Development of a process-based nitrogen mass balance model for a Virginia (USA) Spartina alterniflora salt marsh:implications for net DIN flux

Abstract

Primary production is nitrogen limited in most salt marshes with the possible exception of those impacted by high anthropogenic inputs of nitrogen. It is hypothesized that mature salt marshes which receive only small inputs of 'new' nitrogen from the atmosphere, surface water runoff, groundwater, tidal creek, and nitrogen-fixation will have a conservative nitrogen cycle. We have developed a process-based N mass balance model for a short-form Spartina alterniflora marsh in Virginia, USA. Data for the model included rates of gross mineralization, nitrification, denitrification, nitrogen fixation, above-and belowground macrophyte production, and benthic microalgal production. The annual balance between sources (mineralization, nitrogen fixation, tidal creek flux, atmospheric deposition, and sediment input) and sinks (above-and belowground macrophyte uptake, sedlment microalgal uptake, sediment burial, microbial immobilization, denitnfication, and nitrificat~on) of dissolved inorganic nitrogen (DIN) was determined for both interior S. alterniflora-vegetated sites and unvegetated creek bank sites. S e d i m e n t h a t e r exchanges of DIN species, predicted by results of the mass balance analysis, were compared to measured exchanges. Annually, sources and sinks of DIN in the vegetated marsh were in close balance. The vegetated marsh imported DIN from the adjacent creek during most of the year; the unvegetated creek bank exported NH,-to overlying tidal water during July and imported NH,' during other seasons. The net flux of DIN wds 5 7 g N m-' yr-l from overlying water into the marsh; however, this flux was small relative to rates of internal N-cycling processes. The sediment NH,+ pool turned over rapidly as a result of the high rate of gross mineralization (84 g N m-'yr.'). Other microbial N-cycling rates were low (0 6 to 4 g N m-2 yr-l). The NH,+ supplied by mineralization was more than sufficient to support both macrophyte (33 g N m-2 yr-l) and benthic microalgal (5 g N m-' yr.') uptake. We propose that in order to maintain steady state in the system approximately half of the DIN mineralized is immobilized into a readily remineralizable particulate organic N pool. Since rnineralization and macrophyte uptake are temporally out of phase, the labile organic N pool may serve to temporarily sequester NH4' until it is required for plant uptake.

Carbon cycling in a tidal freshwater marsh ecosystem:a carbon gas flux study

“…Estimates from a Spartina alterniilora salt marsh show that, given the assumptions and limitations inherent in each approach, the gas exchange and ecophysiological techniques provide similar production rates (Anderson et al 1997).…”

“…Although there are potential errors with this approach (see 'Discussion'), we believe this estimate will be more accurate than one based solely on CO, and CH, efflux into sediment chambers. Gross nitrogen mineralization was determined by 15NH4+ isotope pool dilution as described in Anderson et al (1997). Sediment cores (10 cm deep) were collected seasonally in triplicate at 5 randomly selected points along each transect using polycarbonate core tubes (20 cm tall X 25.5 cm2).…”

“…turnover and translocation) and therefore provides a more reliable estimate of total production than harvest methods. If carbon fluxes from the sediments are measured in situ, sediment rnicroalgal production can also be calculated (Anderson et al 1997).…”

“…Ecological simulation (i.e. Buzzelli 1996) or mass balance models (Chalmers et al 1985, Hopkinson 1988, Anderson et al 1997 can be used to predict overall carbon or nitrogen balances in a marsh system, but accurate production estimates must first be obtained. Sediment microalgae are highly productive (Sullivan & Moncreiff 1988, Pinckney & Zingmark 1993, can be suspended and exported from the marsh by tidal waters (Gallagher 1975), and are often an important food source in aquatic systems (Sullivan & Moncreiff 1990, Hamilton et al 1992, Currin et al 1995, Deegan & Garritt 1997.…”

Quantification of Ammonia-Oxidizing Bacteria and Factors Controlling Nitrification in Salt Marsh Sediments

“…Two distinct microbial populations are involved in nitrification: ammonia-oxidizing bacteria (AOB), which catalyze the oxidation of ammonia (NH 3 ) to nitrite, and nitrite-oxidizing bacteria, which catalyze the oxidation of nitrite to nitrate. Although studies of nitrification rates in salt marsh sediments have found considerable variability across horizontal, vertical, and temporal scales, the environmental factors controlling nitrification rates in salt marsh sediments have not been investigated (3,42,43). In other coastal marine sediments, nitrification is influenced by a suite of environmental parameters, including oxygen and dissolved sulfide concentrations, overall rates of C metabolism, and the presence or absence of vegetation and macrofauna (19).…”