title: Nitrogen removal in a small constructed wetland: an isotope mass balance approach.
abstract: The nitrogen (N) removal potential of constructed wetlands is increasingly used to lower the N load from agricultural nonpoint sources to inland and coastal waters. To determine the removal efficiency and key factors limiting wetland N removal, N fluxes were studied in a small constructed wetland in Central Switzerland. With an isotope mass balance approach integrating the natural isotope signature of nitrate (NO3-, ammonium (NH4+), and particulate nitrogen (PN), the N transformations such as assimilation, mineralization, nitrification, and denitrification were quantified. On average, the wetland removed 45 g m(-2) yr(-1) N during the studied 2.5 years, corresponding to a removal efficiency of 27%. Denitrification contributed 94% to the N removal, while only 6% of the removed N accumulated in the sediments. Denitrification was most efficient during periods with an oxic water column overlying anoxic sediments, as NH4+ released during mineralization of sediment organic matter was completely nitrified and subsequently denitrified at the sediment-water interface. During water column anoxia, NH4+ accumulated in the water and fueled assimilation by duckweed and internal recycling. The NO3-N isotope signature in the wetland mainly reflected the mineralization of sediment organic matter and subsequent nitrification, while denitrification at the sediment-water interface produced no fractionation.
authors.affiliation: Swiss Federal Institute of Aquatic Science and Technology (Eawag) and Swiss Federal Institute of Technology (ETH), CH-6047 Kastanienbaum, Switzerland. miriam.reinhardt@eawag.ch
authors.forename: Miriam
authors.initials: M
authors.lastname: Reinhardt
authors.forename: Beat
authors.initials: B
authors.lastname: Müller
authors.forename: René
authors.initials: R
authors.lastname: Gächter
authors.forename: Bernhard
authors.initials: B
authors.lastname: Wehrli
mesh_terms: D000641:Ammonia / Q000032:analysis / Q000378:metabolism; D001673:Biodegradation, Environmental; D001711:Biotransformation; D003247:Conservation of Natural Resources; D017753:Ecosystem; D019015:Geologic Sediments / Q000737:chemistry; D008903:Minerals / Q000737:chemistry / Q000378:metabolism; D009573:Nitrites / Q000737:chemistry / Q000378:metabolism; D009584:Nitrogen / Q000302:isolation & purification* / Q000378:metabolism; D009587:Nitrogen Isotopes; D009930:Organic Chemicals / Q000302:isolation & purification / Q000378:metabolism; D012621:Seasons; D013557:Switzerland; D013696:Temperature; D014867:Water / Q000737:chemistry; D014872:Water Movements; D018508:Water Purification / Q000379:methods*
chemical_list: D008903:Minerals; D009573:Nitrites; D009587:Nitrogen Isotopes; D009930:Organic Chemicals; D014867:Water; D000641:Ammonia; D009584:Nitrogen