Fate of Nitrogen and Phosphorus in a Waste‐water Retention Reservoir Containing Aquatic Macrophytes

<jats:title>Abstract</jats:title><jats:p>Potential use of retention/detention reservoirs stocked with vascular aquatic macrophytes was evaluated, using a microcosm reservoir for reducing the N and P levels of agricultural drainage effluents (waste water). The treatments evaluated were reservoirs stocked with (i) pennywort (<jats:italic>Hydrocotyle umbellata</jats:italic> L.), (ii) water hyacinth (<jats:italic>Eichhornia crassipes</jats:italic> [Mart] Solms), (iii) cattails (<jats:italic>Typha latifolia</jats:italic> L.) and elodea (<jats:italic>Egeria densa</jats:italic> P), and (iv) control (no macrophytes). Labeled <jats:sup>15</jats:sup>N was used to differentiate preferential uptake of <jats:sup>15</jats:sup>NH<jats:sub>4</jats:sub><jats:sup>+</jats:sup> and <jats:sup>15</jats:sup>NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>, and to follow the fate of added <jats:sup>15</jats:sup>NH<jats:sub>4</jats:sub><jats:sup>+</jats:sup> and <jats:sup>15</jats:sup>NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>.</jats:p><jats:p>Results showed that 34 to 40% of the added inorganic <jats:sup>15</jats:sup>N (<jats:sup>15</jats:sup>NH<jats:sub>4</jats:sub><jats:sup>+</jats:sup> + <jats:sup>15</jats:sup>NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>) was removed through plant uptake, while 45 to 52% of the added <jats:sup>15</jats:sup>N was unaccounted for, presumably lost through NH<jats:sub>3</jats:sub> volatilization and nitrification‐denitrification processes. In the control reservoir, algal biomass removed 4.4% of added <jats:sup>15</jats:sup>N, while 41% of the added <jats:sup>15</jats:sup>N was not accounted. Pennywort and cattail‐elodea systems were found to be most effective, with about 50% inorganic N removal in a 4‐day detention period. All aquatic macrophytes preferred <jats:sup>15</jats:sup>NH<jats:sub>4</jats:sub><jats:sup>+</jats:sup> over <jats:sup>15</jats:sup>NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>, but the difference in uptake was not significant, except for pennywort and cattails, which removed 84 and 92% of the added <jats:sup>15</jats:sup>NH<jats:sub>4</jats:sub><jats:sup>+</jats:sup> as compared to 16 and 8% of the added <jats:sup>15</jats:sup>NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>, respectively. About 25 to 29 d were required by the systems with macrophytes to remove 50% of the wastewater P. Plant removal of P was in the range of 3 to 65% of added P, while 7 to 87% of the added P was lost through precipitation and adsorption reactions.</jats:p>