Effects of Nitrogen Loading on Wetland Ecosystems with Particular Reference to Atmospheric Deposition

Wetlands fulfill an important role in global biogeochemical cycles. For example, they transfer to the atmosphere globally significant quantities of CH4 (60, 61) and reduced sulfur gases (127). Coastal marshes may function as net sinks for N20 (46). Because of the anaerobic nature of their soils, wetlands act as sinks for organic carbon. It has been estimated that wetlands once sequestered a net of 57 to 83 x 106 metric tons of carbon per year worldwide, but recent widespread drainage of wetland soils has shifted the carbon balance (7). Although this rate of carbon uptake is small in comparison to other global carbon fluxes, such as the annual release of carbon from combustion of fossil fuel (5-6 x 109 metric tons yr1) (112) or the net uptake of C02-C by the ocean (1.6 x 109 metric tons yr-1) (129), it is important when the net balance between large fluxes is considered, and it is certainly important over geologic time scales. Locally, wetlands function as habitats for wildlife, as flood control systems, as stabilizers and sinks for sediments, as storage reservoirs for water, and as biological filters that maintain water quality. For example, riparian