Effects Of Pinus Massoniana/Erythrophleum Fordii Uneven-aged Mixied Transformation On Rhizosphere Nad Non-rhizosphere Soils Phosphorus Fractions And Its Potential Regulatory Mechanisms

Phosphorus（P）is not only one of the most essential elements that involved in the synthesis and metabolism of important compounds in plants,but also a core part of the soil nutrient cycle.The cycles of P and nitrogen（N）are closely related due to they are living elements which driving the transformation and turnover of other nutrients.However,P limitation in degraded red soil is a serious challenge for sustainable management of subtropical plantation forests in China at present.The rhizosphere soil is under the direct of root exudates,nutrient concentration gradients,and the biological activity of soil fauna and microbiota that are interacting with the plant roots.In turn,the metabolic activities of rhizosphere soil organisms affect plant physiology,health and productivity.Therefore,the availability of rhizosphere soil P has been a common concern of many scholars.This study is based on how to regulate the soil P transformation process in a targeted manner,activate more insoluble P,and improve the availability of soil P.The outcomes of this study aim to provide the scientific knowledge on long-term and sustained promotion of net primary productivity of plantations,improve species diversity and maintain the stability of ecosystem functions.The study area was selected from the southern subtropical region,which has a wide plantation area,a long plantation history and a series of diverse forest stand types,and the soil in this area has two characteristics which are highly weathered and P deficiency.By combining field investigations and laboratory analysis,using molecular biological and physiological and biochemical technology on rhizosphere and non-rhizosphere soils which introducting of precious native N₂-fixing tree specie（Erythrophleum fordii Oliv.）into degraded artificial coniferous tree species（Pinus massoniana Lamb.）（PP）to form needle-broad mixed-age plantations（MP）,the objectives of the study are to determine and elucidate（1）the variation characteristics of understory vegetation and soil physicochemical properties,（2）the changes in microbial biomass,community structure and soil enzyme activity and their influencing factors,and（3）changes in P components（Include inorganic phosphorus（Pi）fractions,organic phosphorus（Po）and residual phosphorus fractions）and their microbial regulation mechanisms.By researching the above issues,we plan to reveal the main regulatory processes and key influencing factors of the coupling relationship between above-ground plants,soil and below-ground microorganisms on P transformation at the level of biochemical and microbial specific functional groups.The outcomes of this study aim to provide the scientific knowledge on the sustainable management of plantations,which directly regulates the P transformation in degraded red soil and aim to improve its effectiveness.The main results of this study are as follows:（1）After 9 years of Pinus massoniana/Erythrophleum fordii uneven-aged mixed plantations（MP）,compared with PP,the biomass of MP fine roots increased significantly by 68.24%（p<0.05）,and the C/N of litter and fine roots was respectively remarkably reduced by 54.40%and 43.18%（p<0.05）,while both N/P was appreciably increased by 122.54%and 23.77%（p<0.05）.（2）Compared with PP,the basic physicochemical properties of the rhizosphere and non-rhizosphere soils of MP have changed to different degrees.Among them,the organic carbon of soil（SOC）,ammonium nitrogen（NH₄⁺-N）,nitrate nitrogen（NO₃^--N）,available phosphorus（AP）and water content of soil（SWC）of MP rhizosphere and non-rhizosphere soils were considerably improved（p<0.05）,while the total phosphorus（TP）content was remarkably reduced（p<0.05）.The total nitrogen（TN）was considerably increased in rhizosphere soil（p<0.05）,whereas there was no significant change in non-rhizosphere soils except for p H,other soil physicochemical indicators showed that the content of rhizosphere soils was higher than that of non-rhizosphere soils.（3）After 9 years of Pinus massoniana/Erythrophleum fordii uneven-aged mixed plantations,the content and effectiveness of N and the microbial biomass（MBC,MBN and MBP）have been changed.The contents of MBC,MBN and MBP in rhizosphere and non-rhizosphere soils in MP were significantly higher than those of PP（p<0.05）.In both rhizosphere and non-rhizosphere soils,the total microbial biomass（Toal PLFAs）,bacteria（Bacteria PLFAs）,Gram-positive bacteria（G⁺PLFAs）,Gram-negative bacteria（G^-PLFAs）,fungi（Fungi PLFAs）,arbuscular mycorrhizal fungi（AMF PLFAs）,actinomyces PLFAs,and the C,N and P related hydrolases（β-glucosidase（BG）,N-acetyl-glucosidase（NAG）,leucine aminopeptidase（LAP）,acid phosphatase（ACP）and phosphodiesterase（PDE））were mostly notably increased in MP than those in PP（p<0.05）.Redundant analysis（RDA）results showed that fine root biomass（FRB）was the most significant environmental factor（p<0.05）driving microbial communities variation in rhizosphere soils,while p H,TN,NO₃^--N,and C/N were the most significant environmental factors（p<0.05）driving microbial community structure variation in non-rhizosphere soils.We analyzed the correlation among each hydrolases and theirs with microbial communities and found that there was a strong positive correlation between hydrolases associated with C,N and P transformation in rhizosphere soils.（4）The Pi fractions（Resin-P,Na HCO₃-Pi,Na OH-Pi and Na OHs-Pi）in rhizosphere soils of MP significantly increased repersentively by 13.17%,13.48%,31.38%and 36.90%（p<0.01）,while in non-rhizosphere soils of MP,of four contents of the Pi fractions,only the Resin-P and Na HCO₃-P were appreciably increased by 23.67%and 11.48%（p<0.05）,and Pi fractions in rhizosphere soils were higher than those in non-rhizosphere soils in both stands.Changes in soil Po and Residual-P fractions mainly occurred among Na OH-Po,Na OHs-Po and Reidual-P in rhizosphere soils.That was presented by the fact that the Na OHs-Po and Residual-P fraction of MP rhizosphere soils were remarkably reduced by13.58%and 28.60%（p<0.01）,respectively,while the Na OH-Po fraction was appreciably increased by 17.26%（p<0.05）.The increase in the content of Pi fractions in rhizosphere soils was associated with the decrease in Residual-P content following the increase in N content and effectiveness of the soil.The RDA results showed that the factors affecting the transformation of Pi fractions were nitrogen of litterfall（N_L）,N/P,NH₄⁺-N（p<0.05）,while the factors affecting the transformation of Po and Residual-P fractions were BG and PDE（p<0.05）;In non-rhizosphere soils,the factors affecting the transformation of Pi fractions were LAP and MBN（p<0.05）,while the factors affecting the transformation of Po and Residual-P fractions were NAG,LAP and G^-（p<0.05）.Moreover,compared with PP,the activation coefficients（PAC）of rhizosphere and non-rhizosphere soils P of MP were respectively significantly higher by 48.50%and 89.80%（p<0.05）.（5）After 9 years of Pinus massoniana/Erythrophleum fordii uneven-aged mixed plantations,the structural equation model（SEM）showed that introducing N2-fixing tree species may be an effective method for increasing SOC content,N availability,and AMF colonization,and thereby promoting P accumulation and transformation in rhizosphere and non-rhizosphere soils in Pinus massoniana plantations.In summary,introducing the N₂-fixing tree species into Pinus massoniana plantation can not only increased the content of N in the soil and its availability,but also notably improved the soil quality.Furthermore,it can remarkably change the microbial community structure,and improved the conversion rate and effectiveness of soil P.The outcomes of this study aim to provide important scientific knowledge on the construction of tree species selection,optimization mode and sustainable management measures to promote soil P transformation and improve soil N and P effectiveness in Pinus massoniana plantations of south subtropical China.