Use of stable nitrogen isotope fractionation to estimate denitrification in small
constructed wetlands treating agricultural runoff
Anne Kristine Søvik and Pål Tore Mørkved
Introduction
Constructed wetlands (CWs) in the agricultural landscape reduce non-point source pollution
through removal of nutrients and particles. The mechanisms contributing to removal of nitrogen (N) in CWs are considered to be sedimentation, assimilation in plants and microorganisms, and denitrification. Nitrogen assimilated in wetland vegetation will to a large extent be
re-mineralized sooner or later, unless the vegetation is harvested. Thus, the most important
removal mechanism for N and the only one which is permanent is microbial denitrification.
Nitrogen removal in ponds is often found to be enhanced in the presence of plants, as denitrification is stimulation by supply of organic material from the plants, which may either be used
directly by the denitrifying bacteria or indirectly by contributing to a lower redox potential.
However, we still have a limited quantitative knowledge regarding the role of denitrification
versus other processes in small CWs, and particularly how this shifts during the year.
Biological discrimination between the two stable isotopes 14N and 15N leads to natural isotopic fractionation, i.e., denitrification yields an enriched pool of 15NO3-, as bacteria preferentially reduce 14NO3- over 15NO3-. The isotopic signature has been used to identify regions of
significant denitrification in e.g., groundwater aquifers, riparian buffer zones and CWs. As far
as we know, the method has until now only been used in one study regarding constructed wetlands. More studies are needed to identify the limits and possibilities of this method in ecosystems such as CWs.
Study site
The experiments were performed in the Lier experimental wetland system located 40 km west
of Oslo (Fig.1). The water flows into a sedimentation chamber and is further distributed into 8
parallel wetland trenches (40 m long and 3 m wide). Nitrogen removal was studied in three
wetland trenches filled with different filter materials (T1 - sand and gravel; T3 - mixture of
peat, shellsand, and light-weight aggregates; T8 - barley straw) and a trench formed as a shallow pond (T4). Water samples were collected during all seasons at six equally spread locations along each wetland trench. All samples were analysed for total nitrogen, NO3-, and dissolved organic carbon (DOC), as well as δ15N-NO3-.
Channel
T8. Barley straw
Sedimentation
chamber
T4. Standard wetland (depth 0.5 m)
T3. Filter with LWA, peat and shellsand
Distribution chamber
T1. Mineral filter (sand and gravel)
40 m
Figure 1. The Lier experimental wetland
Results
The removal of N was found to be highest during summer and lowest during autumn and winter. Trench T8 was found to have the highest N removal during summer. Measurements of the
natural abundance of 15N in NO3- showed that denitrification was not significant during autumn / winter, while it was present in all trenches during summer, but only important for nitrogen removal in trench T8. The 15N enrichment factors of NO3- in this study ranged from 2.5 to -5.9‰ (T3 and T8, summer), thus smaller than enrichment factors found in laboratory
tests of isotope discrimination in denitrification, but similar to factors found for denitrification
in groundwater and a large CW. The low enrichment factors compared to laboratory studies
was attributed to assimilation in plants / microbes as well as diffusion effect. Based on a modified version of the method presented by Lund et al. (2000), denitrification and assimilation
was estimated to account for 53 - 99 and 1 - 47%, respectively, of the total N removal in
summer.
Conclusion
Our results show that use of stable nitrogen isotope fractionation is a valuable tool for verifying whether denitrification is taking place in a given system. Use of the method to quantify
the contribution of denitrification to the N removal is more questionable. Uncertain assumptions regarding nitrification, microbial assimilation and equal diffusion in all trenches irrespective of filter material and for all season suggest that there are problems with this approach
to denitrification estimates. Thus there is a need for a better knowledge of the effect of plant
uptake, microbial assimilation as well as nitrification on N isotopic fractionation before this
method can be used to evaluate the contribution of dinitrification in CWs. The role of diffusion in these systems should be investigated further as well. Finally our work shows that the
standard way of constructing wetlands in Norway is not very efficient for removing N from
agricultural runoff. Filling parts of the wetland volume with barley straw might improve the N
removal of these wetland systems.
References
Lund, L.J., A.J. Horne, and A.E. Williams. 2000. Estimating denitrification in a large
costructed wetland using stable nitrogen isotope ratios. Ecol. Engineer. 14: 67-76.