Nitrogen and phosphorus dynamics in tidal freshwater marshes.

摘要

Nutrient dynamics in tidal freshwater marshes along the Chickahominy River in Virginia were investigated, with special emphasis on exchanges of dissolved inorganic species of nitrogen and phosphorus in surface and subsurface water flow. An initial survey documented that the distribution of organic carbon in plants and soils was closely correlated with nitrogen but not phosphorus, suggesting that P diagenesis in these soils was separate from C and N. Further, soil porewater concentrations of phosphate were higher along creekbanks than in marsh interiors. Experiments demonstrated that iron-phosphorus dynamics contributed to nutrient retention in high marsh creekbanks, where advection of nutrient-rich porewater to oxidized surface water occurs. As anoxic water oxidizes in creekbanks, iron oxyhydroxides precipitate, adsorbing phosphate from solution. This mechanism accounts for the build-up of dissolved P in regions which are sequentially oxidized and reduced, and explains the presence of higher total phosphorus concentrations in creekbank soils.; Nutrient exchange between a low marsh and flooding tidal water was examined using tidally influenced experimental chambers. There was no obvious retention of ammonium or phosphate in a decomposing layer of plant litter on the marsh surface. Processes proposed to be influential on observed net dissolved nutrient exchange were diffusion, microbial activity and geochemical sorption, modeled using general physical measurements of tidal height, temperature and nutrient concentration of flooding water. In comparison with salt marsh soils, more phosphate, but less ammonium was released from the tidal freshwater marsh soils into the flooding water column.; Information on water and nutrient movement among different components of the tidal freshwater marsh complex was used to construct a conceptual model of nutrient dynamics in vegetated marsh soils. Mobilization of nutrients into aboveground plant tissue was the most important method of nutrient removal from the soil. The magnitude of subsurface nutrient exchange due to drainage and evapotranspiration was roughly equivalent to surficial exchange with flooding tidal water. Dissolved nutrient pools in soil porewater, derived from in situ mineralization of organic matter, must turn over rapidly to support seasonal growth of the wetland plant Peltandra virginica.