title: Fine-scale in situ measurement of riverbed nitrate production and consumption in an armored permeable riverbed.
abstract: Alteration of the global nitrogen cycle by man has increased nitrogen loading in waterways considerably, often with harmful consequences for aquatic ecosystems. Dynamic redox conditions within riverbeds support a variety of nitrogen transformations, some of which can attenuate this burden. In reality, however, assessing the importance of processes besides perhaps denitrification is difficult, due to a sparseness of data, especially in situ, where sediment structure and hydrologic pathways are intact. Here we show in situ within a permeable riverbed, through injections of (15)N-labeled substrates, that nitrate can be either consumed through denitrification or produced through nitrification, at a previously unresolved fine (centimeter) scale. Nitrification and denitrification occupy different niches in the riverbed, with denitrification occurring across a broad chemical gradient while nitrification is restricted to more oxic sediments. The narrow niche width for nitrification is in effect a break point, with the switch from activity "on" to activity "off" regulated by interactions between subsurface chemistry and hydrology. Although maxima for denitrification and nitrification occur at opposing ends of a chemical gradient, high potentials for both nitrate production and consumption can overlap when groundwater upwelling is strong.
authors.affiliation: School of Geography, Queen Mary University of London , Mile End Road, London E1 4NS, United Kingdom.
authors.forename: Katrina
authors.initials: K
authors.lastname: Lansdown
authors.forename: Catherine M
authors.initials: CM
authors.lastname: Heppell
authors.forename: Matteo
authors.initials: M
authors.lastname: Dossena
authors.forename: Sami
authors.initials: S
authors.lastname: Ullah
authors.forename: A Louise
authors.initials: AL
authors.lastname: Heathwaite
authors.forename: Andrew
authors.initials: A
authors.lastname: Binley
authors.forename: Hao
authors.initials: H
authors.lastname: Zhang
authors.forename: Mark
authors.initials: M
authors.lastname: Trimmer
mesh_terms: D002712:Chlorides / Q000032:analysis; D058440:Denitrification; D004784:Environmental Monitoring / Q000379:methods*; D009566:Nitrates / Q000378:metabolism*; D058465:Nitrification; D009584:Nitrogen / Q000378:metabolism; D010539:Permeability; D016062:Porosity; D012044:Regression Analysis; D045483:Rivers / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D002712:Chlorides; D009566:Nitrates; D014867:Water; D009584:Nitrogen