Nitrogen removal and cycling in restored wetlands used as filters of nutrients for agricultural runoff

Four restored wetlands dominated by Phragmites australis, Typha latifolia and Scirpus lacustris were used to improve the quality of agricultural runoff in the Delta of the Ebro River (NE Spain) in 1993. The wetlands were continuously flooded with water from a ricefield irrigation network during the growing season and received water with between 0-270 mg m⁻²d⁻¹ of total nitrogen, 29-105 mg m⁻²d⁻¹ of dissolved inorganic nitrogen and 0-27 mg m⁻²d⁻¹ of dissolved organic nitrogen. Surface outflows contained between 0-80 mg m⁻²d⁻¹ of total nitrogen, 0-12 mg m⁻²d⁻¹ of dissolved inorganic nitrogen and 1-19 mg m⁻²d⁻¹ of dissolved organic nitrogen. The nitrogen retention efficiency was close to 100% of the input, except for dissolved organic nitrogen at the end of the growing season. The denitrification rates measured by the acetylene reduction in the sediment ranged between 0 and 3.46 mg N m⁻² d⁻¹ and represented between 0 and 12% of the inflowing dissolved inorganic nitrogen. Emergent macrophytes accumulated between 20 and 100 mg N m⁻² d⁻¹, which accounts for between 66 and 100% of the inflowing dissolved inorganic nitrogen. The wetland sediment accumulated between 111 and 250 mg N m⁻² d⁻¹ during the six month growing season. The removal rate constants calculated according to a first - order plug - flow kinetics, were between 0.01-0.075 m d⁻¹ for total nitrogen and 0.01-0.3 m d⁻¹ for dissolved inorganic nitrogen. Plant uptake, detritus accumulation and decomposition, and nitrogen recycling in the sediment are major processes for nitrogen retention and recycling in the wetlands. This type of wetlands, restored from ricefields, act as highly efficient water polishing filters for agricultural runoff and, at the same time, can contribute to increase the habitat biodiversity of large areas where rice is cultivated extensively.