The retention of nitrogen and phosphorus in aboveground biomass of plants growing in constructed wetlands treating agricultural drainage

Abstract

Three horizontal subsurface flow constructed wetlands (CW1, CW2 and CW3) for treatment of agricultural tile drainage water were built in 2018. Two wetlands were filled with a mixture of washed gravel and birch woodchips (10:1 volume ratio), in the third one the layer of woodchips was on top of a gravel layer. All wetlands were planted with Gyceria maxima and Phalaris arundinacea in parallel bands and parallel to water flow. Four-year study (2019–2022) was aimed at the amount of nutrients retained in aboveground plant biomass. The results revealed that the biomass of both species was lower as compared to biomass of these species growing in constructed wetlands for municipal sewage. Due to low aboveground macrophyte biomass, the nitrogen and phosphorus standing stocks were low (maximum 1.3 g P m−2 and 11.1 g N m−2). When comparing the nitrogen standing stocks with the inflow annual loadings and removed load, it represented on average only 2.38% of the removed load and 0.85% of the inflow load because of very high inflow loading (1298, 866 and 842 g N m−2 yr−1 in CW1, CW2 and CW3, respectively). On the other hand, the respective values for phosphorus amounted to 43.9% and 21.6% due to very low inflow loading (8.8, 6.6 and 6.0 g P m−2 yr−1 in CW1, CW2 and CW3, respectively). The study confirmed that plant contribution to nutrient removal in constructed wetlands expressed as removal percentage is strongly affected by inflow loading. The study also revealed that under low load conditions the plant uptake may substantially contribute to nutrient removal.