Nitrogen transformations and balance in a constructed wetland for nutrient-polluted river water treatment using forage rice in Japan

A pilot-scale surface-flow wetland planted with a new rice variety (Oryza sativa 'Kusahonami') developed for livestock feed was constructed for treating nutrient-polluted river water. To calculate the balance between nitrogen removal and rice plant uptake of nitrogen, nitrogen removal from river water and nitrogen interactions among plants, soil water, and soil were investigated for this constructed wetland over two growing seasons in 2004 and 2005. The constructed wetland removed 33% of the total nitrogen entering with the river water. Rice plants were found to constitute the major nitrogen storage, with plant uptake being the major removal mechanism. The total inorganic nitrogen concentration in the rhizosphere changed seasonally because of plant uptake. Most nitrogen taken up by rice plants was contained in the aboveground biomass, with the mean amount being 34.0 g N m−2. However, the nitrogen balance calculation suggested that rice plants uptake some nitrogen from soil, decreasing the available nitrogen in the soil of the lined impermeable wetland.