Can N₂O stable isotopes and isotopomers be useful tools to characterize sources and microbial pathways of N₂O production and consumption in tropical soils?

Abstract

Nitrous oxide (N2O) is an important greenhouse gas in which the main sources are tropical rainforest and agricultural soils. N2O is produced in soils by microbial processes, which are enhanced by the application of nitrogenous fertilizers. The soil N2O bulk isotopic composition (δ15Nbulk and δ18O) and the “site‐specific,” or intramolecular, 15N isotopic composition, i.e., the 15N/14N ratio at the cenral (α) or terminal (β) nitrogen position, expressed in this study as δ15Nα and δ15Nβ could help identify both the sources (natural and anthropogenic) and microbial pathways of N2O production and consumption prior to emission.We report new isotope measurements of soil N2O emissions and from soil air collected during the rainy season in a mature tropical forest (Tapajos National Forest, Para, Brazil) and in a tropical agricultural corn field (“Fundo Tierra Nueva,” Guárico State, Venezuela). The statistically different δ15Nbulk emission weighted average between the mature forest (−18.0‰ ± 4.0‰, n = 6) and agricultural corn field (−34.3‰ ± 12.4‰, n = 17) suggest that the δ15Nbulk data are useful for distinguishing N2O fluxes from fertilized agricultural and natural “background” soils. They also demonstrate that the site‐specific δ15N measurements have the potential to provide a new tool to differentiate between the production and consumption N2O microbiological processes in soils. This study further demonstrates that the observed correlations (or lack thereof) between δ15Nα, δ15Nβ, and δ18O can be used to estimate the relative proportion of N2O that would have been emitted to the air but was consumed via reduction of N2O to N2 within the soil.

Constraining N<sub>2</sub>O emissions since 1940 using firn air isotope measurements in both hemispheres

“…However, the latest IPCC Assessment Report (Ciais et al, 2013) for the first time separated oceanic emissions into a natural component and an anthropogenic component, e.g., due to atmospheric N deposition to rivers (Syakila and Kroeze, 2011;Duce et al, 2008;Kroeze et al, 2005). The oceanic fraction of the anthropogenic source was estimated as 1 Tg year −1 N. N 2 O emitted from agricultural soils and biomass burning is more depleted in δ 15 N av and δ 18 O than the tropospheric background (Park et al, 2011;Goldberg et al, 2010;Ostrom et al, 2010;Tilsner et al, 2003;Pérez et al, 2001Pérez et al, , 2000, while N 2 O emitted from other minor sources, such as automobiles, coal combustion, and industry, has values closer to tropospheric N 2 O values (Syakila and Kroeze, 2011;Toyoda et al, 2008;Ogawa and Yoshida, 2005a, b). An increase in strongly depleted agricultural emissions in the first part of our reconstruction, followed by a decreasing relative contribution from agriculture and increasing contributions from more enriched sources like industry, automobiles, and coal combustion, could qualitatively explain the reconstructed changes in isotope signatures of both the total source and the anthropogenic component.…”

Isotopic signatures of N2O produced by ammonia-oxidizing archaea from soils

“…Studies using pure AOB cultures have indicated that a positive SP (30-38%) is consistent with a N 2 O source from hydroxylamine oxidation (ammonia oxidation), whereas nitrifier denitrification (and denitrification) is thought to produce N 2 O with a SP value near or below zero (-10 to 0%) (Frame and Casciotti, 2010;Sutka et al, 2006). The N 2 O emissions from various soils have positive (0-35%) SP signatures (Pérez et al, 2001;Park et al, 2011Park et al, , 2012Toyoda et al, 2011), which indicates that ammonia oxidation is an important source of soil N 2 O. The SP values of soil AOA strains (MY1-3 and JG1) were distinct and they ranged from 20 to -30%, except strain CS (Table 2).…”

Continuous measurements of nitrous oxide isotopomers during incubation experiments

“…However, occasionally (e.g., D-O event 15 and 17) the N 2 O concentration increases long before the onset of the dramatic temperature change (Schilt et al, 2010), providing a potential early warning for rapid climate change. Isotopomers of N 2 O provide information on the sources (Pérez et al, 2000(Pérez et al, , 2001Park et al, 2011), i.e., whether N 2 O originates predominately from nitrification or denitrification processes. As the conditions/circumstances leading to emissions from the two processes differ both for the marine and terrestrial sources, measuring isotopomers potentially improves our understanding of the climate conditions leading to the release of N 2 O over rapid climatic changes.…”