No abstract

No abstract

High productivity and waterlogged conditions make many freshwater wetlands significant carbon sinks. Most wetland carbon studies focus on boreal peatlands, however, with less attention paid to other climates and to the effects of hydrogeomorphic settings and the importance of wetland vegetation communities on carbon sequestration. This study compares six temperate wetland communities in Ohio that belong to two distinct hydrogeomorphic types: an isolated depressional wetland site connected to the groundwater table, and a riverine flow-through wetland site that receives water from an agricultural watershed. Three cores were extracted in each community and analyzed for total carbon content to determine the soil carbon pool. Sequestration rates were determined by radiometric dating with 137 Cs and 210 Pb on a set of composite cores extracted in each of the six communities. Cores were also extracted in uplands adjacent to the wetlands at each site. Wetland communities had accretion rates ranging from 3.0 to 6.2 mm yr À1 . The depressional wetland sites had higher (P < 0.001) organic content (146 ± 4.2 gC kg À1 ) and lower (P < 0.001) bulk density (0.55 ± 0.01 Mg m À3 ) than the riverine ones (50.1 ± 6.9 gC kg À1 and 0.74 ± 0.06 Mg m À3 ). The soil carbon was 98-99% organic in the isolated depressional wetland communities and 85-98% organic in the riverine ones. The depressional wetland communities sequestered 317 ± 93 gC m À2 yr À1 , more (P < 0.01) than the riverine communities that sequestered 140 ± 16 gC m À2 yr À1 . The highest sequestration rate was found in the Quercus palustris forested wetland community (473 gC m À2 yr À1 ), while the wetland community dominated by water lotus (Nelumbo lutea) was the most efficient of the riverine communities, sequestering 160 gC m À2 yr À1 . These differences in sequestration suggest the importance of addressing wetland types and communities in more detail when assessing the role of wetlands as carbon sequestering systems in global carbon budgets.

This paper summarizes the importance of climate on tropical wetlands. Regional hydrology and carbon dynamics in many of these wetlands could shift with dramatic changes in these major carbon storages if the inter-tropical convergence zone (ITCZ) were to change in its annual patterns. The importance of seasonal pulsing hydrology on many tropical wetlands, which can be caused by watershed activities, orographic features, or monsoonal pulses from the ITCZ, is illustrated by both annual and 30-year patterns of hydrology in the Okavango Delta in southern Africa. Current studies on carbon biogeochemistry in Central America are attempting to determine the rates of carbon sequestration in tropical wetlands compared to temperate wetlands and the effects of hydrologic conditions on methane generation in these wetlands. Using the same field and lab techniques, we estimated that a humid tropical wetland in Costa Rica accumulated 255 g C m -2 year -1 in the past 42 years, 80% more than a similar temperate wetland in Ohio that accumulated 142 g C m -2 year -1 over the same period. Methane emissions averaged 1,080 mg-C m -2 day -1 in a seasonally pulsed wetland in western Costa Rica, a rate higher than methane emission rates measured over the same period from humid tropic wetlands in eastern Costa Rica (120-278 mg-C m -2 day -1 ). Tropical wetlands are often tuned to seasonal pulses of water caused by the seasonal movement of the ITCZ and are the most likely to be have higher fire frequency and changed methane emissions and carbon oxidation if the ITCZ were to change even slightly.

The succession of vegetation, soil development, water quality changes, and carbon and nitrogen dynamics are summarized in this article for a pair of 1-hectare flow-through-created riverine wetlands for their first 15 years. Wetland plant richness increased from 13 originally planted species to 116 species overall after 15 years, with most of the increase occurring in the first 5 years. The planted wetland had a higher plant community diversity index for 15 years, whereas the unplanted wetland was more productive. Wetland soils turned hydric within a few years; soil organic carbon doubled in 10 years and almost tripled in 15 years. Nutrient removal was similar in the two wetlands in most years, with a trend of decreased removal over 15 years for phosphorus. Denitrification accounted for a small percentage of the nitrogen reduction in the wetlands. The wetlands were effective carbon sinks with retention rates of 1800-2700 kilograms of carbon per hectare per year, higher than in comparable reference wetlands and more commonly studied boreal peatlands. Methane emission rates are low enough to create little concern that the wetlands are net sources of climate change radiative forcing. Planting appears to have influenced carbon accumulation, methane emissions, and macrophyte community diversity.