Processes Controlling the Stable Isotope Composition of N2O

Stable isotope compositions are presented in this chapter using <5 notation, where 5 = ((^ampie/^standard) - 1), and R = 15N/14N or 180/160. The standards used are atmospheric N2 for 15N/14N, and standard mean ocean water (SMOW) for 180/160. The S values are conveniently expressed in parts per thousand or per mil (%o).

Effect of Substrate Availability on the S15N of N20

Mass-dependent isotope effects occur during chemical transformations, with molecules containing the lighter isotope (14N) reacting faster than the molecules containing the heavier isotope (loN). As a consequence the product of a reaction is depleted in l3N relative to the isotope composition of the substrate pool. Assuming complete removal of products and no further inputs of substrate, the substrate pool will become enriched in the heavy isotope as the reaction proceeds over time. The change in the isotope composition of the substrate pool will in turn result in a progressive increase in the heavy isotope composition of the product pool as the substrate is consumed. Changes in the availability of substrates (ammonium and nitrate) for N2O production should cause, therefore, temporal shifts in the 515N of N2O produced in soils. This idea has been tested under field conditions by adding pulses of substrate (urine, fertilizer) to soil and monitoring temporal shifts in the isotope composition of N2O emitted from soils, as will be discussed below.

After urea fertilization in agricultural fields it has been observed that the final product of nitrification (NOJ) has a lower ¿l3N value than the substrate (NH^), and nitrate becomes more enriched in 15N as time progresses when substrate availability is limiting (Nadelhoffer and Fry, 1994). The <515N of N2O from fertilized soils and agricultural fields has also been shown to increase with time after soil amendments (Pérez et al., 2001; Yamulki et al., 2001). For example, in a subtropical area of Mexico the isotope composition of N2O emitted from an agricultural soil shifted dramatically over a 2-week period after urea fertilization and irrigation, with S l3N values ranging from highly depleted values (—46%o) during the first week when N2O emissions were high, to enriched <515N values (+5%o) at the end of the second week when emissions were low (Pérez et al., 2001). It was also observed that the <515N values of substrate NH|" changed from 2 to 25%o during the 2-week period. This suggests that the N2O isotopic composition changes are related to the substrate availability, if nitrification was the major pathway of N2O production. Although it is difficult to deduce that all the N2O emitted was derived from nitrification in this study, there was remarkable similarity

Figure 5,1 Comparison of measurements of the stable isotope composition of nitrous oxide and ammonium made in a field study with urea-fertilized soil (Perez et al., 2001) and a lab study using a pure culture of ammonium-oxidizing bacteria Nitrosomonas sp. (Ueda et al., 1999). The stable isotope data (%o) are plotted as a function of changes in the concentration of ammonium during the experiment (NH^ t), expressed relative to initial starting concentration of ammonium (NH^o)- For the field study the filled and open circles are the ¿■"'N-N2O and S O-NgO values, respectively; and the filled triangles are the S15N-NH+ values. The data for the incubation study of Ueda et al. (1999) are shown as regression lines; the black and dashed lines represent the ¿>15N-N20 and <5180-N20 values, respectively; and the dot-dash line represents the<515N-NH+ values.

Figure 5,1 Comparison of measurements of the stable isotope composition of nitrous oxide and ammonium made in a field study with urea-fertilized soil (Perez et al., 2001) and a lab study using a pure culture of ammonium-oxidizing bacteria Nitrosomonas sp. (Ueda et al., 1999). The stable isotope data (%o) are plotted as a function of changes in the concentration of ammonium during the experiment (NH^ t), expressed relative to initial starting concentration of ammonium (NH^o)- For the field study the filled and open circles are the ¿■"'N-N2O and S O-NgO values, respectively; and the filled triangles are the S15N-NH+ values. The data for the incubation study of Ueda et al. (1999) are shown as regression lines; the black and dashed lines represent the ¿>15N-N20 and <5180-N20 values, respectively; and the dot-dash line represents the<515N-NH+ values.

between these results and those of a study conducted with a pure culture of Nitrosomonas sp. (Ueda et al., 1999; Fig. 5.1), which strongly suggests that the N20 isotopic changes in the agricultural field were mostly a product of nitrification. Nitrosomonas bacteria are exclusively involved with nitrification. In another agricultural study the 515N value of N2O emitted from a soil fertilized with urine showed a change over a 24-hour period from —12 to 0%o (Yamulki et al, 2001). The magnitude of the emission rate did not correlate with the isotopic shift, but the enrichment was progressively higher with time. This suggests that substrate consumption led to a progressive enrichment of the produced N2O. Unfortunately, in this study no measurements were made of either the concentration or <515N of NH|" in order to support this conclusion.

