Nutrient management of nursery runoff water using constructed wetland systems

Substantial quantities of water and nutrients are required for the production of high value nursery and greenhouse crops. As water quality criteria are strengthened locally and nationally, horticultural enterprises win have to meet stricter limits on nutrients in discharge water. This study examined the efficacy of an established vegetated surface-flow constructed wetland to mediate nitrogen (N) and phosphorus (P) in runoff water from a commercial nursery over a period of 38 months. Maximum oxidized nitrogen [nitrate-N (NO3-N) + nitritc-N (NO2-N)] inputs occurred during the spring fertilization period of March through May (11.1 to 29.9 mg.L-1 N) and minimum inputs occurred during winter plant dormancy between December and February (2.8 to 5.2 mg(.)L(-1) N). Nitrogen remediation efficiency averaged 94.7% for March through November sampling dates but declined to a mean of 70.7% between December and February when mean wetland water temperature dropped below 15 degrees C. Orthophosphate phosphorus (PO4-P) concentrations in nursery runoff showed no dramatic changes over months, seasons, or years. Mean wetland influent orthophosphate concentration ranged from 0.7 to 2.2 mg.L-1 PO4-P with an overall mean of 1.41 mg-L-1 PO4-P for all months sampled. Mean discharge orthophosphate concentration ranged from 0.5 to 2.1 mg.L-1 PO4-P with a mean of 1.45 mg.L-1 PO4-P. Phosphorus remediation efficiency varied widely and there was no correlation with water temperature. This 9.31-acre surface-flow constructed wetland was highly efficient at removing N from nursery runoff from a 120-acre catchment (large container production area), although it failed to consistently lower orthophosphate levels in runoff. This type of constructed wetland is suitable for removing oxidized N forms from nursery runoff and, depending on size, is capable of handling the large volumes of runoff generated by medium to large nursery and greenhouse operations.