In Europe, salt marsh ecology developed around floristics and phytosociology. In USA, research on the Atlantic and Gulf Coasts characterized salt marsh ecosystem functioning, especially productivity, microbial activities, outwelling of organic matter, food webs, and support of commercial fisheries, while on the Pacific Coast, studies concern the impacts of invasive species of Spartina and effects of extreme events on vegetation dynamics. In Canada, effects of geese damaging vegetation are a research focus. Studies of USA's inland salt marshes have contributed knowledge of waterfowl support functions and halophyte salt tolerance. In South Africa's small estuaries, Spartina productivity and shifts of vegetation in response to altered freshwater inflows have been explored. In Asia, widespread plantings of S. alterniflora have been undertaken in order to extend coastal land area, provide forage, and produce grass for human use. In general, salt marshes of Asia, Central America, and South America are poorly known.
Salt marshes develop primarily on fine sediments, but salt marsh plants can grow on sand and sometimes gravel. Older salt marshes have peaty soils, especially in cooler latitudes where decomposition is slow.
Both roots and burrowing invertebrates affect soil structure by creating macropores in soil. Invertebrates also cause bioturbation, a process whereby sediments are re-suspended and potentially eroded away. This activity can be countered by algae and other microorganisms, which form biofilms on the soil surface. Biofilms cement soil particles and reduce erosion; they also add organic matter, and those that contain cyanobacteria fix nitrogen.
Salt marsh soils are often anoxic just below the surface due to high organic matter content and abundant moisture for microorganisms. This is especially so in lower intertidal areas and in impounded marshes. Tidal marsh soils are typically high in sulfur, which forms sulfides that blacken the soil, emit a distinctive rotten-egg smell, and stress many plants. Across intertidal elevation ranges, soil microorganisms, sulfides, and inundation regimes reduce species richness where inundation is most prolonged, often to a single, tolerant species.
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