HR: 0800h
AN: B21A-0855    [Abstracts]
TI: Apportionment of Nitrous Pxide Flux From a Successional Midwest Grassland to Denitrification and Nitrification Based on Isotopomers.
AU: * Ostrom, N E
EM: ostromn@msu.edu
AF: Department of Zoology, 203 Natural Science Building Michigan State University, East Lansing, MI 48824-1115 United States
AU: Sutka, R L
EM: sutkarob@msu.edu
AF: Department of Zoology, 203 Natural Science Building Michigan State University, East Lansing, MI 48824-1115 United States
AU: Grandy, A S
EM: grandya1@msu.edu
AF: Department of Crop and Soil Sciences, Kellogg Biological Station, KELLOGG BIOLOGICAL STATION 3700 E GULL LAKE DR, Hickory Corners, MI 49060 United States
AU: Hauzinga, K M
EM: huizing9@msu.edu
AF: Department of Microbiology, 203 Natural Science Building Michigan State University, East Lansing, MI 48824-4320 United States
AU: Ostrom, P H
EM: ostrom@msu.edu
AF: Department of Zoology, 203 Natural Science Building Michigan State University, East Lansing, MI 48824-1115 United States
AU: Robertson, G P
EM: robertson@kbs.msu.edu
AF: Department of Crop and Soil Sciences, Kellogg Biological Station, KELLOGG BIOLOGICAL STATION 3700 E GULL LAKE DR, Hickory Corners, MI 49060 United States
AB: Nitrous oxide (N$_{2}$O) is an important greenhouse gas that in agricultural environments may contribute more to global warming than CO$_{2}$. We investigated nitrogen cycling and N$_{2}$O flux following the initial tillage of a previously uncultivated successional Midwest grassland. This grassland has now been tilled annually for a total of 3 years. Tillage resulted in a 5-10-fold increase in NO$_{3}$-N; a 5-fold increase in nitrifier enzyme activity (0.61 vs. 3.04 mg N kg$^{-1}$ soil d$^{-1}$); and increased N$_{2}$O flux during the first two years. Following each tillage event, NH$_{4}$+ concentration increases, followed successively by increases in NO$_{3}$- concentration and N$_{2}$O flux. Soil trace gas flux chambers were placed on a tilled plot and a control plot that had not been tilled approximately 1.5 weeks after the third annual tillage. Samples for isotopic analysis of N$_{2}$O were taken over the course of one-hour chamber closings during four different times of day from early morning to evening. Fluxes of 14.6 to 22.8 g N$_{2}$O-N ha$^{-1}$ day$^{-1}$ were measured in the tilled treatment plot as compared to 0.6 to 1.9 g N$_{2}$O-N ha$^{-1}$ day$^{-1}$ in the non-tilled control treatment. Fluxes increased in the control and tilled plots and were highest at the afternoon (4 PM) and evening (8:30 PM) sampling points. The isotopomer composition of N$_{2}$O was evaluated to determine the relative importance of nitrification and denitrification to soil-derived N$_{2}$O. The bulk $\delta$$^{15}$N-N$_{2}$O from the tilled field decreased during each closing and ranged from - 10.2 to 1.3 $\permil$. The nitrogen isotopic composition in the $\alpha$, or central, N atom similarly decreased during each closing and ranged from - 4.7 to 11.2 $\permil$. Values for the isotopomer site preference ($\delta$$^{15}$N$\alpha$ - $\delta$$^{15}$N$\beta$) varied between 9.8 to 19.7 $\permil$ and decreases with time were also evident. Variation in isotopomer values during closings reflected mixing of atmospheric and soil-derived N$_{2}$O. Based on the measured site preference and prior knowledge of values for site preference for tropospheric N$_{2}$O (18.7 $\permil$) and for N$_{2}$O derived from denitrification (0 $\permil$) and nitrification (33 $\permil$) isotope-mixing models were used to determine soil-derived isotopomer values (Keeling plot) and to apportion soil-derived N$_{2}$O to nitrification and denitrification. Within the first closing in the tilled plot N$_{2}$O originated completely from denitrification whereas a mixture of 70$%$ and 30$%$ from denitrification and nitrification, respectively, was determined for the fourth closing.
DE: 1030 Geochemical cycles (0330)
DE: 1094 Instruments and techniques
DE: 1610 Atmosphere (0315, 0325)
DE: 0330 Geochemical cycles
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting