Nitrogen enrichment increases net silica accumulation in a temperate salt marsh

Two New England salt marshes exposed to different N availability were assessed seasonally for 1 yr, creating the first complete salt marsh Si budgets. Triplicate seasonal measurements were collected for aboveground biomass of dominant Spartina species (S. alterniflora Loisel. and S. patens (Aiton) Muhl.), belowground vegetation (roots and rhizomes), sediment, and pore water. Measured Si values (reported as SiO2) diverged in several respects to that of other wetlands. Biogenic Si (BSi) concentrations in S. alterniflora were > 1% of dry weight (dry wt), values higher than previously reported in a temperate salt marsh. Rhizomes had significantly less BSi than roots (0.45% and 1.24% dry wt in the rhizomes and roots, respectively). Roots at the high-N marsh had higher BSi concentrations (average 1.44 +/- 0.11% dry wt) than reported in other wetland plant species. Likewise, sediment amorphous Si (ASi) concentrations (> 12% ASi) were elevated compared with most other temperate wetlands, although the average (4.4% +/- 0.5% ASi dry wt) was within the range of other reported values. A clear pattern toward more Si accumulation at the N-enriched salt marsh was found in the roots, sediment, and pore water, and occasionally in the aboveground vegetation. Unlike other aquatic systems, N enrichment does not result in salt marsh Si limitation, despite the presence of Si-accumulating organisms (marsh grasses). Increased soil respiration, sediment availability, and rates of organic matter decomposition are hypothesized to cause higher Si accumulation at the high-N marsh. This relationship was most pronounced in the summer for S. patens, leading us also to hypothesize that Si accumulation in marsh grasses may be related to temperature stress.