HR: 11:35h
AN: H51G-05    [PDF]
TI: Nitrogen Isotopic Fractionation by In-Stream Populations of Ammonia-Oxidizing Bacteria
AU: * Casciotti, K L
EM: kcasciot@usgs.gov
AF: US Geological Survey, 431 National Center, Reston, VA 20192 United States
AU: Voytek, M A
EM: mavoytek@usgs.gov
AF: US Geological Survey, 431 National Center, Reston, VA 20192 United States
AU: Bohlke, J K
EM: jkbohlke@usgs.gov
AF: US Geological Survey, 431 National Center, Reston, VA 20192 United States
AB: Nitrogen cycling in the upper Illinois River basin is heavily impacted by agriculture and the application of nitrogenous fertilizer. Nitrate concentrations in many of its tributaries in Indiana (e.g. Iroquois River, Sugar Creek) have large seasonal fluctuations, ranging from less than 10 micromolar in the late summer to greater than 650 micromolar in the spring. We have performed a series of experiments to study the impact of this seasonal influx of nitrogen on bacterial populations and corresponding biogeochemistry related to nitrogen cycling in small streams. In this study, microcosms were used to measure nitrogen isotopic fractionation during nitrification by natural communities of water-column bacteria amended with NH$_{4}$$^{+}$. The in-stream nitrifier communities were also studied using molecular biological techniques to expand a previously observed correspondence between ammonia monooxygenase sequence and isotopic fractionation. The isotope effect ($\epsilon$) for ammonia oxidation (14 permil) in June was calculated from a Rayleigh model based on changes in $\delta^{15}$NH$_{4}$$^{+}$ during the microcosm incubation. Supporting analyses indicated that NH$_{4}$$^{+}$ was oxidized primarily to NO$_{2}$$^{-}$. The evolution of the $\delta^{15}$N in the [NO$_{2}$$^{-}$ + NO$_{3}$$^{-}$] pool was also consistent with an $\epsilon$ of 14 permil for ammonia oxidation. This value is similar to previous reports of NH$_{4}$$^{+}$ fractionation in the Chesapeake Bay and for {\it Nitrosomonas} species cultured from low- NH$_{4}$$^{+}$ environments, but is significantly different from {\it Nitrosomonas} species cultured from eutrophic environments. These results introduce a relationship between microbial community structure and isotope fractionation that may be used to better understand nitrogen cycling in streams.
DE: 0400 Biogeosciences
DE: 1040 Isotopic composition/chemistry
DE: 1615 Biogeochemical processes (4805)
SC: Hydrology [H]
MN: 2003 Fall Meeting