Nutrient retention and nutrient-use efficiency in Phragmites australis stands after wasterwater application

The capacity of Phragmites australis to extract nutrients from its environment and to store them in plant tissue was studied in an infiltration wetland used for wastewater treatment. The aims of the study were to estimate the contribution of plant uptake to the overall nutrient removal efficiency of the wastewater treatment system and to determine measures to enhance the nutrient removal efficiency. The functional response of Phragmites australis to nutrient addition was studied at two levels of nutrient availability by comparing a wetland used for wastewater treatment with a natural, undisturbed wetland. In the nutrient-rich wastewater wetland, biomass production, nutrient content of plant tissue, shoot/root ratios, and translocation rates were much greater than in the natural wetland. In contrast, plant nutrient-use efficiencies of both nitrogen and phosphorus were much lower in the wastewater wetland. The lower nutrient-use efficiencies were probably due to (1) lower nutrient productivity, which was probably due to self-shading resulting in a lower photosynthetic activity, and (2) lower mean nutrient residence time, which can be explained by a higher litter production (higher shoot and leaf turnover). In the wastewater wetland, harvesting during winter removed only 9% of the total nitrogen input and 6% of the total phosphorus input associated with the loading of wastewater. The low nutrient removal efficiency by aboveground plant harvesting is caused by translocation of nutrients from shoots and leaves to rhizomes and roots in autumn. Nutrient removal efficiency of the wastewater wetland could be enhanced by 40–50% by harvesting at peak nutrient storage levels in the aboveground plant parts in August or September.