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1.3.2. Mass Balance Approach
Mass balance has long been used to estimate various terrestrial N fluxes at the field scale
and more recently at stream reach, watershed, or regional scales (Groffman et al., 2006).
In some studies, denitrification is estimated using literature values and is one component
of the mass balance (Hantzche & Finnemore, 1992); others estimate denitrification by the
difference in the mass balance (e.g., David & Gentry, 2000, Tesoriero et al., 2000 Pribyl
et al., 2004).
As this method can be applied at a range of scales, from small plots to large watersheds,
lakes, estuaries, and open marine systems, it is a useful method to assess denitrification.
However, in order to estimate denitrification by mass balance, direct measurements or
estimates of all other N fluxes and changes in storage for the system are needed. The use
of this method requires the assumption of a steady state system and only inputs and
outputs are quantified. Major inputs to terrestrial systems typically include fertilizer,
biological N2 fixation, and atmospheric deposition, with outputs including crop or
biomass harvest and export from the area or leaching, runoff and riverine export.
(Groffman et al., 2006). These assumptions place a limitation on the usefulness of mass
balance as a predictive tool.
While certainly useful in providing some insight into the potential importance of
denitrification, the mass balance approximations can be improved if multiple years of
data are used and averaged. This method is perhaps better suited to verify the validity of
new methods of measuring the rate of denitrification.
1.3.3. Isotopes
Kinetic isotopic fractionation of
−
NO 3 during bacterial denitrification has been
documented in laboratory and field studies (Mariotti 1981; Mariotti et al., 1988; Altabet
et al., 1995; Barford et al., 1999; Voss et al., 2001). The bacteria responsible for
denitrification preferentially utilize the lighter isotopes than the heavier isotopes during
denitrification, thus leaving the remaining elements in the system enriched in the heavier
isotopes.
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