Restored wetlands as filters to remove nitrogen

Four wetlands established in abandoned ricefields and dominated by Phragmites australis, Typha latifolia and Scirpus lacustris were used to improve the quality of agricultural runoff in the Ebro Delta (NE Spain) in 1993, 1994 and 1995. The wetlands were continuously flooded with water from a ricefield irrigation network during the growing season and received water with between 5 and 200 mg N m−2 d−1 in the form of dissolved inorganic nitrogen (DIN), between 0 and 67 mg N m−2 d−1 in the form of dissolved organic nitrogen (DON) and between 1.2 and 225 μg M−2 d−1 in the form of particulate nitrogen (PN). Surface N outflows contained between 0 and 12 mg N m−2 d−1 of DIN, between 1 and 86 mg N m−2 d−1 of DON and between 1 and 40 μg m−2 d−1 of PN. The nitrogen retention efficiency was always positive 100% of the input, except for DON and PN at low inlet loadings. The emergent macrophytes accumulated between 20 and 100 mg N m−2 d−1, which accounted for between 66 and 100% of the inflowing DIN. The removal rate constants calculated according to first-order plug-flow kinetics, were between 0.003–0.09 m day−1 for total nitrogen, and 0.005–0.3 m day−1 for DIN. Plant uptake, detritus accumulation and decomposition, and nitrogen recycling in the sediment are the major processes which could explain nitrogen retention in the wetlands. Wetlands restored from ricefields act as highly efficient water polishing filters for agricultural runoff and, at the same time, can contribute to the habitat biodiversity of large areas where rice is cultivated extensively.