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Abstract

The isotopic signatures of N-15 and O-18 in N2O emitted from tropical soils vary both spatially and temporally, leading to large uncertainty in the overall tropical source signature and thereby limiting the utility of isotopes in constraining the global N2O budget. Determining the reasons for spatial and temporal variations in isotope signatures requires that we know the isotope enrichment factors for nitrification and denitrification, the two processes that produce N2O in soils. We have devised a method for measuring these enrichment factors using soil incubation experiments and report results from this method for three rain forest soils collected in the Brazilian Amazon: soil with differing sand and clay content from the Tapajos National Forest (TNF) near Santarem, ParA, and Nova Vida Farm, Rondonia. The N-15 enrichment factors for nitrification and denitrification differ with soil texture and site: -111 parts per thousand +/- 12 parts per thousand and -31 parts per thousand +/- 11 parts per thousand for a clay-rich Oxisol (TNF), -102 parts per thousand +/- 5 parts per thousand and -45 parts per thousand +/- 5 parts per thousand for a sandier Ultisol (TNF), and -10.4 parts per thousand +/- 3.5 parts per thousand (enrichment factor for denitrification) for another Ultisol (Nova Vida) soil, respectively. We also show that the isotopomer site preference (delta N-15(alpha) - delta N-15(beta) where alpha indicates the central nitrogen atom and beta the terminal nitrogen atom in N2O) may allow differentiation between processes of production and consumption of N2O and can potentially be used to determine the contributions of nitrification and denitrification. The site preferences for nitrification and denitrification from the TNF-Ultisol incubated soils are: 4.2 parts per thousand +/- 8.4 parts per thousand and 31.6 parts per thousand +/- 8.1 parts per thousand, respectively. Thus, nitrifying and denitrifying bacteria populations under the conditions of our study exhibit significantly different N-15 site preference fingerprints. Our data set strongly suggests that N2O isotopomers can be used in concert with traditional N2O stable isotope measurements as constraints to differentiate microbial N2O processes in soil and will contribute to interpretations of the isotopic site preference N2O values found in the free troposphere.