Multiple levels of nitrogen applied to an oligohaline marsh identify a plant community response sequence to eutrophication

We enriched experimental plots in a Sagittaria lancifolia L. dominated oligohaline marsh for 4 yr with one of 4 levels of nitrogen (N) (0, 50, 200, or 1200 kg N ha(-1) yr(-1)) in combination with one of 2 levels of phosphorus (P) (0 or 131 kg P ha(-1) yr(-1)) to investigate nutrient limitation of primary production and plant community-and species-level responses to nutrient enrichment. Overall, significant changes in ecosystem structure and function occurred with N enrichment only; P enrichment had no significant effect alone or in interaction with N. Both 200 and 1200 kg N ha(-1) yr(-1) stimulated above-ground plant production. Enrichment with 1200 kg N ha(-1) yr(-1) also increased S. lancifolia tissue N:P ratio, reduced S. lancifolia N and P resorption during senescence, and altered the relative dominance of the 3 dominant species, but had no effect on species richness. We conclude that (1) N limits above-ground primary production in this oligohaline marsh; (2) the plant response to N enrichment is sequential: moderate N loading stimulated plant production only, while high N loading maintained the elevated production and also altered plant tissue nutrients and species dominance, but not species richness; (3) N enrichment beyond the assimilation capacity of the vegetation drives changes in ecosystem structure caused by altered plant nutrient cycling; and (4) linear changes in species dominance with increasing N enrichment suggest that further nutrient enrichment in time or quantity may result in a shift in species dominance and reduced species richness.