THE FLORIDA STATE UNIVERSITY ARTS AND SCIENCES ...

denitrification, nitrate would not be enriched in δ 15 N and in effect there would be no
enrichment of the heavy isotope. This implies that the enrichment factor would be zero.
We assume that at any stage of denitrification in a given region or location, the
enrichment factor for the system due to denitrification would be any where between the
two extremes of complete denitrification and no denitrification. As there are two
pathways for the reduction of nitrate in an ecosystem (DNRA and denitrification) the
enrichment factor may have a value that lies between these two extremes.
In order to obtain an estimate of the nitrate removed due to denitrification we may
1) Plot the δ 18 O vs. δ 15 N, to estimate the slope b (slope ~ 0.48)
2) Plot δ 15 N vs. ln (Ct/Co) , i.e. the log of the fraction of nitrate remaining to estimate
ε
3) Plot ε for 0, max denitrification and the value obtained
4) Determine the percent of nitrate lost due to denitrification from the resulting
graph
The example (Figure 6.1) shows how the δ 15 N–NO3 − isotope enrichment at a point on a
curve with a certain enrichment factor may be attributed to denitrification and
assimilation (plants and microorganisms). An analytical derivation of the process is
presented by Wang (2011).
A Rayleigh curve with enrichment factor due to only denitrification (ε D ) is − 17‰
(literature value from Blackmer and Bremner (1977), a curve with ε A = 0‰ represents a
system with only assimilation, and a curve with ε W = − 8‰ represents a wetland system
with both denitrification and assimilation. If the final
157
−
NO 3 concentration represents a
loss of 70% (remaining fraction of 0.3), about 61% (0.43/0.70) of the NO3 − loss would be
attributed to denitrification while about 39% (0.27/0.70) of the loss would be due to
assimilation ( Lund et al., 2000).