The fate of carbon resulting from pore water exchange in a mangrove and Spartina alterniflora ecozone

Mangrove and salt-marsh wetlands are important coastal carbon sinks.In order to quantify carbon export via pore water exchange and to evaluate subsequent fate of the exported carbon,we carried out continuous observations in a mangrove-Spartina alterniflora ecozone in the Zhangjiang River Estuary,China.The carbon fluxes via pore water exchange were estimated using 222Rn and 228Ra as tracers to be(2.15±0.63)mol/(m2∙d)for dissolved inorganic carbon(DIC)and(-0.008±0.07)mol/(m2∙d)for dissolved organic carbon(DOC)in the wet season and(3.02±0.65)mol/(m2∙d)for DIC and(-0.15±0.007)mol/(m2∙d)for DOC in the dry season in the mangrove-dominated creek(M-creek),while(2.52±0.82)mol/(m2∙d)for DIC and(0.02±0.09)mol/(m2∙d)for DOC in the dry season in the S.alterniflora-dominated creek(SA-creek).The negative value means that pore water was a sink of DOC in the creek.The total carbon via pore water exchange in the tidal creeks in the mangroves accounted for 41%-55%of the net carbon fixed by mangrove vegetation and was 3-4 times as much as the soil carbon accretion in the mangroves.The exported carbon in the form of DIC contributed all of the carbon outwelling from the M-creek and 79%of the carbon outwelling from the SA-creek,implying effective fixation of carbon by the wetland ecosystem.Moreover,it resulted in 54%in the dry season,75%in the wet season of the carbon dioxide released from the M-creek to the atmosphere,and 84%of the release from the SA-creek.Therefore,quantification of pore water exchange and related soil carbon loss is essential to trace the fate of carbon fixed in intertidal wetlands.