Long-term nitrogen removal
Denitrification is largely dependent on wetland vegetation, but it is still unclear which type of wetland vegetation most favours nitrogen removal. Therefore, in this study, we aimed to test the effects of wetland maturation and planting on wetland nitrogen removal.
To do this, we used data collected in our experimental wetland facility near Halmstad, in south-western Sweden. The facility consists of 18 small wetlands designed to simulate semi-natural wetlands in an agricultural landscape. All wetlands have one inlet pipe where flow rates can be controlled, and one outlet pipe where wetland depth can be altered. During this study, wetlands were between 0.5 and 0.8 m deep and the surface area of each wetland was between 22 and 29 sqm.
At the time of construction, the 18 wetlands were divided into three treatments. One group was planted with emergent vegetation (EV), one was planted with submerged vegetation (SV), and one group was left unplanted for free development (FD).
Overview of the facility and its 18 wetlands. All water is taken from ground water source A for distribution to inlet tanks B, C and D. From B, C and D, water is then fed to the six wetlands encircled by the same broken line.
From the year of construction to 11 years afterwards, flow rates and nitrogen contents were measured on average biweekly. We have used this data to compare nitrogen removal in the different treatments while the wetlands progressed from newly constructed to more mature ecosystems.
Pictures from the experimental wetland facility, taken a few years after construction. 1) Some of the wetlands in the facility. 2) One wetland inlet. 3) Close-up of one inlet pipe. 4) The ends of two outlet pipes which lead water from the wetlands to a drainage ditch.
The main results from this study are summarized in the figure below. This figure shows that nitrogen removal in emergent vegetation wetlands initially was higher compared to nitrogen removal in the other treatments. There was no general difference between nitrogen removal in submerged vegetation and free development wetlands, so they are represented by the same line. Over time, the differences in nitrogen removal between the three treatments disappeared.
To be more specific, our wetlands passed three distinct phases of nitrogen removal: (1) initial phase, year 0–1, (2) colonization phase, year 2–7, and (3) mature phase, year 8–11 (see figure below).
The expected boosting effect on nitrogen removal by initial planting was visible in year 0, when no vegetation had yet established in free development wetlands. This effect was, however, transient. From year 1 on, when vegetation had started to grow in all wetlands, nitrogen removal in submerged vegetation and free development wetlands never differed. In addition, variability in nitrogen removal was exceptionally high within all vegetation types in year 1.
During the colonization phase, both emergent vegetation coverage and nitrogen removal increased in submerged vegetation and free development wetlands. In emergent vegetation wetlands, however, there was no change in nitrogen removal during the colonization phase, even though emergent vegetation coverage increased. Our results thus indicate a positive correlation between the proportion of emergent vegetation covering the wetlands and wetland nitrogen removal, but only until wetlands reach ca. 30 % emergent vegetation cover. Above 30 % emergent vegetation cover, all our wetlands entered a mature phase with stable nitrogen removal. For submerged vegetation and free development wetlands, the maturation process took 8 years. In emergent vegetation wetlands it only took 2 years. It therefore seems nitrogen removal in emergent vegetation wetlands passed over the colonization phase and progressed directly from the initial phase to the mature phase. Thus, by planting emergent vegetation, we seem to have accelerated the maturation process fourfold.
Nitrogen removal, expressed as the first-order area-based nitrogen (N) removal rate coefficient k (m d−1), in three wetland categories of initial planting (EV = emergent vegetation wetlands, SV = submerged vegetation wetlands, FD = free development wetlands) from the year of wetland construction to the 11th year after wetland construction. Error bars represent standard error. Stars (*) mark significant differences (p < 0.05) and the cross (+) marks a marginally insignificant difference (p < 0.07) between groups. Vertical dotted lines mark the different phases which are fitted according to visual examination.
We conclude that wetlands can be planted with emergent vegetation in order to quickly achieve high nitrogen removal, but in older wetlands, initial planting of emergent vegetation will no longer have an effect on nitrogen removal.
This study is finished and the article (Nilsson, J.E., Liess, A., Ehde, P.M. & Weisner, S.E.B. (2020) Mature wetland ecosystems remove nitrogen equally well regardless of initial planting. Science of the Total Environment, 716, 137002) is available in full text at https://doi.org/10.1016/j.scitotenv.2020.137002.
Josefin Nilsson, PhD student
Antonia Liess, Senior Lecturer
Per Magnus Ehde, Research Engineer
Stefan Weisner, Senior Professor