Used for Measurements of Nitrogen Cycle Process Rates

Methodologies Used for Measurements OF Nitrogen Cycle Process Rates 

Because of the steep redox gradients in many sediments and the often rapid removal of the products of NTR by DNF and other dissimilatory processes, measurements of NTR remain problematic. NTR has been estimated in the presence and absence of specific inhibitors by the accumulation of NH4 (Henriksen et al., 1980 Caffrey et al., 2003) or by dark lT COj uptake into SOM (Dore and Karl, 1996). The inhibitors used include nitrapyrin (N-serve), methyl fluoride, dimethyl ether, acetylene, and aUylthiourea. Problems involved with these methods include nonspecificity of inhibitors and difficulty in determining the correct conversion factor for uptake to N1T4 oxidized, which has been observed to range from 8.3 to 42 mol N1T4 oxidized per mole fixed. Dore and Karl (1996) used an independent chemical assay of nitrite oxidation to verify the conversion factor used. NTR has also been estimated by the accumulation of nitrite in the presence versus absence of chlorate, an inhibitor of nitrite oxidation (Belser and Mays, 1980). NOa isotope dilution techniques, similar to those used to measure NMIN (see Section 5.2) have been used to measure NTR (Anderson et al., 1997 Risgaard-Petersen et al., 1994) however, when DNRA rates are high, the added NOa may disappear faster than it is sufficiently diluted to accurately estimate NTR. 

The product of DNF, dinitrogen (N2), is a major component of the atmosphere, meaning that natural waters contain high concentrations of N2 ( 300 pM) when in equilibrium with the atmosphere. This high background makes measuring the 

Indirect methods used to estimate DNF include the acetylene block method (S0rensen, 1978), metabolite stoichiometry (Dollar et al., 1991), or stable isotope tracers (Nielsen, 1992). Acetylene (C2H2) blocks the terminal step of DNF, the conversion of N2O to N2, and the DNF rate is estimated by quantifying the production of N2O on a gas chromatograph with an electron capture detector. Problems with the acetylene block technique include blockage of NTR (which means that rates of coupled NTR—DNF cannot be obtained), inefficacy at low N03 concentration, and interference by H2S. Sulfide appears to alleviate the acetylene block of nitrous oxide reductase and permit full reduction of N2O to N2. 

Stable nitrogen isotopic tracers provide another way to estimate DNF rates and are often used in concert with the direct approaches (described above) to better constrain rates of coupled NTR—DNF. Heavy ( N-labeled) NH4 or NOs is added to samples in tracer quantities and the subsequent production of N-labeled gases ( Nd N or Nd N) is quantified (Nielsen, 1992). The main drawback of this approach is that pore water N pools may not reach isotopic equilibrium during the incubation, which complicates calculation of DNF rates. 

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