Plant nutrient uptake and biomass accumulation in a constructed wetland

We examined the role of plants in the nutrient cycle of a 0.3 ha constructed wetland that received tile drainage water from agricultural fields. The objectives were to determine: 1) above- and below-ground production of wetland macrophytes; 2) production of algae; 3) accumulation and uptake rate of N and P by vegetation during the growing season; and, 4) role of wetland vegetation in the overall N and P budgets. Total biomass ranged seasonally from 12000 to 30000 kg ha^-1 in the wetland, reaching a maximum in September, with roots accounting for 54 to 77% of the total. Above-ground macrophyte biomass ranged from 2000 to 5700 kg ha^-1, and also reached a maximum in September. Algae were only present early in the growing season and had a maximum biomass of 233 kg ha^-1 at the end of May. During the 1998 water year, tile input transported 715 kg ha^-1 total N and 10 kg ha^-1 total P into the wetland, whereas wetland output was 256 kg total N ha^-1 (256 kg ha^-1 in outlet flow and 120 kg ha^-1 in seepage) and 7.3 kg total P ha^-1 Therefore, the wetland removal efficiencies for N and P were 47 and 29%, respectively. Total N and P in biomass reached maxima of 367 and 57 kg ha^-1 respectively, with below-ground biomass accounting for most of the N and P found in plants. Although the N accumulation by wetland plants was equal to the difference between the wetland input and output for N, most of the plant growth occurred after tile flow ceased. Plant removal of N and P from the water column was likely a small component of the overall effectiveness of the wetland due to the lack of synchronization between plant growth and tile flow.