Nitrous oxide (N(2)O) is a major greenhouse gas that is mainly produced but also reduced by microorganisms in soils. We determined factors for N and O isotope fractionation during the reduction of N(2)O to N(2) in soil in a flow-through incubation experiment. The absolute value of the fractionation factors decreased with increasing reaction rate constant. Reaction rates constants ranged from 1.7 10(-4) s(-1) to 4.5 10(-3) s(-1). The minimum, maximum and median of the observed fractionation factors were for N -36.0 per thousand, -1.0 per thousand and -9.3 per thousand and for O -74.0 per thousand, -6.9 per thousand and -26.3 per thousand, respectively. The ratio of O isotope fractionation to N isotope fractionation was 2.4 +/- 0.3 and it was independent from the reaction rate constants. This leads us to conclude that fractionation factors are variables while their ratio in this particular reaction might be a constant.

Abstract. Soils are capable to consume N 2 O. It is generally assumed that consumption occurs exclusively via respiratory reduction to N 2 by denitrifying organisms (i.e. complete denitrification). Yet, we are not aware of any verification of this assumption. Some N 2 O may be assimilatorily reduced to NH 3 . Reduction of N 2 O to NH 3 is thermodynamically advantageous compared to the reduction of N 2 . Is this an ecologically relevant process? To find out, we treated four contrasting soil samples in a flow-through incubation experiment with a mixture of labelled (98%)

Abstract. Soils are capable to consume N2O. It is generally assumed that consumption occurs exclusively via respiratory reduction to N2 by denitrifying organisms (i.e. complete denitrification). Yet, we are not aware of any verification of this assumption. Some N2O may be assimilatorily reduced to NH3. Reduction of N2O to NH3 is thermodynamically advantageous compared to the reduction of N2. Is this an ecologically relevant process? To find out, we treated four contrasting soil samples in a flow-through incubation experiment with a mixture of labelled (98%) 15N2O (0.5–4 ppm) and O2 (0.2–0.4%) in He. We measured N2O consumption by GC-ECD continuously and δ15N of soil organic matter before and after an 11 to 29 day incubation period. Any 15N2O assimilatorily reduced would have resulted in the enrichment of soil organic matter with 15N, whereas dissimilatorily reduced 15N2O would not have left a trace. None of the soils showed a change in δ15N that was statistically different from zero. A maximum of 0.27 % (s.e. ±0.19%) of consumed 15N2O may have been retained as 15N in soil organic matter in one sample. On average, 15N enrichment of soil organic matter during the incubation may have corresponded to a retention of 0.019% (s.e. ±0.14%; n=4) of the 15N2O consumed by the soils. We conclude that assimilatory reduction of N2O plays, if at all, only a negligible role in the consumption of N2O in soils.

Reduction of nitrous oxide (N 2 O) is an autonomous respiratory pathway. Nitrous oxide is an alternative electron acceptor to O 2 when intensive biological activity and reduced diffusivity result in an O 2 deficit. Hypoxic or anoxic micro sites may form even in well-aerated soils, and provide a sink for N 2 O diffusing through the gas-filled pore space. We reproduced similar in vitro conditions in suboxic (0.15% O 2 ) flow-through incubation experiments with samples from a Stagnosol and from a Histosol. Apparent half-saturation constants (k m ) for N 2 O reduction were similar for both soils and were, on average, 3.8 mmol mol À1 at 5°C, 5.1 mmol mol À1 at 10°C, and 6.9 mmol mol À1 at 20°C. Respiration of N 2 O was estimated to contribute a maximum proportion of 1.7% to total respiration in the Stagnosol (pH 7.0) and 0.9% in the Histosol (pH 2.9).