Limited response of cordgrass (Spartina foliosa) to soil amendments in a constructed marsh

Constructed marshes in San Diego Bay (California, USA) have short canopies of the cordgrass Spartina foliosa and soils that are low in organic matter and nitrogen, compared to natural salt marshes of this area. Thus, we hypothesized that organic and nitrogen amendments would accelerate plant growth and enrich soil nitrogen pools. A field experiment in a newly excavated mitigation site examined the effects of adding ammonium sulfate (N), nitrogen-rich alfalfa (with or without added N), and nitrogen-poor straw (with or without added N) prior to planting. Aboveground biomass and stem densities of cordgrass were proportional to the amount of N added, whether in the organic or inorganic form.Alfalfa decomposed more rapidly than straw, but in both cases, N was lost quickly (50-66% in 2 wk). Sediment nitrogen and carbon pools were not increased significantly by the amendments. High decomposition rates and sandy soils apparently prevented accumulation of N in the nutrient and/or organic matter pools. Less than 5% of the nitrogen potentially available for uptake was actually recovered in aerial tissue. With N-poor soil and high nutrient-loss rates, the experimental plots developed less than half the aboveground biomass found in reference natural cordgrass marshes. Amending soils prior to transplantation did not achieve the goal of functional equivalency with natural marshes, and after 2 yr there was no indication that nutrient pools or plant growth rates would achieve natural levels.