The role of soil and vegetation processes in the control of organic and mineral fluxes in some western European salt marshes

Comparative studies of salt marsh productivity, the pool of nutrients in ma rsh soils, and the associated fluxes of nutrients, sediments and organic ma tter have been made in the UK, The Netherlands, Portugal and France. These have shown contrasting relationships between the functional maturity of a m arsh and the direction and magnitude of associated fluxes. Actively-extendi ng marshes tend to be flood-dominated systems and net importers of sediment and organic matter, while exporting mineral nutrients. Marshes that are no t extending, however, are usually ebb-dominated, net exporters of organic m atter and even sediment. These latter marshes can best be regarded as 'matu re' or, if they are showing signs of decay, as 'over-mature', in contrast t o 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 con centrations are themselves partly controlled by the origin and amount of se dimentation on the marsh surface. In contrast, soil inorganic nitrogen show ed a strong seasonal trend, with minimum concentrations, related to plant u ptake, occurring in early summer. The breakdown of organic matter, both above and below the soil surface, app ears 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 m icrobial activity, immobilization of mineral nitrogen and the slow breakdow n of plant litter would seem to characterize degenerating 'over-mature' mar shes. Despite these factors, there was little or no difference in the Net A erial Primary Productivity (300-800 g m(-2) y(-1)) measured at two marshes, one of each type, in the UK.