Effects Of Flooding,Organic Acids And Heavy Metals Treatment On Root Exudates,Rhizosphere Bacteria And Rhizoplane Fe Plaque Of Kandelia Obovata

Plant rhizosphere is the center of soil life activities.Plant root exudates（REs）provide a continuous input of nutrients and energy to soil microbial life,which is important for maintaining the structure and function of soil microbial community.Mangrove wetland soil contains a large amount of iron（Fe）and its oxidation products.Active Fe oxides can combine with heavy metals/metal-like elements and nutrients（such as phosphorus）in the soil,affecting the migration and bioavailability of these elements.Reactive Fe oxides can also form Fe plaque on the rhizoplane of mangrove plants under the action of radial oxygen loss and root exudation,which plays an important role in regulating the absorption and transport of heavy metals and nutrients in plants.The dynamic transformation of the formation and decomposition of Fe minerals in mangrove wetland soil was particularly active in the rhizosphere of mangrove plants under the influence of plant root exudation and the activities of Fe redox cycling bacteria（Fe OB and Fe RB）.The following questions remain to be solved regarding the morphological transformation of Fe oxides in mangrove rhizosphere（including the formation and decomposition of root Fe plaque）.1)The coupling relationship between REs and mangrove rhizosphere bacterial community,especially Fe OB and Fe RB;2)Whether this coupling relationship affects the formation of Fe plaque on the rhizoplane of mangrove plants;3)What is the response of their coupling and formation of Fe plaque to heavy metal stress?Organic acids（OAs）in mangrove REs are the main carbon source of soil functional microorganisms.In this study,Kandelia obovata,a typical mangrove plant in the coastal areas of southern China,was used to simulate periodic tidal flooding with a small-scale flooding device,and REs were simulated artificially with mixed OAs treatment solution.High-throughput sequencing and high-performance liquid chromatography were used.The effects of heavy metal Cd and exogenous OAs treatment on REs,bacterial community diversity and structure in rhizoplane and rhizosphere of K.obovata,were studied in a controlled laboratory experiment.The main results are as follows:（1）The diversity and structure distribution of bacteria in the rhizoplane and rhizosphere of K.obovata showed obvious rhizosphere effect under different periods of flooding.The diversity of bacteria in the rhizosphere was higher than that in the rhizoplane,while the relative abundance of rhizoplane dominant bacteria such as Cyanobacteria and Spirochaetota was higher than that in the rhizosphere.OAs and flooding can enhance this rhizosphere effect.Exogenous addition of OAs significantly promoted the exudation of dissolved organic carbon（DOC）,NO₃^--N NH₄⁺-N,and dissolved inorganic phosphorus（DIP）in REs,and with the increase of flooding time,these REs contents increased with the increase of OAs concentration more obviously.Exogenous OAs can promote endogenous root exudation of LMWOAs（low molecular weight organic acids）.Exogenous OAs treatment ran to process the relative abundance of Fe OB,such as Gallionella,unclassified＿f＿＿Gallionellaceae and Sideroxydan to increase in the rhizoplane,but to decrease in the rhizosphere,presenting a chemotactic colonization from the rhizosphere to the rhizoplane.Citric acid and other LMWOAs could act as induction factors to promote the growth and chemotaxis of related bacteria,while flooding and exogenous OAs could promote the exudation of LMWOAs in roots.（2）Exogenous OAs significantly inhibited the formation of Fe plaque on the rhizoplane of K.obovata,while exogenous Cd and flooding significantly promoted the formation of Fe plaque.Under 10h flooding condition,the Fe plaque increases gradually with the increase of Cd concentration.The content of Fe plaque increased firstly and then decreased with with increasing Cd concentration under 5h flooding condition.The addition of Cd increased the relative abundance of Proteobacteria to about two times of that of the control group,while Cyanobacteria decreased to one third of that of the control group.Compared with the rhizosphere,the diversity and structure of rhizoplane bacteria were more responsive to Cd,which might be related to the adsorption of REs and Fe plaque on rhizoplane Cd²⁺.（3）The effects of different flooding time on root exudation and rhizosphere bacterial structure were different.This may be because flooding directly or indirectly changes the bioavailability of Cd in soil through chelation and precipitation of Cd²⁺.Cd and flooding treatment can promote the root exudation of DOC,NH₄⁺-N,NO₃^--N,DIP and LMWOAs such as oxalic acid,acetic acid and propandioic acid.Both Fe oxides and OAs can chelate with heavy metal ions,thus adsorbing and blocking the uptake and transport of Cd ions in plants.Therefore,the exudation of root LMWOAs and the formation of Fe plaque on the rhizoplane are likely to be important adaptive mechanisms for mangrove plants to cope with Cd stress.In conclusion,both exogenous OAs and Cd treatment can promote root exudations of dissolved organic matter such as DOC,NH₄⁺-N,NO₃^--N,DIP and LMWOAs under periodic flooding condition.It inhibited and promoted the formation of Fe plaque on rhizoplane,respectively.It also affected the diversity of bacterial community in the rhizoplane and rhizosphere.The rhizosphere Fe OB showed a trend of chemotactic colonization from the rhizosphere to the rhizoplane,which may be related to the induction of LMWOAs such as citric acid under the treatment of exogenous OAs.Root exudation of OAs and Fe plaque may be the adaptive mechanisms of mangrove plants to cope with Cd stress.This study can enrich the existing theoretical system of plant-microorganism interaction,and further clarify the role and significance of wetland plant root exudates in rhizosphere soil bacterial diversity,soil Fe cycling and plant heavy metal resistance.