Mature wetland ecosystems remove nitrogen equally well regardless of initial planting

Restored and constructed semi-natural wetlands are increasingly used in the agricultural landscape to intercept nutrients from surface waters. Vegetated surface-flow wetlands remove more nitrogen (N) than those without vegetation. However, changes in N removal over time as differently vegetated wetlands progress from early successional stages to mature systems are less investigated. We monitored three different types of initial planting over the course of 12 years, with the aim to examine how planting of newly constructed wetlands affects long-term N removal. All our data were collected in an experimental wetland facility in south-western Sweden. The facility consists of 18 identical small (ca. 25 m2) surface-flow wetlands, simulating semi-natural wetlands in an agricultural landscape. Initially, the 18 wetlands were randomly divided into three treatments (vegetation types) with six replicates each and planted with (1) emergent vegetation, (2) submerged vegetation and (3) no vegetation for free development. Vegetation succession afterwards progressed uninhibited in all wetlands. Emergent vegetation wetlands initially removed more N than both submerged vegetation and free development wetlands. We found 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 vegetation types converged when the wetlands reached a more mature state, around 8 years after wetland construction. However, although all wetlands contained emergent vegetation in year 8, the proportion of emergent vegetation cover and vegetation composition still differed substantially between wetland types. Our study indicates that it is not the cover of emergent vegetation per se which promotes higher N removal in more mature wetlands, but the maturation process itself; mature wetlands despite differing emergent vegetation coverage achieved equally high N removal. In conclusion, once wetlands reach maturity, beneficial effects of initial planting on N removal disappear.