Effects of Phragmites australis removal on marsh nutrient cycling

Abstract

In the northeast US removal of exotic and invasive plant species is a common wetland restoration activity and the invasive common reed (Phragmites australis) is often the target of control efforts. We examined effects of reed removal on sediment nutrient pools and denitrification potential in a tidal freshwater marsh on the Connecticut River. In the first year after herbicide application and cutting of a reed stand, porewater ammonium concentrations in the removal area were about 4× higher relative to extant reed or cattail. Denitrification potentials were 50% lower than in a reference stand of reed. Denitrification activity had “recovered” by the second growing season after reed removal but porewater ammonium continued to accumulate. By the third growing season following reed removal, plant regrowth had occurred over approximately half the experimental plot and porewater ammonium had declined to pre-manipulation levels. Sediment organic content, moisture and porewater phosphate showed no significant response to reed removal over the four-year course of this study. Reed removal allowed regrowth of a more diverse plant community thereby achieving one of the goals of this restoration effort but patterns in ammonium accumulation and denitrification suggest a reduction in the capacity of this site to act as a sink for nitrogen.