The Role of Soil and Vegetation Processes in the Control of Organic and Mineral Fluxes in Some Western European Salt Marshes
Keywords:Salt marshes, nutrient fluxes, soils, sediments, primary productivity, ecosystem function.
Comparative studies of salt marsh productivity, the pool of nutrients in marsh soils, and the associated fluxes of nutrients, sediments and organic matter have been made in the UK, The Netherlands, Portugal and France. These have shown contrasting relationships between the functional maturity of a marsh and the direction and magnitude of associated fluxes. Actively-extending marshes tend to be flood-dominated systems and net importers of sediment and organic matter, while exporting mineral nutrients. Marshes that are not extending, however, are usually ebb-dominated, net exporters of organic matter and even sediment. These latter marshes can best be regarded as 'mature' or, if they are showing signs of decay, as 'over-mature', in contrast to the 'immature' marshes that are still growing.
Soil available phosphorus (PO4-P) concentrations measured ranged from 15 to 200 mg/kg, and influenced the primary productivity of the marsh. PO4-P concentrations are themselves partly controlled by the origin and amount of sedimentation on the marsh surface. In contrast, soil inorganic nitrogen showed a strong seasonal trend, with minimum concentrations, related to plant uptake, occurring in early summer.The breakdown of organic matter, both above and below the soil surface, appears more rapid in a healthy mature marsh than in a degenerating marsh, and physical processes in the creeks are more pronounced. Low levels of soil microbial activity, immobilization of mineral nitrogen and the slow breakdown of plant litter would seem to characterize degenerating 'over-mature' marshes. Despite these factors, there was little or no difference in the Net Aerial Primary Productivity (300-800 g m-2 y-1) measured at two marshes, one of each type, in the UK.