To　better　understand　how　seasonal　change　and　the　presence　of　plants　affect　biogeochemical　iron　(Fe)　cycling　in　tidal　marsh　sediments,　we　examined　the　seasonal　dynamics　of　microbial　Fe　reduction　(FeR)　and　related　Fe　speciation　from　2012　to　2014　using　in　situ　vegetated　and　unvegetated　mesocosms　created　with　macrobenthos-proof　enclosures.　The　pools　of　Fe(III)　oxides,　Fe　sulfides,　porewater　Fe2+,　and　rates　of　microbial　sulfate　reduction　(SR)　peaked　in　summer　(hot　and　wet),　whereas　rates　of　FeR　and　non-sulfidic　Fe(II)　levels　peaked　in　winter　(mild　and　dry).　Sedge　presence　greatly　increased　the　FeR　rate,　organic　matter　pools,　and　Fe(III)　oxide　levels,　but　decreased　carbon:nitrogen　ratios　and　sulfide　abundances.　FeR　rates　were　mainly　affected　by　the　SR　level,　and　temperature　and　plant　primary　productivity　had　comparable　effects　on　SR　rates.　The　dominant　organic　carbon　mineralization　pathway　changed　from　SR　in　summer　to　FeR　in　winter　in　both　vegetated　and　unvegetated　mesocosms.　FeR　was　more　important　in　the　vegetated　mesocosms　than　in　the　unvegetated　mesocosms,　and　seasonal　change　contributed　more　to　the　total　variability　of　each　anaerobic　pathway　than　the　presence　of　sedge.　Our　study　reveals　that　temperature　and　plants　both　mediate　variations　in　the　organic　carbon　mineralization　pathways　of　a　subtropical　estuarine　tidal　marsh.　©　The　authors　2017.