be one of the nutrient elements that limit microbial activity,community composition,and function in subtropical forest soil.Focusing on the effects and mechanisms of phosphorus nutrients on the structure and function of soil microbial has become a"high priority area"for research in the subtropical region.However,most of the existing studies focus on the effects of phosphorus addition on the composition and diversity of regional forest soil bacterial or fungal microbial communities,how the intra-boundary and inter-boundary interactions and functions of forest soil microbial communities will change after phosphorus addition,what are the potential driving mechanisms,there are still unclear.Therefore,in this study,in situ phosphorus addition experiment in Chinese fir plantation were established.The changes in the composition,diversity,intra-boundary and inter-boundary interactions,enzyme activity,and functional gene abundance of soil microbial were observed using biomarkers（phospholipid fatty acids）and molecular biology techniques.Combined with the change of soil physical and chemical properties after phosphorus addition,the influence and mechanism of soil phosphorus availability change on soil microbial population structure and function of Chinese fir plantation were clarified.This study is helpful to accurately predict the response of ecosystem productivity and ecological function of Chinese fir plantation to phosphorus nutrients,provides basic data for incorporating phosphorus nutrients into the earth system model,and also provides scientific guidance for the rational management and management of Chinese fir plantation.The main results are as follows:（1）Phosphorus addition significantly changed soil physicochemical properties.Among them,the soil p H,total phosphorus（TP）and available phosphorus（AP）contents increased significantly after phosphorus addition,while nitrate-nitrogen（NO₃^--N）and ammonium-nitrogen（NH₄⁺-N）decreased significantly.The interaction between phosphorus addition and seasonal variation significantly reduced soil organic carbon（SOC）,indicating that phosphorus addition was expected to slow down the soil acidification process caused by nitrogen deposition and reduce soil nitrogen leaching loss while alleviating the soil phosphorus limitation of Chinese fir plantation,but it might be not conducive to the maintenance of soil organic carbon.（2）Phosphorus addition significantly affected soil microbial biomass carbon and phosphorus and the characteristic ratio of different types of microbial phospholipid fatty acid content.Among them,soil microbial biomass phosphorus（MBP）increased significantly after phosphorus addition,microbial biomass carbon（MBC）and microbial biomass carbon nitrogen phosphorus ratio（MBC/MBN,MBC/MBP,and MBN/MBP）decreased significantly,and the ratio of fungi to bacteria（F:B）increased significantly after phosphorus addition,and the ratio of Gram-positive bacteria to Gram-negative bacteria（GP:GN）decreased significantly,indicating that phosphorus addition changed the nutrient demand status of soil microbial carbon,nitrogen and phosphorus and the composition of soil microbial community in Chinese fir plantation,and the microbial community tends to use refractory organic carbon.Bacterial communities tend to change in the direction of using plant-derived organic carbon.（3）Phosphorus addition significantly affected soil microbial community composition and diversity.The alpha diversity of bacterial species increased significantly after phosphorus addition,and the abundance of Bacteroidetes at the phylum level increased significantly.The alpha diversity of fungal species was significantly decreased,and the abundance of Ascomycota at the phylum level was significantly increased.In addition,the interaction between phosphorus addition and seasonal variation significantly affected the beta diversity of bacteria and fungi.Random forest and correlation analysis showing that SOC,soluble organic nitrogen（DON）,soil carbon to phosphorus ratio（C/P）,and soil nitrogen to phosphorus ratio（N/P）were the key factors indicated the change in bacterial community structure,while p H,AP,C/P,and N/P were the main environmental factors affecting the change of fungal community structure,indicating that phosphorus addition directly or indirectly affected the composition and diversity of the microbial community by changing soil p H as well as carbon and nitrogen nutrient limitation status.（4）Phosphorus addition significantly affected interactions within the soil microbial community and interactions between bacterial and fungal communities.The number of total nodes,total connections,modularity index,and negative correlation ratio in the topological parameters of bacterial networks increased significantly after phosphorus addition,while the changes in network topological parameters of fungal communities and fungal-bacterial interactions were opposite to those of bacteria,indicating that the complexity of network structure of bacterial communities increased,signaling efficiency increased,intra-boundary and inter-boundary interactions of bacterial communities weakened and competition strengthened after phosphorus addition;on the contrary,the complexity of network structure of fungal communities and fungal-bacterial interactions decreased,signaling efficiency slowed down,intra-boundary and inter-boundary interactions of fungal communities strengthened and competition weakened.（5）The soil enzyme vector angle（VA）was greater than 45°and the enzyme vector length（VL）was greater than 1 in the phosphorus addition and control treatments.After phosphorus addition,the activity of soil acid phosphatase（Ac P）and VA significantly reduced,but the ratio of soil carbon acquisition enzyme to phosphorus acquisition enzyme(E_C/P)significantly increased.The correlation analysis showed that Ac P was negatively correlated with soil TP and AP,and positively correlated with the abundance of Actinobacteria phylum.Those results showed that soil microorganisms in Chinese fir plantations were limited by carbon energy and phosphorus nutrients,and phosphorus addition alleviated the phosphorus limitation of soil microorganisms to some extent.But it may exacerbate the carbon limitation of soil microbes,and microbes in the Actinobacteria phylum played an important role in regulating soil acid phosphatase activity.（6）Phosphorus addition significantly increased the abundance of soil carbon cycle genes(including methane metabolism（mmox,em GDH）,carbon sequestration（frd A,kor A,mct）,carbon degradation（hemicellulose degradation（man A）,starch degradation（gam）,and lignin degradation（mnp）genes）and nitrogen cycle（including nitrogen fixation（nif H）,nitrification（amo B）,denitrification（nar G,nir S-3,nos Z-1）,ammonia oxidation（hao,hzs B）genes）,but reduced the abundance of inorganic phosphorus solubilization genes（gm GDH,pqq C）.Correlation analysis revealed that p H,TP,AP,and C/P,N/P were the key factors affecting the abundance of carbon and nitrogen cycle genes,suggesting that phosphorus addition may regulate the carbon,nitrogen,and phosphorus cycle functional genes by changing soil nutrient supply status and p H.In addition,changes in the abundance of Bacteroidetes and Elusimicrobia could be used to predict the functional potential of soil microorganisms in the carbon,nitrogen and phosphorus cycles.In summary,phosphorus addition directly or indirectly affected soil microbial community structure and function by changing soil environmental factors.The results showing that phosphorus addition not only changed the composition and diversity of soil microbial communities and the interaction between microbial communities;but also changed the soil enzyme activity and the abundance of functional genes related to carbon,nitrogen,and phosphorus transformation.This study clarified the response mechanism of microbial community structure and function to phosphorus addition,which is expected to provide a theoretical basis for phosphorus nutrient incorporation into the earth system model and provide theoretical support for the management and management of subtropical Chinese fir plantation.