Effects Of Short-term Nitrogen Application On Soil Ecological Stoichiometric In Pinus Taiwanesis Forest And Its Microbial Mechanisms

Soil microorganisms play an important role in nutrient cycling,fertility maintenance and carbon sequestration in terrestrial ecosystems.Soil nutrient is the basis of maintaining vegetation growth,which soil ecological stoichiometry method is a good method to measure the status of soil nutrients,and the quantification of soil nutrients limitation.However,subtropical forest soil is "rich in nitrogen and low of phosphorus",with the nitrogen deposition increasingly aggravating due to economic growth and urbanization,which the soil nutrient limitation is becoming more and more serious.In this context,how does nitrogen deposition affect soil nutrient stoichiometry in subtropical forests? How does soil microbial community structure respond to nitrogen deposition and nutrient limitation? What are the biomarkers of soil microbial communities that respond to nitrogen deposition,and what are the response mechanisms? It is still unclear.Taking the Pinus Taiwanensis forest in Daiyun Mountain Nature Reserve in Fujian Province as the research object,this study carried out nitrogen application experiment with different gradient to simulate nitrogen deposition.During the experiment,samples were taken once a year for 3 years,in order to explore the effect of nitrogen application induced by soil ecological stoichiometric imbalance on microorganisms of P.Taiwanensis forest in Dayyun Mountain.This paper attempts to explain the microbial mechanism of maintaining ecological stoichiometric balance in P.Taiwanensis forest under the background of nitrogen deposition,so as to provide scientific basis for improving soil nutrient utilization.(1)The results showed that the effects of nitrogen application on soil properties,microbial biomass,enzyme activity and nutrient limitation were as follows: nitrogen application led to the increase of TP and AN,and changed the soil stoichiometric ratio,indicating that nitrogen application significantly affected the soil nutrient status.Nitrogen application significantly altered the microbial biomass carbon,nitrogen and phosphorus,and changed the value of the stoichiometry ratio MBN:MBP,indicating that nitrogen application caused the change of soil microbial biomass,and stimulated the demand for phosphorus by microorganisms and vegetation.Nitrogen application resulted in the increase of BG enzyme and Ac P enzyme,the significant change of enzyme stoichiometry Ln C:P,Ln N:P and the significant increase of vector Angle,demonstrating that nitrogen application caused the phosphorus limitation of soil microbial metabolism in Daiyun Mountain area.The homeostasis model showed that soil microorganisms did not have homeostasis characteristics,representing that soil microorganisms may not follow ecological stoichiometry theory,but microbial nutrient mining theory or resource allocation theory.The effect of nitrogen application on soil microbial TERc:x implied that the microbial community mainly shifted from carbon utilization efficiency to nutrient utilization efficiency,which alleviated the phosphorus limitation of microbial metabolism.(2)The effects of nitrogen application on soil microbial community structure are as follows: In the third year of nitrogen application,soil bacterial diversity Shannon descreased.The observed species and PD whole tree index of soil bacteria increased significantly,and the observed species,Shannon and Simpson index of soil fungal increased significantly.In the third year of nitrogen application,Myxococcota decreased significantly due to high nitrogen application.The results exhibited that soil microbial αdiversity was significantly affected by nitrogen application,while soil Fungal Divers.was significantly affected by available nutrients.Low nitrogen significantly increased Chloroflexi,Planctomycetota in soil B horizon,while high nitrogen significantly increased WPS-2 in soil B horizon.Nitrogen addition also significantly changed the composition of soil fungal community,leading to Basidiomycota,Mucoromycota decreased significantly,while Ascomycota increased significantly.Redundancy analysis(RDA)showed that the changes of soil bacterial community were mainly correlated with soil properties TC and AP,and were also correlated with microbial biomass C and P with nitrogen addition.However,the variation of fungal community was mainly regulated by increased nitrogen availability rather than soil acidification,and there was a close relationship between fungi and microbial biomass C/N and microbial biomass C/P.The structural equation model indicated that the composition of soil bacterial community was significantly regulated by soil availability factors after nitrogen application,and the composition of soil fungal community was indirectly affected by microbial biomass factors.(3)LEfSe analysis showed that the LDA threshold of bacteria was 4.5,indicating a total of 8 bacterial evolutionary clades with significant differences,while the LDA threshold of fungi was 5,displaying a total of 11 bacterial evolutionary clades with significant differences.Chloroflexi and Trimorphomycetaceae belonging to Tremellales family in the class Tremellomycetes are biomarkers(differential species)that cause structural changes in microbial communities and may be key microbial groups that cause microbial phosphorus limitation.PICRUSt2 function prediction found that nitrogen application shifted Ac P pathway,which also supported the possible mechanism of soil bacteria secreting phosphatase to promote soil organophosphorus decomposition to alleviate phosphorus limitation.FUNGuild predicted that Symbiotroph,Saprotroph and their interaction were the main nutritional types of fungi,accounting for 71.74% of the total nutritional types.Ectomycorrhizal fungi and their interaction types accounted for43.48% of the total,and were the main groups(Guild).The results manifested that nitrogen application changed soil nutrient stoichiometry and microbial ecological stoichiometry,and exacerbated the phosphorus limitation of microorganisms,and soil microorganisms adapted to phosphorus deficiency by changing microbial enzyme activity and community composition.The results are helpful to clarify the microbial theory of soil nutrient imbalance in subtropical forests under nitrogen deposition,provide a basis for further exploring the impact of nitrogen deposition on nutrient stoichiometry and imbalance in subtropical forests in the future,and provide a reference for the development of management measures in subtropical forests.