Production and dynamics of salt marsh vegetation and the effects of experimental treatment with sewage sludge. Biomass, production and species composition.
Fertilization with a sewage sludge fertilizer containing 10-6-4 NPK increased the total peak standing crops of salt marsh vegetation, despite the presence of considerable amounts of heavy metals and chlorinated hydrocarbons in the sludge. Increases in standing crop were detected in the 2nd year of treatment and remained for 3 years of treatment. The increases in biomass were considered to be due to the added N. Spartina alterniflora, which was the dominant species in low marsh areas, progressively excluded Salicornia spp. from the fertilized plots after an initial increase of Salicornia. In high marsh areas, fertilization initially increased the standing crop of Distichlis spicata but this was later replaced by Spartina patens. S. alterniflora did not respond to fertilization in the high marsh. The amount of dead matter in both high and low marsh was highest in autumn, after death of the sward, and decreased slowly to a low point in midsummer. The amount of dead matter was never equal to peak live biomass, implying decomposition or tidal export. The production levels achieved by the most heavily fertilized plots (1.13-1.53 kg DM/m2) are among the highest recorded for marsh plants, except that of the tall form of S. alterniflora in exceptionally favourable sites. The treatments converted low marsh vegetation, consisting mainly of dwarf-form S. alterniflora, into a sward approaching the biomass and morphology of tall form. It was concluded that the so-called 'forms' are a response to N supply.