NITROUS OXIDE NITRIFICATION AND DENITRIFICATION¹⁵N ENRICHMENT FACTORS FROM AMAZON FOREST SOILS

Abstract

The isotopic signatures of 15N and 18O 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 Santarém, Pará, and Nova Vida Farm, Rondônia. The 15N enrichment factors for nitrification and denitrification differ with soil texture and site: -111 per thousand +/- 12 per thousand and -31 per thousand +/- 11 per thousand for a clay-rich Oxisol (TNF), -102 per thousand +/- 5 per thousand and -45 per thousand +/- 5 per thousand for a sandier Ultisol (TNF), and -10.4 per thousand +/- 3.5 per thousand (enrichment factor for denitrification) for another Ultisol (Nova Vida) soil, respectively. We also show that the isotopomer site preference (delta15Nalpha - delta15Nbeta, 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 per thousand +/- 8.4 per thousand and 31.6 per thousand +/- 8.1 per thousand, respectively. Thus, nitrifying and denitrifying bacteria populations under the conditions of our study exhibit significantly different 15N 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.

Stable nitrogen isotope patterns of trees and soils altered by long-term nitrogen and phosphorus addition to a lowland tropical rainforest

“…The remaining nitrate pool becomes 15 N-and 18 O-enriched because the lighter isotopomers are enzymatically favored by denitrifiers (Pérez et al 2006;Wunderlich et al 2013). In the Gigante Fertilization Experiment, positive correlations between d 15 N and d 18 O values of soil nitrate following N addition provide evidence for the imprint of gaseous losses on soil nitrate ( Supplementary Fig.…”

“…Foliar and total soil d 15 N values show strong latitudinal gradients (Martinelli et al 1999;Amundson et al 2003), promoted by enhanced loss of isotopically light forms of N (Pérez et al 2006) and more rapid microbial processing at lower latitudes (Kramer et al 2003;Dijkstra et al 2008). Once thought common to industrialized regions alone, isotopic signals of atmospheric N deposition have now been detected in plant tissue from tropical regions as well (Hietz et al 2010(Hietz et al , 2011.…”

Deforestation of watersheds of Panama: nutrient retention and export to streams

“…There is the additional imponderable associated with within-ecosystems losses via denitrification, which could also affect ''leakiness'' of these tropical ecosystems. It is clear that future work needs to address N-cycle rates before we fully understand function of these affected ecosystems (Houlton et al 2006;Perez et al 2006;Holtgrieve et al 2006;Barron et al 2009;Reed et al 2011;Wurzburger et al 2012).…”

Effect of soil saturation on denitrification in a grassland soil

“…Moreover, δ 18 O, δ 15 N and SP of emitted N 2 O depend on the denitrification product ratio (N 2 O / (N 2 +N 2 O)) and hence provide insight into the dynamics of N 2 O reduction (Well and Flessa, 2009;LewickaSzczebak et al, 2014LewickaSzczebak et al, , 2015. Koster et al (2013), for example, recently reported δ 15 N bulk values of N 2 O between −36.8 and −31.9 ‰ under the conditions of their experiment, which are indicative of denitrification according to Perez et al (2006) and Well and Flessa (2009), who proposed the range −54 to −10 ‰ relative to the substrate. Baggs (2008) summarised that values between −90 and −40 ‰ are indicative of nitrification.…”