Effects Of Introducing Broadleaf Tree Species Mixed Transformation On Soil Phosphorus Fractions And Their Transformation In Pinus Massoniana Plantation

Phosphorus（P）is an essential nutrient for plant growth and development and an important component of soil fertility.However,especially in highly weathered subtropical forest soils,P is a limiting factor for plant growth in many terrestrial ecosystems,and addressing soil P limitation is key to achieving sustainable development of subtropical plantation forests.Pinus massoniana is an important pioneer afforestation species in the subtropics,but long-term single plantations are prone to soil degradation.Therefore,the selection of suitable tree species allocation patterns for the transformation of Pinus massoniana plantations,so as to improve the effectiveness of soil P and promote soil nutrient cycling,has received the common attention of many scholars.This study was conducted in the southern subtropical region,which has a wide area of plantation forest and a variety of stand types.A mixed plantation（MP）was formed by introducing a broadleaf tree（Castanopsis hystrix）into a pure plantation of Pinus massoniana,and a multiple species mixed plantation（MMP）was formed by introducing a variety of broadleaf trees（Castanopsis hystrix Erythrophleum fordii,Michelia hedyosperma and Quercus griffithii）.A combination of field monitoring and indoor experiments was used to investigate the 0-10 cm and 10-20 cm soil layers of Pinus massoniana pure plantation（PP）under different transformation models:（i）Characteristics of changes in plant properties and soil physicochemical properties;（ii）Characteristics of changes in soil microbial biomass,microbial diversity and microbial community structure and their main influencing factors;（iii）Characteristics of changes in the abundance of functional genes related to soil nitrogen（N）and P cycles and soil enzyme activities;（iv）Distribution characteristics of soil P fractions and the main regulators of soil P transformation.The key influencing factors and main regulatory pathways of soil P transformation were revealed from the linkage between plant-soil-soil microorganism.This will provide a scientific basis for effectively promoting P conversion in Pinus massoniana plantations and a sustainable forest management model to improve P effectiveness.The main findings are as follows:（ⅰ）Fourteen years after the introduction of mixed transformation of broad-leaved species into the Pinus massoniana plantations,the carbon/nitrogen ratio of litterfall(C/N_litter)and carbon/nitrogen ratio of root(C/N_root)of both MP and MMP decreased to different degrees compared to PP.MP and MMP changed to varying degrees in the basic physicochemical properties of the soil in both the0-10 cm and 10-20 cm soil layers.In particular,soil organic carbon（SOC）,nitrate nitrogen（NH₄⁺-N）and available phosphorus（AP）were significantly increased in the 0-10 cm soil layer in both MP and MMP.Total nitrogen（TN）,total phosphorus（TP）and AP were significantly higher in MP than in MMP,while NH₄⁺-N,carbon/nitrogen ratio of soil(C/N_soil),carbon/phosphorus ratio of soil(C/P_soil)and p H were significantly lower than in MMP.Significant increases in SOC,TN and AP and significant decreases in soil bulk density（BD）in both MP and MMP compared to PP in the 10-20 cm soil layer;TP and AP were significantly higher in MP than in MMP,while NH₄⁺-N,C/P_soil and nitrogen/phosphorus ratio of soil(N/P_soil)were significantly lower than in MMP.（ⅱ）Soil microbial biomass carbon（MBC）,microbial biomass nitrogen（MBN）and microbial biomass phosphorus（MBP）were significantly higher in MP and MMP than in PP.Significantly higher diversity indices（Feature,ACE,Chao1）and significantly higher copy numbers of both 16Sr RNA and ITS genes in the soil bacterial and fungal communities of MP and MMP relative to PP.Principal coordinate analysis（PCo A）showed highly significant changes in the structure of the bacterial and fungal communities of both MP and MMP relative to PP,and the results of linear discriminant analysis（LEf SE）showed that the biomarkers of soil bacterial and fungal communities of PP and MP and MMP differed significantly（LDA>4.0）.Redundancy analysis（RDA）further showed that in the 0-10 cm soil layer,NH₄⁺-N and C/N_root were the main environmental factors affecting changes in bacterial community structure,while the main environmental factors affecting changes in fungal community structure were C/N_root and AP.SOC is the main environmental factor affecting changes in bacterial community structure in the 10-20 cm soil layer.（ⅲ）Functional genes and hydrolase activities associated with the N and P cycles were increased to different degrees in both MP and MMP compared to PP.The denitrification gene（nir S）,the organic phosphorus mineralization gene（bpp）,and the inorganic phosphorus activation gene（pqq C）showed significant increases.Similarly,the activities of leucine aminopeptidase（LAP）and acid phosphatase（ACP）were also significantly elevated.Pearson correlation analysis showed that there was a significant positive correlation between the abundance of functional genes and enzyme activity related to soil N and P cycling.（ⅳ）Both MP and MMP had significantly and more easily decomposable phosphorus（E-P）and moderately decomposable phosphorus（M-P）than PP,while MP had significantly more E-P and M-P than MMP.The hard decomposable phosphorus（H-P）was significantly higher in MP than in PP,but there was no significant difference between MMP and PP.In addition,the soil phosphorus activation coefficients（PAC）were significantly higher for MP and MMP.Redundancy analysis（RDA）showed that the main environmental factors affecting the phosphorus fraction of the soil（0-10 cm）were AP and MBC,while in the 10-20 cm soil layer was AP,TP and MBC.In the 0-10 cm soil layer,the soil phosphorus fraction was significantly correlated with the bacterial community（Chloroflexi,Bacteroidota）and the fungal community（Kickxellomycota,Mucoromycota）.In the 10-20 cm soil layer,the soil phosphorus fraction was significantly correlated with the bacterial community（Gemmatimonadota,Firmicutes）and the fungal community（Mucoromycota）.The research results indicate that 14 years after the introduction of mixed transformation of broad-leaved species into the Pinus massoniana plantations,the quality of litter has been improved,and soil nutrients and their effectiveness have been increased.The soil microbial biomass and diversity index have enhanced,and the microbial community structure has changed,accelerating the soil N cycle and P transformation process.Moreover,soil P components may transform from H-P to E-P,and the interaction between plant,soil,and soil microbial properties is the most important driving factor for soil P transformation.The introduction of broad-leaved tree species alters plant and soil properties,as well as microbial diversity and biomass,further affecting functional genes and enzyme activities related to N and P cycling of microbial communities,ultimately affecting soil P transformation and improving its effectiveness.