Soil Organic Carbon Mineralization In Root Zone Of Tidal Marshes And Its Response To Sea-Level Rise In The Min River Estuary

Organic carbon mineralization is an important part of soil carbon cycle in tidal wetlands.Wetland plants influence soil organic carbon mineralization rate and main pathways(microbial Fe(Ⅲ)reduction,sulfate reduction and methanogenesis)through their root activities(root oxygen loss and root exudates).Phragmites.australis and Cyperus.malaccensis are main indigenous species,and Spartina alterniflora is invasive species in tidal marsh of the Min River estuary in the East China Sea.To assess root activity on soil organic carbon(SOC)mineralization rates and pathways,in-situ rhizoboxes and field sampling across plant growth stages(seedling,shooting,flowering,and maturity)of S.alterniflora and P.australis were conducted in the Shanyutan of the Min River estuary,southeast China.The spatial change pattern in-situ rhizoboxes and temporal change pattern across plant growth stages of SOC mineralization rate and pathway were studied from the perspective of electron acceptor,organic matter substrate and microorganism associated with SOC mineralization.In addition,to study the rates and pathways of SOC mineralization in rhizosphere soil of tidal marsh in response to sealevel rise in future,we established mesocosms loaded with an indigenous plant(C.malaccensis)from a Tajiaozhou tidal freshwater wetland and subjected to three salinity treatments(fresh control,oligohaline,mesohaline)plus three flooding(i.e.,flooding water-level height)treatments.(1)From the spatial distance of the root zone soil of S.alterniflora and P.australis,the concentrations of DOC and Fe(Ⅲ),abundance of iron reducing bacteria,microbial Fe(Ⅲ)reduction rate and SOC mineralization rate in the soil layer showed significant changes,as follows: rhizosphere soil > inner bulk soil(0–5 mm)> outer bulk soil(5–100mm),but the abundances of total bacteria,iron-reducing bacteria(Geobacter),sulfatereducing bacteria(dsr A)and methanogen(mcr A)in outer bulk soil were lower than those in inner bulk soil and rhizosphere soil.Root activity significantly increased the rates of SOC mineralization(an increase of ca.0.8 times),microbial Fe(Ⅲ)reduction(an increase of ca.5 times),sulfate reduction(an increase of ca.0.5 times)and methanogenesis(an increase of ca.0.7 times)from rhizosphere soil to bulk soil,but root activity showed a greater preference for microbial Fe(Ⅲ)reduction.Moreover,root activity shifted the dominated mineralization pathway from sulfate reduction in bulk soil to microbial Fe(Ⅲ)reduction and sulfate reduction in rhizosphere soil.The shift in the terminal metabolic pathways was associated with the bacterial community composition across the rhizoboxes.The relative contribution of methanogenesis in both rhizosphere soil and bulk soil were negligible.Plant species in this study did not affect the partitioning of terminal metabolic pathways,but did influence the total SOC mineralization rates as well as the total bacterial abundance.Although the root activity and that of plant species both influence the bacterial community,our study shows that plant species had a less pronounced impact on the bacterial community when compared with that of the rhizosphere effect.(2)Across different growth stages of S.alterniflora and P.australis,based on changes in root activity,the rates of SOC mineralization and microbial Fe(Ⅲ)reduction in rhizosphere soil increased from seedling stage to flowering stage,but decreased at maturity stage.The relative contribution of microbial Fe(Ⅲ)reduction to SOC mineralization in rhizosphere soil increased from seedling stage to flowering stage(from21% to 41%),and decreased to 32% at maturity stage.Based on changes in soil temperature across growth stages,the rates of SOC mineralization and sulfate reduction in bulk soil increased from seedling stage to flowering stage,but decreased at maturity stage.The relative contribution of sulfate reduction to SOC mineralization in bulk soil increased from seedling stage to flowering stage(from 50% to 63%),and decreased to56% at maturity stage.Sulfate reduction is the pathway of SOC mineralization to compete with microbial Fe(Ⅲ)reduction,and its change across growth stages in rhizosphere and bulk soils was opposite to that of microbial Fe(Ⅲ)reduction.Root activity shifted the dominated pathway of SOC mineralization in rhizosphere soil from sulfate reduction during other growth stages to microbial Fe(Ⅲ)reduction and sulfate reduction during flowering stages.In bulk soil,the pathway of SOC mineralization was dominated by sulfate reduction in different growth stages.The relative contribution of methanogenesis in both rhizosphere soil and bulk soil were negligible.Plant species did not affect the partitioning of terminal metabolic pathways across different growth stages,but did influence the total SOC mineralization rates as well as the total bacterial abundance in rhizosphere soil.In conclusion,the changes of SOC mineralization rate and pathways during plant growth stages were jointly controlled by plant root activity and environmental factors(soil temperature).(3)From the interactive effects of increased salinity and flooding level induced by stimulated sea-level rise,increased salinity had a greater effect on rate and pathways of SOC mineralization compared to increased flooding.The oligohaline treatment elevated root Fe(Ⅲ)plaque abundance,C-degrading enzyme activities,microbial Fe(Ⅲ)reduction and SOC mineralization rates relative to fresh control,while the mesohaline treatment suppressed them.The dominated pathways of SOC mineralization were microbial Fe(Ⅲ)reduction under oligohaline treatment and sulfate reduction under mesohaline treatment,but the relative contribution of methanogenesis under oligohaline and mesohaline treatments were negligible.Under the effects of salinity and flooding,root Fe(Ⅲ)plaque abundance could be the main factor affecting the microbial Fe(Ⅲ)reduction rate and Cdegrading enzyme activities.And salinity and flooding affect SOC mineralization in rhizosphere soil by mediating C-degrading enzyme activities and microbial Fe(Ⅲ)reduction rate.Altogether,under future sea-level rise,root Fe(Ⅲ)plaque abundance could be as an indicator of SOC mineralization rates in tidal freshwater wetland soils in response to salinity and flooding.