/ In salt marshes, most biomass plus large reserves of biologically important N, P, Fe, and S are sequestered below ground under saline, waterlogged, and anaerobic conditions. Thus, hydrologic alterations such as diking and ditch drainage that reduce salinity and increase peat aeration can cause radical changes in the composition of salt marsh soils.Experimental short-term desalination and drainage of salt marsh cores in greenhouse microcosms caused Spartina production to increase after one growing season, reflecting decreased salt stress and sulfide toxicity. However, production thereafter declined, likely due to pyrite oxidation and acidification in drained treatments and sulfide accumulation in waterlogged treatments.A survey of longer-term (decadal) effects of diking on peat composition of Cape Cod, Massachusetts, USA, marshes revealed acidification, Fe(II) mobilization, and decreased organic content in drained sites. Despite the aerobic decomposition of organic matter, abundant nutrients remained as sorbed NH4 and mineral-bound PO4. In diked, seasonally waterlogged sites, porewater alkalinity, sulfide, ammonium and orthophosphate were much lower, and organic solids higher, than in adjacent natural marsh.Seawater was added to cores from diked marshes to study the effects of tidal restoration. Salination of the drained peat increased porewater pH, alkalinity, ammonium, orthophosphate, Fe, and Al; copious ammonium N, and Fe(II) for sulfide precipitation favored Spartina growth. Salination of diked-waterlogged peat increased sulfate reduction and caused 6-8 cm of sediment subsidence. The resulting increase in porewater sulfides and waterlogging decreased vigor of transplanted Spartina alterniflora. Results indicate that seawater restoration should proceed cautiously to avoid nutrient loading of surface waters in drained sites or sulfide toxicity in diked-waterlogged marshes.KEY WORDS: Salt marsh; Diking; Biogeochemical cycling; Restoration; Massachusettshttp://link.springer-ny.com/link/service/journals/00267/bibs/24n1p111.html