Holtan-Hartwig et al. 2002

Holtan-Hartwig L, Dörsch P, Bakken LR. 2002. Low temperature control of soil denitrifying communities: kinetics of N2O production and reduction. Soil Biology & Biochemistry 34: 1797-1806.

These authors measured the activation energies of N2O production and reduction in soils taken from agricultural settings in Finland, Sweden, and Germany. The underlying observation is that temperate soils show an unexpectedly large emission profile of N2O in late winter and early spring. Other authors have attributed this release to freeze-thaw effects, such as release of N2O trapped in frozen soils. Differences in activation energies could also explain these observations if these activation energies are asymmetrical at low temperatures, such that the activation energy of N2O reduction is much higher than that for N2O production.

N2O is both a greenhouse gas and an ozone-layer depleter. From soils, it is produced in the penultimate step in a series of reactions known collectively as the denitrification pathway: these reactions when run to completion convert nitrate (NO3-) to N2. N2 of course represents a net loss of nitrogen from an ecosystem, since it is no longer available to organisms. However, N2 is utterly harmless, while N2O has important physical effects on the atmosphere. The basic biochemistry and temperature response of this pathway is described in Firestone (1982).

Another underlying observation for this study is that the product ratio of N2O/N2 increases with decreasing temperature; in other words, proportionally more N2O is released from the system compared to N2. If N2O reduction (to N2) has a higher activation energy than N2O production (from NO), this would explain this observation. Another possibility is that the enzyme responsible for N2O reduction is strongly inhibited at some critical low temperature threshold.

The laboratory analyses carried out for this paper are strongly divergent from field conditions, involving anaerobic slurries with an excess of electron acceptors and the removal of NO3-. However, the differences observed between the different soils support other conclusions that N2O emissions from soils varies strongly with soil types and soil sources.

Previously reported activation energies for NO3- loss by denitrification range between about 41 and 89 kJ/mol; a similar range of activation energies was found here for both N2O production and reduction. This suggests that asymmetrical activation energies are not driving the observed changes in N2O flux by season. It seems temperatures close to 0ºC represent a particular challenge to the microbial communities of these soils, but the nature of this challenge remains unclear – this study did not examine community dynamics in any detail. Some details of the methods of preparing soils used here may be important in this regard.

Another possibility, not mutually exclusive with this threshold effect, is that strong decreases in metabolic rates at low temperatures (60-70% per 10ºC) combined with weak decreases in N2O diffusion rates (20-25% per 10ºC in water) allow N2O to escape the biological pathway as temperatures approach zero.

This paper was recommended to me by Dr. Steven Siciliano, as a guide to some of the calculations and comparisons we are doing with soils and greenhouse gases (including N2O) from Alexandra Fjord.