Carbon and Nutrient (N, P) Cycling of Created and Restored Wetlands

Abstract

Wetlands are created or restored for a number of purposes, including flood control, water purification, sediment and nutrient retention, and biodiversity. Restoration of wetlands is the process of recreating former conditions at a site that once contained a wetland. Restoration and creation of wetlands involve the creation or restoration of hydrology, but may also include activities that alter soil composition, manipulate vegetative communities, or promote target wetland functions. Carbon accumulation in restored and created wetlands is dependent on hydrology, vascular vegetation, and microbial communities. Once hydrology is restored, consistent periods of soil inundation reduce rates of microbial decomposition by creating anoxic soils. Wetlands prior to restoration and terrestrial soils prior to development of wetland hydrology are usually carbon poor, and created wetlands often retain soil properties from the soils from which they are created, potentially taking tens of decades to build up carbon reserves similar to natural wetlands. Initial nitrogen pools reflect past land use. Agricultural sites are increasingly being converted into wetlands, resulting in many restored or created wetlands with large initial nitrogen pools due to legacy effects of fertilizer use. Since nitrogen removal via denitrification is often desirable in restored and created wetlands, increasing the duration of inundation in restored or created wetlands may be used to encourage denitrification, but microbial community biomass often is still limited by available organic carbon. Unlike carbon and nitrogen, phosphorus cycling in wetlands is primarily controlled by abiotic processes. Increasing water residence time and depth have been shown to increase soil phosphorus sorption. However, in restored or constructed wetland phosphorus release may occur in sites that initially contain elevated phosphorous levels in the soil, such as sites with agricultural legacies. Overall, the retention and recycling of carbon, nitrogen, and phosphorus are controlled by a variety of biotic and abiotic processes, themselves influenced by ambient oxygen and for restored wetlands initial nutrient concentrations that become less important over time.