Hamersley Michael R.

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Hamersley
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Michael R.
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  • Article
    Coupled nitrification–denitrification measured in situ in a Spartina alterniflora marsh with a 15NH4+ tracer
    (Inter-Research, 2005-09-01) Hamersley, Michael R. ; Howes, Brian L.
    Measurements of N losses by denitrification from saltmarsh sediments have proved difficult because of the importance of plant metabolism and tidal cycles to sediment N cycling. In vitro approaches often do not measure the dominant coupled nitrification–denitrification pathway and/or alter in situ plant growth and redox conditions. We developed an in situ 15NH4+ tracer approach to measure coupled nitrification–denitrification fluxes in an undisturbed New England Spartina alterniflora saltmarsh. The tracer was line-injected into sediments underlying natural S. alterniflora stands and in similar areas receiving long-term N amendment (up to 11.2 mol organic N m–2 yr–1 for 16 to 23 yr), and 15N retention and loss routes were followed for 1 to 5 d. Denitrification losses in unfertilized grass stands ranged from 0.4 to 11.9 mmol N m–2 d–1 (0.77 ± 0.18 mol N m–2 yr–1). Denitrification in unfertilized sediments remained low until late summer, but underwent a ca. 4-fold increase in August and September, although sediment temperatures and respiration rates were high throughout the summer. Plant N uptake may limit the availability of N to support denitrification during the early summer, and denitrification may be released from competition with plant uptake in late summer, when plant growth slows. Denitrification rates in fertilized areas ranged from 22 to 77 mmol N m–2 d–1 (10.5 ± 4.9 mol N m–2 yr–1), and denitrification was likely controlled by the availability of fertilizer N rather than by competition with plants, since N was added in excess of plant demand. Our results emphasize the importance of in situ measurements of denitrification in understanding the dynamics of saltmarsh N cycling.
  • Thesis
    The role of denitrification in the nitrogen cycle of New England salt marshes
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2001-12) Hamersley, Michael R.
    I used direct measurements of nitrogen gas (N2) fluxes and a 15N stable isotope tracer to determine the contribution of denitrification to salt marsh sediment N cycling. Denitrification in salt marsh tidal creekbottoms is a major sink for groundwater nitrate of terrestrial origin. I studied creekbottom denitrification by direct measurements of N2 fluxes in closed chambers against a low-N2 background. I undertook experiments and simulation modeling of sediment N2 fluxes in closed chambers to optimize the key experimental parameters of this approach. Denitrification in these sediments was driven by the degradation of labile organic matter pools which are depleted during long incubations. Sediment thickness was the most important parameter controlling the required incubation time. Errors of up to 13% with gas headspaces and 80% with water headspaces resulted from headspace N2 accumulation and the resulting collapse of the sediment-water diffusion gradient. These errors could be eliminated by using headspaces of sufficient thickness. Headspace flushing to reduce ammonium accumulation did not affect denitrification rates, but caused transient disturbance of N2 flux rates. Direct measurements of O2, CO2, N2, and inorganic N fluxes from the sediments of a salt marsh tidal creek were made in order to examine the interaction of denitrification with the carbon, oxygen, and N cycles. Organic carbon concentration and lability were the primary controls on metabolic rates. CO2/N flux ratios averaged 6.1, indicating respiration driven by algal biomass. Allochthonous denitrification accounted for 39% of total sediment denitrification (2.7 mol N m-2 yr-I). 46% of remineraIized ammonium was denitrified, while the contribution of autochthonous denitrification to O2 and CO2 fluxes was 18% and 10%, respectively. A 15N-ammonium tracer was used to study competition between plants and nitrifying bacteria for remineralized ammonium. In undisturbed sediments of Spartina alterniflora, plant uptake out-competed nitrification-denitrification, with plant uptake accounting for 66% of remineralized ammonium during the growing season. Under N fertilization (15.5 mol m-2 yr-1), both plant N uptake and denitrification increased, but denitrification dominated, accounting for 72% of the available N. When plant uptake was hydrologically suppressed, nitrification-denitrification was stimulated by the excess N, shifting the competitive balance toward denitrification.