Nitrogen and phosphorus in the sediments of a tidal,freshwater marsh in Massachusetts

Total organic nitrogen (TON) and phosphorus (TOP) were measured as a function of depth in 14 cores taken from a New England, tidal, freshwater marsh. TON and TOP ranged from 1.56 to 1.97% and 0.11 to 0.30% of dry weight sediments, respectively. The variation in both pool sizes over time was small and TON varied inconsistently with depth; however, TOP decreased regularly down to 20 cm. Consequently, the TON: TOP ratio increased linearly from 14∶1 at the surface to 32∶1 at 20 cm, then was nearly constant to 70 cm. This pattern may be a general feature of marsh sediments and may indicate 1) that phosphorus is recycled less efficiently than nitrogen, 2) that over time proportionately more introgen than phosphorus is incorporated into recalcitrant compounds, or 3) that phosphorus is more mobile than nitrogen in these marsh sediments. The total inorganic nitrogen pool was measured in this marsh also and was dominated by ammonium (97% of total). The annual average free ammonium concentration was 3.70±0.64 mg N per 1 at the surface and decreased to 0.92±0.18 mg N per 1 at 20 to 22 cm in the sediments. Sorptiondesorption studies showed that, on a fresh sediment volume basis, sediment sorbed ammonium was roughly equivalent to free porewater ammonium (K=0.8). The relationship between free and sorbed ammonium was linear between 0.4 and 24.0 mg NH₄·N per 1 of pore water. The depth distribution of ammonium in these sediments is probably maintained by a dynamic balance between net microbial mineralization of litter, plant uptake, transpiration, diffusion, and porewater advection.