Isotope Effects (<515N) During Diffusion of N2O in Soil

The effect of diffusion on the isotopic composition of N2O has been studied in Brazilian tropical rain forest soils (Perez et al, 2000). Soil air was collected at different depths and the <515N values of N2O in the soil air were measured. The authors found an enrichment of 15N in N2O at a depth interval of 75-100 cm and the S1''N values of N2O decreased in soil layers above and below this depth. The high <515N values in the 75-100 cm depth interval suggested that this was a zone of N2O production via denitrification and that N2O diffused both upward towards the surface and downward to greater soil depths from this zone. Molecular diffusion of N2O would be expected to have an isotope effect of 4.35%o for S15 N. The soil showed a difference in <515 N values of 4.6 ± 0.6%o between 75 cm and deeper soil layers, which was consistent with that expected from fractionation caused by molecular diffusion. Therefore the isotopic composition of the N2O emitted from these soils should reflect the <515N of N2O production and the isotopic shift due to molecular diffusion.

Isotope Effects (<515N) During Nitrification and Denitrification

As discussed above, normal mass-dependent effects result in products of a chemical reaction being depleted in heavy isotopes relative to the initial substrate pool. Consistent with this, it has been observed that N2O formed during nitrification is depleted in 15N relative to NH^ (Letolle 1980; Mariotti et al 1981; Yoshida, 1988), and that N2O produced during denitrification is depleted in 15N relative to NOg (Table 5.1). Since different bacterial populations are involved in nitrification and denitrification, the magnitudes of the isotope effects are different for these two production pathways. There is a much larger isotope effect for nitrification than for denitrification, when the isotopic composition of N2O is compared to that of the substrates ammonium and nitrate, respectively (Table 5.1; Fig. 5.2).

If it was assumed that the nitrogen cycle processes started with ammonium and proceeded in a linear sequence, NH^ -» N2O —>■ NOg —» N2O, the (515N of N2O produced by denitrification should be lower than that of nitrous oxide produced by nitrification, because of mass-dependent isotope fractionation. However, a number of studies have reported that N2O produced via denitrification has higher <515N values than that produced via nitrification (WahlenandYoshinari, 1985; Yoshida, 1988; Yoshinari and Koike, 1994; Webster and Hopkins, 1996; Barford, 1997; Yoshinari et al, 1997; Barford et al, 1999). The apparent contradiction can be resolved by considering two other isotope effects that operate simultaneously. First, as N2O is formed some may be lost from the soil via diffusion to the atmosphere. Fractionation during diffusion results in the remaining soil pool of N2O becoming enriched in 15N by approximately 4.35%o. The isotopic composition of the soil N2O pool is also influenced by the extent to which it is consumed to form nitrate (duringnitrification) andN2 (duringdenitrification).

Table 5.1 Nitrogen and Oxygen Isotope Effects for Several Reactions Involved in N2O Production and Consumption During Nitrogen Cycle Reactions

Stable isotope

Reaction

Organism

Isotope effect (%o)

Reference

15n

nh+ n20

Nitrosomonas europeae

-66.5 ± 2.3

Yoshida etal (1989)

Nitrosomonas sp.

-45.3 to -46.6 (range)

Ueda etal. (1999)

nh2oh -»• n2o

Nitrosomonas europeae

Methylococcus capsulatus (methane oxidizer)

-26.0 2.3

Sutka et al. (2003)

no2 -> n2o

Nitrosomonas europeae

-35.9

Sutka et al. (2003)

noj n2o

Paracoccus denitrificans

-28.6 ± 1.9

Barford et al. (1999)

Soil denitrifier

—27 to —16 (range)

Wada and Ueda (1996) (and references therein)

n2o -> n2

Paracoccus denitrificans

-13 ±2.6

Barford etal. (1999)

Soil denitrifier

-27

Wada and Ueda (1996)

180

no^ n20

Paracoccus denitrificans

-105

Barford (1997)

n2o ->■ n2

Pseudomonas aeruginosa

-37 to -42 (range)

Wahlen andYoshinari (1985)

The isotope effects are expressed as:

The isotope effects are expressed as:

where a is the ratio ßproduct/^substate» and/? = 15N/14N (or180/^0) for the reaction product and the reaction substrate. The negative isotope effects indicate that the reaction product is depleted in the heavy isotope relative to the reaction substrate.

Nitrification Denitrification step 1 Denitrif¡cation step 2

NHJ-iNgO N03-+N20 N20->N2

[NHil

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