Will planting of surface-flow wetlands improve nitrogen removal in the long run?

• Initial planting of created wetlands is common practice in order to, for instance, improve nitrogen (N) removal. It has been shown that vegetated surface-flow wetlands remove more N than non-vegetated surface-flow wetlands. However, changes in N removal as differently vegetated wetlands progress from an early successional stage to a mature system are less investigated.In our study, we followed three different wetland types of initial planting over the course of 12 years, with the aim to examine how planting of newly created wetlands affects long-term N removal. All our data were collected in the experimental wetland facility near Halmstad in south-western Sweden. The facility consist of 18 small (ca. 25 m2) surface-flow wetlands, equal in age, shape and size. At the time of creation, the 18 wetlands were randomly divided into three types. One type was then planted with emergent vegetation, one was planted with submerged vegetation and the last type was left unplanted for free development. Succession of vegetation was thereafter allowed to progress uninhibited in all wetlands.Our results confirmed that emergent vegetation wetlands initially removed more N than submerged vegetation and free development wetlands. In addition, our results showed that N removal in submerged vegetation and free development wetlands increased with ecosystem age, whereas N removal in emergent vegetation wetlands did not. N removal in all three wetland vegetation types converged when the wetlands reached a more mature state, around year 9 after wetland creation. However, although all wetlands contained emergent vegetation in year 9, proportion cover of emergent vegetation and vegetation composition still differed substantially between wetland types.We therefore conclude planting of created surface-flow wetlands with emergent vegetation will have a positive effect on N removal, but only during an early successional stage. Our study indicates it is not the emergent vegetation per se which results in higher N removal in more mature wetlands, but the maturation process in itself, since mature wetlands with different emergent vegetation cover achieved similar N removal. Initial planting will not result in higher N removal once the system has reached maturity.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Naturgeografi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Physical Geography (hsv//eng)

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