An on-line method for S-isotope analysis is described. Samples are combusted in an elemental analyzer. SO2 is separated from other combustion gases by gas chromatography, and the gases enter the ion source of the mass spectrometer through a split interface. Integrated peak areas for 32S02+ and 34SC>2+ are compared to the response for a standard gas sample to determine the S^S value. 534S values of samples analyzed using the on-line method correspond linearly with those achieved from the same sample prepared off-line, where Kiba reduction followed by oxidation of the sulfur to SO2 is carried out prior to S-isotope analysis against a known standard. With the described on-line method, the amount of sulfur necessary for S-isotope analysis is reduced to about 10 #tg of S per analysis. The time needed for on-line preparation and measurement is less than one-third of the off-line procedure.Stable sulfur-isotope analysis is often associated with many problems concerning sample preparation and mass spectrometric determination. The 34S/32S ratios are most commonly determined after SO2 has been generated out of natural materials. Sample preparation always requires several chemical transformation steps to finally produce SO2 out of the S-containing compounds. The standard preparation techniques used for most sulfur-bearing samples start with conversion of all sulfur to BaSO*, which is then reduced to H2S by one of the following three procedures: graphite reduction at 1000 °C,' reduction with tin(II)-strong phosphoric acid (Kiba's reagent) at 300 °C in a stream of nitrogen,2 or application of a HI-H3PO4-HCI reduction solution.3 The generated H2S is converted to Ag2S, which is finally oxidized to SO2. One major disadvantage of these methods is that they all need relatively large amounts of the original material to obtain sufficient sulfur as the numerous chemical processes require rather high amounts of sulfur (3-7 mg of S). Although

Abstract. Stable isotopic analyses of soil-emitted N2O (δ15Nbulk, δ18O and δ15Nsp = 15N site preference within the linear N2O molecule) may help to quantify N2O reduction to N2, an important but rarely quantified process in the soil nitrogen cycle. The N2O residual fraction (remaining unreduced N2O, rN2O) can be theoretically calculated from the measured isotopic enrichment of the residual N2O. However, various N2O-producing pathways may also influence the N2O isotopic signatures, and hence complicate the application of this isotopic fractionation approach. Here this approach was tested based on laboratory soil incubations with two different soil types, applying two reference methods for quantification of rN2O: helium incubation with direct measurement of N2 flux and the 15N gas flux method. This allowed a comparison of the measured rN2O values with the ones calculated based on isotopic enrichment of residual N2O. The results indicate that the performance of the N2O isotopic fractionation approach is related to the accompanying N2O and N2 source processes and the most critical is the determination of the initial isotopic signature of N2O before reduction (δ0). We show that δ0 can be well determined experimentally if stable in time and then successfully applied for determination of rN2O based on δ15Nsp values. Much more problematic to deal with are temporal changes of δ0 values leading to failure of the approach based on δ15Nsp values only. For this case, we propose here a dual N2O isotopocule mapping approach, where calculations are based on the relation between δ18O and δ15Nsp values. This allows for the simultaneous estimation of the N2O-producing pathways' contribution and the rN2O value.

The study reveals that for future research on N2O isotopocules, standardisation against N2O reference material is essential to improve interlaboratory compatibility. For atmospheric monitoring activities, we suggest N2O in whole air as a unifying scale anchor.

Our results confirm that SP of N2O is a promising tool to differentiate between fungal and bacterial N2O from denitrification. Modelling of oxygen isotope fractionation processes indicated that the contribution of the NO2(-) and NO reduction steps to the total oxygen exchange differed among the various fungal species studied. However, more information is needed about different biological orders of fungi as they may differ in denitrification enzymes and consequently in the SP and δ(18)O values of the N2O produced.

N 2 O gas is involved in global warming and ozone depletion. The major sources of N 2 O are soil microbial processes. Anthropogenic inputs into the nitrogen cycle have exacerbated these microbial processes, including nitrification. Ammonia-oxidizing archaea (AOA) are major members of the pool of soil ammonia-oxidizing microorganisms. This study investigated the isotopic signatures of N 2 O produced by soil AOA and associated N 2 O production processes. All five AOA strains (I.1a, I.1a-associated and I.1b clades of Thaumarchaeota) from soil produced N 2 O and their yields were comparable to those of ammonia-oxidizing bacteria (AOB). The levels of site preference (SP), d 15 N bulk and d 18 O -N 2 O of soil AOA strains were 13-30%, À 13 to À 35% and 22-36%, respectively, and strains MY1-3 and other soil AOA strains had distinct isotopic signatures. A 15 N-NH 4 þ -labeling experiment indicated that N 2 O originated from two different production pathways (that is, ammonia oxidation and nitrifier denitrification), which suggests that the isotopic signatures of N 2 O from AOA may be attributable to the relative contributions of these two processes. The highest N 2 O production yield and lowest site preference of acidophilic strain CS may be related to enhanced nitrifier denitrification for detoxifying nitrite. Previously, it was not possible to detect N 2 O from soil AOA because of similarities between its isotopic signatures and those from AOB. Given the predominance of AOA over AOB in most soils, a significant proportion of the total N 2 O emissions from soil nitrification may be attributable to AOA.