Fractionation factors for stable isotopes of N and O during N2O reduction in soil depend on reaction rate constant

Nitrous oxide (N2O) 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 N2O 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 parts per thousand, -1.0 parts per thousand and -9.3 parts per thousand and for 0 -74.0 parts per thousand -6.9 parts per thousand and -26.3 parts per thousand, respectively. The ratio of 0 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. Copyright (c) 2007 John Wiley & Sons, Ltd.