Contribution of Denitrification to C, O, and N cycling in Sediments of a New England Salt Marsh

M. Robert Hamersley and Brian L. Howes

Introduction

Salt marsh productivity is limited by nitrogen (N) availability. In populated areas, inputs are dominated by sewage effluent and agricultural fertilizer N. This allochthonous N has altered salt marsh C and N cycling, primarily by enhancing denitrification. Denitrification, the microbial conversion of nitrate (NO3-) to N2 gas, plays an important role in the saltmarsh N cycle and in regulating the availability of N to primary producers. N transported in groundwater and surface flows is delivered primarily to the regularly flooded bottoms of salt marsh creeks, which account for ~1/3 of the area of a New England salt marsh. In these shallow salt marsh creeks, sediment processes predominate over water-column processes. Organic material fixed by the enhanced primary production creates a strongly reducing environment in the sediments capable of supporting the denitrification of autochthonously-generated NO3- (from mineralization and nitrification of sediment organic matter) as well as allochthonous (water-column) NO3-.

The purpose of this study was to determine:
1) controls on the cycling of sediment-remineralized NH4+,
2) uptake of watercolumn NO3-, and the 3) contribution of nitrification and denitrification to the overall cycling of O2, C, and N within the salt marsh sediments.

Methods

Study Site: Mashapaquit Marsh, a small (5.7 ha) salt marsh in Cape Cod, Massachusetts, receiving 32 mmol N m-2 d-1 in groundwater N inputs, primarily from sewage disposal in the watershed (Figure 1). Water-column NO3- concentrations averaged 43 mM. Sediment Flux Measurements: Sediment-water fluxes of N2, O2, total CO2, NH4+ and NO3-, were measured in cores collected and incubated in gas-tight chambers (Figure 2). N2 fluxes were measured against a low-N2 atmosphere. In such a system, initial N2 fluxes out of the sediment are predominated by diffusion of dissolved porewater N2 pools. Paired anoxic chambers, where coupled nitrification-denitrification was inhibited, were used to control for sediment off-gassing (Nowicki, B.L. 1994. Estuar. Coast. Shelf Sci. 38:137.).

Results

1) O2 consumption, CO2 production, and autochthonous denitrification within the salt marsh creek sediments all followed a seasonal cycle of biological activity with winter minima and late summer maxima (Figure 4).
2) Although sediments of the two sites differed in density, porosity, sediment C and pigment composition, mean rates of C, O, and N cycling did not differ between the two sites (Table 1). Although sandy sediments had lower organic C concentration, they contained a higher proportion of metabolically active algal biomass.
3) Metabolic rates were highly dependent on sediment C concentrations (Figure 5). Sediment O2 consumption, CO2 production, autochthonous denitrification, and allochthonous denitrification (in experiments where NO3- was not limiting) rose with increasing sediment C for both sandy and muddy sediments, but the sediment C of muddy sediments was less metabolically active than that of sandy sediments.
4) The elemental ratios of CO2, O2 and N fluxes across the sediment-water interface were highly consistent across all seasons and sediment types (Figure 6). The C/N molar flux ratio was 6.1, near to the Redfield ratio for algal biomass (6.6).
5) Coupled nitrification-denitrification contributed 18% to total O2 fluxes and 10% to total CO2 fluxes, while 46% of N mineralized in the sediments was nitrified and denitrified (Figure 7).

Summary and Conclusions

1) Metabolic rates were controlled by sediment organic C lability between sites, and by organic C concentration within sites. Allochthonous denitrification is limited by NO3- availability.
2)The CO2 / N flux ratios and d13C evidence support a sediment metabolism supported primarily by algal biomass.
3) Although the sediment C content of sandy sediments was one half that of muddy sediments, sandy sediment C was twice as labile, so that mean rates elemental cycling did not differ between the two sediment types. Allochthonous denitrification accounted for 39% of the total denitrification.
4)Total denitrification rather than allochthonous denitrification alone represents the true size of the sink for groundwater NO3-, since the only significant source of N to support benthic algal production was groundwater NO3-.
5)Denitrification was a significant contributor to sediment elemental cycling, accounting for 18% of O2 fluxes and 10% of CO2 fluxes. 46% of NH4+ mineralized within the sediments was denitrified.

Acknowledgements: Support for this research was provided by the Education department of the Woods Hole Oceanographic Institution and by the Coastal Systems Group of the School for Marine Science and Technology, University of Massachusetts. The authors would also like to acknowledge the analytical work of Paul Henderson and the contributions of field and laboratory personnel too numerous to name individually.