Wetland ecosystems are significant carbon sinks. Their high productivity and presence of water gives them the ability to efficiently sequester carbon in the soil, serving as a potential tool to mitigate the net greenhouse effect of carbon emissions to the atmosphere and abate climate change. We explored the efficiency of freshwater wetlands sequestering carbon under different climates, wetland types, and vegetation communities, in order to assess the conditions that favor carbon accumulation. We also studied the ability of created freshwater wetlands to sequester carbon and the effect of their vegetation communities on this task.
We found significant differences on carbon sequestration between wetland types in temperate and tropical regions, being consistently higher in the studied forested wetlands (260 ± 58 gC m-2 y-1) than the riverine ones (113 ± 27 gC m-2 y-1), indicating the importance of wetland productivity and the type of organic matter entering the system. Our temperate wetlands were also consistently more efficient in sequestering carbon than similar tropical ones (233 ± 89 and 151 ± 57 gC m-2 y-1, respectively), suggesting that higher temperatures in tropical climates may hinder carbon sequestration by intensifying organic matter decomposition rates. Within the tropical climates, we found that the tropical humid wetland sites had
significantly higher carbon sequestration rates (306 ± 77 gC m-2 y-1) than those located in the tropical dry regions, were there is a marked wet and dry season (63 ± 10 gC m-2 y-1 on average).
Our comparison between vegetation communities show that wetland productivity and permanent anaerobic conditions are key in enhancing soil carbon sequestration, being 214 ± 54 gC m-2 y-1 in the open water sites (with prolonged anaerobic conditions) and 184 ± 72 gC m-2 y-1 in the edges (typically more productive due to their fluctuating water levels). In the tropics, where temperature might to be a limiting factor for carbon sequestration in wetland soils, the presence of water seems to be of critical importance to enhance carbon sequestration rate and thus carbon sequestration rates were highest in the tropical permanently flooded wetland sites (306 ± 77 gC m-2 y-1). In temperate regions, where temperature is unlikely to limit carbon sequestration in wetland soils, permanent anaerobic conditions seem to be the most favorable condition for carbon accumulation as well. However, when organic matter inputs are high or recalcitrant, their role in favoring soil carbon sequestration is as important or more than the permanent presence of water (carbon sequestration rates in the temperate forested sites were, on average, 317 ± 93 gC m-2 y-1, while in the permanently flooded riverine ones were 160 gC m-2 y-1 in the natural wetland and 267 ± 21 gC m-2 y-1 in the created ones).
In the temperate created riverine wetlands carbon sequestration was strongly correlated with aboveground productivity. Our two similar created wetlands in central Ohio differed only in their initial planting (one was planted and the other one was colonized naturally), but otherwise have had the same hydrologic features and nutrient loadings. After 15 years, the originally planted wetland had lower carbon sequestration rate (219 ± 15 gC m-2 y-1) than the naturally colonized one (267 ± 17 gC m-2 y-1), which had also higher aboveground net primary productivity. Soil carbon sequestration had no correlation with the nitrogen concentrations in the water, suggesting that nutrient rich waters do not necessarily favor carbon accumulation in the soil directly, even though they enhance biomass productivity and thus soil carbon inputs. On average, these temperate created wetlands sequestered 243 ± 24 gC m-2 y-1 after 15 years since creation, 26 % more than the rate after 10 years since creation (190 ± 7 gC m-2 y-1) and 55 % more than the similar natural wetland in the same region (140 ± 16 gC m-2 y-1), implying that once created wetlands are fully functional and structured they can successfully sequester carbon, especially in their early years.