『Abstract
Constructed wetlands aid in the removal of NO3-
from surface waters due to enhanced rates of denitrification.
In this study intact sediment cores from a constructed wetland
were used to measure denitrification rates via the isotope pairing
technique and to compare with denitrification rates measured via
a mass balance of NO3- disappearance
from the water column. The traditional application of the isotope
pairing technique calls for the use of mechanical mixing to ensure
the transfer of tracer 15NO3-
into the sediments. However, artificial mixing can disrupt natural
redox processes near the sediment-water interface, and thus may
yield rates that are not representative of field conditions. The
objective of this study was to examine the applicability of the
isotope pairing technique for intact sediment cores and to determine
the adjustments that may be necessary in such applications. The
15NO3- was added to
the overlying water of intact sediment cores and was transported
into sediments only by natural diffusion processes. Using Br-
as a conservative tracer, it was determined that passive diffusion
alone allowed 27% of the added compound to reach the zone of denitrification
in the sediments. Using these results, the enrichment factor (ε)
used in the isotope pairing technique was adjusted to account
for this effect. Also, it was determined that agitation of the
cores at the end of the incubations was necessary to release residual
15N-labelled N2 gas entrained
in the sediment matrix. Only after accounting for these two factors
did the denitrification rate from the isotope pairing technique
agree with mass balance calculations. The isotope pairing technique
can be used to quantify denitrification in intact sediment cores
if diffusion limitations are quantified and entrained N2
is released from the sediment matrix before final isotope analyses
are conducted.』
1. Introduction
2. Materials and methods
2.1. Study site
2.2. Preliminary incubations
2.3. Denitrification experiments
2.4. Ammonium experiment
2.5. Diffusion experiment
2.6. Sample analysis
2.6.1. Ion analysis
2.6.2. Gas analysis
2.7. Denitrification calculations
2.8. Statistics
3. Results
3.1. Mass balance
3.2. Isotope paring technique
4. Discussion
4.1. Mass balance
4.2. Isotope pairing technique
4.3. Implications and limitations
5. Conclusions
Acknowledgements
References