Process Mechanism Of Soil-Plant Involvement In Phosphorus Transformation To Maintain The Effectiveness Of Soil Phosphorus Under The Background Of Nitrogen Deposition In A Subtropical Castanopsis Faberi

Phosphorus is an essential nutrient elements for the growth of plants and microorganisms,It is generally accepted that phosphorus availability is highly limited in subtropical forest soils,where is rich in oxides such as iron and aluminum,for phosphorus is easily adsorbed and fixed by soil minerals.Additionally,nitrogen deposition in subtropical regions of China is as high as 40 kg N ha^-1 a^-1,which may indirectly affect soil phosphorus availability by changing soil-plant processes,is essential to reveal mechanisms that sustain and enhance ecosystem function in subtropical forest.We conducted an experiment to simulated nitrogen deposition with different gradients of nitrogen addition[control(0 kg N·hm^-2·a^-1),low N(40 kg N·hm^-2·a^-1),and high N(80 kg N·hm^-2·a^-1)]in a subtropical Castanopsis faberi forest and sampled after experiment addition.The rhizosphere and non-rhizosphere soil chemical properties（p H,soil,nutrient,exchange cation）,and soil phosphorus fractions,microbial characteristics（microbial biomass,phosphatase activity,fungal community structure）and plant fine root characteristic（nutrients,morphology,colonization rate）were measured.Aims to explore the influence of nitrogen addition on soil phosphorus fractions morphological transformation and absorption utilization in the Castanopsis faberi forest ecosystem,and reveals the potential mechanisms underlying about availability of soil phosphorus in the background of nitrogen addition,and provide a scientific basis for improving the quality of subtropical soil.The results showed that nitrogen addition significantly reduced the soil inorganic phosphorus and Enzyme-P content,but significantly increased the soil availability phosphorus,organic phosphorus,Citrate-P and HCl-P content.indicating that nitrogen addition can maintain phosphorus availability in soil by affecting the morphological transformation of soil phosphorus components.However,the phosphorus content of fine roots tends to decrease first and then increase under nitrogen addition treatment,indicating that the ability of fine roots to absorb effective soil phosphorus is also an important process affecting the availability of soil phosphorus.Nitrogen addition decreased soil p H but significantly increased the content of soil Fe³⁺、Al³⁺、Mn²⁺,This indicates that nitrogen addition treatment significantly changed siol chemical environment.However,soil microbial biomass phosphorus and acid phosphatase activity were significantly increased under the nitrogen addition treatment,indicating that the nitrogen addition increased the microbial demand for phosphorus.Nitrogen addition had no significant effect on fungal diversity and community composition（phylum level）,suggesting that soil fungal community structure is highly adaptable for short-term exogenous nitrogen inputs.However,nitrogen addition significantly affects the functional groups of soil fungi,like increasing the relative abundance of soil and wood saprophytic fungi.These results suggest that nitrogen addition enhances the functional potential of phosphorus cycling in soil fungi.However,the fine root colonization rate was significantly increased under the nitrogen addition treatment.This indicates that both plant root characteristics should be focused on and the characteristics of soil microorganisms,which can help to clarify the contribution of soil-plant in promoting soil phosphorus transformation and uptake processes in maintaining phosphorus availability.Through Variance Partitioning Analysis,it cannot be ignored that the influence of soil chemical properties,soil microbial characteristics and plant fine root characteristics on phosphorus components,and the three play different roles in the morphological transformation and absorption utilization of soil phosphorus.Redundancy analysis found that soil Fe³⁺、Al³⁺、Mn²⁺showed a significant negative correlation with soil inorganic phosphorus content,indicates that soil Fe³⁺、Al³⁺、Mn²⁺are important soil properties affecting soil inorganic phosphorus conversion under nitrogen addition.soil acidic phosphodiesterase activity and soil organic phosphorus,soil microbial biomass phosphorus and soil inorganic phosphorus showed significant positive associations,suggests that soil acidic phosphodiesterase activity and microbial biomass phosphorus are soil microbial features that affect soil organic phosphorus and inorganic phosphorus transformation.Fine root morphological characteristics such as special root area,special root length,special root density are important affecting the change of soil phosphorus component.Moreover,the results of this study showed a significant increase in fine root colonization rate and ectomycorrhizal fungi under nitrogen addition treatment.This suggests that nitrogen addition may enhance penptorhizal and mycorrhizal symbiosis to maintain plant demand for availability phosphorus,possibly acquiring phosphorus through increased soil-plant cooperative relationships,which is supported by the contribution of soil bioavailability phosphorus to soil availability phosphorus.It was shown that nitrogen addition promotes the conversion between soil organic and inorganic phosphorus,while plant fine roots reduce soil phosphorus uptake,which may be the soil-plant process for maintaining availability soil phosphorus.Soil Fe³⁺、Al³⁺、Mn²⁺,soil acid phosphodiesterase,microbial biomass phosphorus,and fine root morphology are the key soil chemical properties,microbial factors,and plant factors that affect the transformation and uptake of soil phosphorus morphology.The results of ectomycorrhizal fungi and fine root colonization rate also further support the involvement of soil-microbial-plant transformation and absorption in soil phosphorus.These results are helpful to clarify the maintenance mechanism of soil phosphorus availability in subtropical forest ecosystem under the background of nitrogen deposition,and provide parameters for further exploring the influence of nitrogen deposition on soil phosphorus cycle in subtropical forest in the future,and provide reference for the development of regional forest management measures.