Ammonium Transformation in a Nitrogen-Rich Tidal Freshwater Marsh

Abstract

The fate and transport of watershedderived ammonium in a tidal freshwater marsh fringing the nutrient rich Scheldt River, Belgium, was quantified in a whole ecosystem 15 N labeling experiment. In late summer (September) we added 15 N-NH 4 + to the flood water entering a 3477 m 2 tidal freshwater marsh area, and traced the ammonium processing and retention in four subsequent tide cycles. In this paper we present the results for the water-phase components of the marsh system and compare them to a similar experiment conducted in spring/early summer (May). Changes in concentration and isotopic enrichment of NO 3 ) + NO 2 ) , N 2 O, N 2 , NH 4 + and suspended particulate nitrogen (SPN) were measured in concert with a mass balance study. All analyzed N-pools were labeled, and 49% of the added 15 NH 4 + was retained or transformed. The most important pool for 15 N was nitrate, accounting for 17% of 15 N-transformation. N 2 , N 2 O and SPN accounted for 2.4, 0.02 and 1.4%, respectively. The temporal and spatial patterns of 15 N transformation in the water phase component of the system were remarkably similar to those observed in May, indicating good reproducibility of the whole ecosystem labeling approach, but the absolute ammonium transformation rate was 3 times higher in May. While the marsh surface area was crucial for nitrification in May this was less pronounced in September. Denitrification, on the other hand, appeared more important in September compared to May.

The stoichiometry of inorganic carbon and nutrient removal in the Mississippi River plume and adjacent continental shelf

“…The marshes and bays could export dissolved Si and import NO − 3 (Cai et al, 2000;Gribsholt et al, 2006;Das et al, 2010), and thus could show high Si : NO − 3 ratios in the waters from the marshes (Ren et al, 2009;Struyf et al, 2011) in contrast to a low Si : NO − 3 ratio (≈ 1) from the Mississippi River (Turner and Rabalais, 2004;Lohrenz et al, 1999). We found that shelf waters close to Barataria Bay had high Si concentrations and low NO − 3 concentrations, especially in June 2003 (Figs.…”

Nitrogen assimilation and short term retention in a nutrient-rich tidal freshwater marsh – a whole ecosystem <sup>15</sup>N enrichment study

“…The higher degree of labelling in September compared to May was due to a combination of significantly lower ammonium concentrations in the flood water (Fig. 2a) and lower tidal height (see Discussion; Gribsholt et al, 2006). Thus only half the volume of water flooded the marsh during September labelling compared to May ( Table 2).…”

“…To maximize contrasts we therefore scheduled our two experiments in spring (May), when plants were young and building up biomass, and late summer (September), when macrophytes were in a flowering or early senescent state, respectively. The detailed results of the water-phase component of these studies have been described previously (Gribsholt et al, 2005(Gribsholt et al, , 2006. In both May and September, nitrification accounted for the largest fraction of ammonium transformation, and the large reactive surface area of the marsh played a crucial role in nitrogen transformation.…”

“…Advective water fluxes in and out of the study area were determined for all the tides except September T 29 (Gribsholt et al, 2005(Gribsholt et al, , 2006. Creek water nitrogen concentrations and 15 N in dissolved (NH as Br − (conservative tracer) were determined 12 times over each main tidal cycle and three times during seepage (Gribsholt et al, 2005(Gribsholt et al, , 2006, and water column stock size for all components subsequently calculated from concentration and discharge measurements (mass balance budget).…”

“…Creek water nitrogen concentrations and 15 N in dissolved (NH as Br − (conservative tracer) were determined 12 times over each main tidal cycle and three times during seepage (Gribsholt et al, 2005(Gribsholt et al, , 2006, and water column stock size for all components subsequently calculated from concentration and discharge measurements (mass balance budget). Dissolved oxygen, specific conductivity, temperature, pH and turbidity were recorded continuously (2 min intervals) using a Hydrolab Datasonde 3.…”