Nitrogen and phosphorus removal from agricultural sewage by wetlands under contrasting hydrologic regimes

Two wetlands dominated by Phragmites australis, which were previously rice fields, were used under contrasting hydrological regimes, intermittently and permanently flooded. The aim was to study their role as filters of nitrogen and phosphorus contained in the water transponed through the network of channels used to irrigate ricefields in the delta of the Ebro River (NE Spain). In both wetlands, most of the forms of nitrogen and phosphorus were retained, except dissolved organic nitrogen in the intermittently flooded wetland and particulate nitrogen and dissolved organic phosphorus in the continuously flooded wetland.

The intermittently flooded wetland retained 70.7 kg N ha^-1 and 1 kg P ha^-1 while the permanently flooded wetland retained 62.1 kg N ha^-1 and 0.55kg P ha^-1. The wetland that was flooded intermittently showed a higher retention efficiency of nitrogen and phosphorus than the continuously flooded wetland.