Intramolecular Stable Isotope Ratios of N 2 O in the Tropical Swamp Forest in Thailand

Abstract

The dry-wet cycle is a common climatic phenomenon in the tropical regions of monsoon-affected Asia. An intermittent increase in N2O was clearly observed in the Bang Nara River, with a tropical swamp catchment at the beginning of rainy season of November 1997. The intramolecular distribution of isotopes clearly demonstrated that nitrification is the major process for the production of N2O. Using stable isotope data, the production mechanism of N2O in the study areas was found to vary in time and space with changes in the redox boundary along the water flow.

Distinguishing Nitrous Oxide Production from Nitrification and Denitrification on the Basis of Isotopomer Abundances

“…In this study, the average SPs of N 2 O produced during NH 2 OH oxidation by N. multiformis and N. europaea were similar (33.5 Ϯ 1.2‰ and 32.5 Ϯ 0.6‰, respectively). In addition, the SPs of N 2 O produced in N. europaea batch cultures with NH 4 ϩ and in concentrated cell suspensions were similar (31.4 Ϯ 4.2‰ and 33.5 Ϯ 1.2‰, respectively). This similarity is particularly startling given that the substrates were similar in ␦…”

“…This result indicates that bulk N isotope fractionation during nitrification occurs mainly during the conversion of NH 4 ϩ to NH 2 OH, and furthermore, that the SP is constant even though differences are evident in bulk ␦ 15 N fractionation. Thus, despite variations in substrates and physiological differences between the genera Nitrosomonas and Nitrosospira, SP values of 32 to 35‰ for N 2 O produced by NH 2 OH and NH 4 ϩ oxidation can be applied to ammonia-oxidizing organisms as a whole.…”

“…In addition, the SPs of N 2 O produced during NH 2 OH oxidation by N. multiformis and N. europaea were similar to those of N 2 O produced by M. trichosporium and M. capsulatus Bath. The results indicate that large SPs of 32 to 35‰ are characteristic of nitrification, regardless of whether it is catalyzed by a methane or NH 4 ϩ oxidizer. Site preference of N 2 O produced during denitrification.…”

“…Ostrom et al (18) proposed that the observed shifts in the ␦ 18 O value of N 2 O with depth in the ocean reflects a predominance of N 2 O derived from NH 4 ϩ oxidation, with the preponderance of N 2 O from NO 2 Ϫ reduction within a comparatively narrow depth interval. Our results are consistent with this dual-source interpretation, as N 2 O produced by nitrifier denitrification was markedly depleted in 18 O relative to that produced by NH 4 ϩ oxidation by N. europaea and NO 2 Ϫ reduction by N. multiformis (Fig.…”

“…Previous attempts to differentiate nitrification-and denitrification-mediated N 2 O production in soils using stable isotope approaches (4,20,30,31,32) relied on the observation that the fractionation factor associated with N 2 O production by denitrifiers (2) is substantially less than that associated with nitrification (34). The assumption was that N 2 O with a high ␦ 15 N value is indicative of denitrification, whereas a lower value reflects nitrification (4). However, this approach is problematic because the isotopic composition of the substrate (nitrate or ammonia) can vary temporally and spatially (20).…”