Harvesting effects on biomass and nutrient retention in Phragmites australis in a free-water surface constructed wetland in western Ireland

The aim of this study was to examine the seasonal variation in biomass, total nitrogen (Tot-N) and total phosphorus (Tot-P) content of Phragmites australis in a three-cell, free-water surface (FWS) constructed wetland in western Ireland and to investigate the effects of harvesting on its biomass and nutrient content. One cell of the wetland was divided into two plots: one plot, measuring 80m², was completely harvested on 16 June 2005, while the other plot, the control plot, remained uncut throughout the study duration. Completely randomised 0.64m² areas within each plot were harvested to water level at intervals of approximately one month over an eight-month study period, and the shoot biomass and nutrient content were measured. In the control plot, the plant biomass, total nitrogen and total phosphorus content peaked in August. In the June-cut plot, the shoot biomass, total nitrogen and total phosphorus content peaked in September. The mean rate of dry matter production, defined as the mean daily rate of dry matter production per unit area per day between harvests, attained maximum rates of 12.8g m^-2d^-1 and 4.2g m^-2d^-1 for the control and June-cut plots, respectively, indicating that annual harvesting of emergent vegetation may not have any beneficial effect on biomass production or nutrient content under Irish climatic conditions.