6 Mar 2003 18 24 AR AR182 EA31 10 tex AR182 EA31 10 sgm LaTeX2e 2002 01 18 P1 GJB 10 1146 annurev earth 31 110502 080901 Annu Rev Earth Planet Sci 2003 31 329 56 doi 10 1146 annurev earth 31 110502 080901 c 2003 by Annual Reviews All rights reserved Copyright First published online as a Review in Advance on January 8 2003 PRODUCTION ISOTOPIC COMPOSITION AND ATMOSPHERIC FATE OF BIOLOGICALLY PRODUCED NITROUS OXIDE Lisa Y Stein1 and Yuk L Yung2 1 Department of Environmental Sciences University of California Riverside California 92521 email lisa stein ucr edu 2 Division of Geological and Planetary Sciences California Institute of Technology Pasadena California 91125 email yly gps caltech edu Key Words microorganisms isotopic discrimination photolysis greenhouse gas global budget Abstract The anthropogenic production of greenhouse gases and their consequent effects on global climate have garnered international attention for years A remaining challenge facing scientists is to unambiguously quantify both sources and sinks of targeted gases Microbiological metabolism accounts for the largest source of nitrous oxide N2O mostly due to global conversion of land for agriculture and massive usage of nitrogen based fertilizers A most powerful method for characterizing the sources of N2O lies in its multi isotope signature This review summarizes mechanisms that lead to biological N2O production and how discriminate placement of 15N into molecules of N2O occurs Through direct measurements and atmospheric modeling we can now place a constraint on the isotopic composition of biological sources of N2O and trace its fate in the atmosphere This powerful interdisciplinary combination of biology and atmospheric chemistry is rapidly advancing the closure of the global N2O budget INTRODUCTION Nitrous oxide N2O is a potent greenhouse gas and the major source of stratospheric reactive nitrogen species that participate in the catalytic destruction of ozone It has an atmospheric residence time of 118 25 years Minschwaner et al 1998 Olsen et al 2001 The infrared radiative forcing of N2O is about 206 times that of CO2 on a per molecule basis at present atmospheric conditions IPCC 2001 This is illustrated in Figure 1 which shows the blackbody radiation of an atmosphere at 273 K and the infrared bands of N2O The fundamental bands are 2 1 and 3 at 589 1285 and 2224 cm 1 respectively the most important for greenhouse warming of the atmosphere is the 1 band at 7 78 m Goody Yung 1989 0084 6597 03 0519 0329 14 00 329 6 Mar 2003 18 24 330 AR STEIN AR182 EA31 10 tex AR182 EA31 10 sgm LaTeX2e 2002 01 18 P1 GJB YUNG Figure 1 a Radiance due to a blackbody at 273 K as a function of wavenumber The units are in W m2 cm 1 sr where sr is steradian b Absorption coefficient for N2O in the infrared The strongest absorption bands are 2 1 and 3 at 589 1285 and 2224 cm 1 respectively The others are overtone or combination bands Data taken from the HITRAN high resolution transmission molecular absorption database as described in Goody Yung 1989 There has been a steady increase in atmospheric N2O since the Industrial Revolution as shown in Figure 2 IPCC 2001 The early data are deduced from gases trapped in air bubbles in polar ice Over the recent few decades the concentration of N2O has been accumulating in the atmosphere see inset of Figure 2 at a rate of 0 6 ppb year 1 adding to the current concentration of about 314 ppb parts per billion by volume IPCC 2001 The radiative forcing approximately on a linear scale for small perturbations is plotted on the right axis There has been an increase of about 0 15 Wm 2 since the Industrial Revolution For comparison the radiative forcing due to CO2 over the same period is about 1 4 Wm 2 Although many sources of N2O have been identified including agricultural soils oceans animal waste biomass burning fuel combustion and industry the combined strength of these sources calculated in 1994 only matched two thirds of the known sinks which are removal and accumulation in the atmosphere IPCC 2001 Upon re evaluation of 6 Mar 2003 18 24 AR AR182 EA31 10 tex AR182 EA31 10 sgm LaTeX2e 2002 01 18 GLOBAL BUDGET OF NITROUS OXIDE P1 GJB 331 Figure 2 N2O mixing ratio for the past 1000 years as determined from ice cores firn and whole air samples Datasets are from Battle et al 1996 Flu ckiger et al 1999 Langenfields et al 1996 Machida et al 1995 and Steele et al 1996 Radiative forcing approximately on a linear scale is plotted on the right axis Deseasonalized global averages are plotted in the inset Butler et al 1998 Figure taken from IPCC 2001 the strength of each source it was found that the application of nitrogen based fertilizers to agricultural soils accounted for a much larger source of N2O than previously thought Mosier Kroeze 1998 Nevison Holland 1997 By 1998 the balance between the total sources and total sinks of N2O added up to a budget of 16 Tg N year 1 IPCC 2001 However large uncertainty remains as to the actual strength of individual sources of N2O Table 1 Increased fertilizer usage over the past several years has stimulated the rates of microbial metabolism especially those processes involved in the nitrogen cycle Figure 3 Nitrification and denitrification are the primary metabolic activities leading to N2O production However several physiological and ecological feedbacks can greatly influence the rates of these metabolic pathways One of the purposes of this review is to examine the diversity of enzymatic organismal community and environmental factors that are simultaneously involved in producing N2O Once we achieve an understanding of the complexity of N2O production by microbial communities the next goal is to quantify how much N2O is produced by nitrification and denitrification under a particular set of environmental conditions To accomplish this feat many researchers are attempting to track discrete sources of N2O accumulating in the atmosphere by using differential isotopic signatures of kNmNnO as produced by specific microbial communities This concept necessarily relies on enzymatic organismal community and environmental factors that influence the isotopic ratios and intramolecular site preference of 15N Here we review current research on organisms and enzymes within the microbial 6 Mar 2003 18 24 332 AR STEIN AR182 EA31 10 tex AR182 EA31 10 sgm LaTeX2e 2002 01 18 P1 GJB YUNG TABLE 1 Sources of N2O in Tg N year 1 Source Anthropogenic sources Agricultural soils Biomass burning Industrial