Regulation of N2O emissions from agricultural soil on the transcriptional level
Final Report Abstract
Denitrifying bacteria in agricultural soil contribute both to the production and to the consumption of the greenhouse gas nitrous oxide (N2O), which is formed as an intermediate of the stepwise reduction from nitrate (NO3-) to dinitrogen gas (N2). Efficiency and gaseous end product composition (N2O versus N2) of the denitrification cascade crucially depend on 1) composition of the denitrifying community and 2) activities of the gene products that mediate the four reaction steps. These two factors in turn depend on environmental conditions including the availability of substrates such as organic carbon and nitrate. In this study, microcosm experiments were used to analyze how different fertilization regimes that bring along different environmental conditions influence denitrification rates, expression of denitrification genes and composition of denitrifying communities. Due to huge variations in RNA extracts, it was not possible to quantify the expression of denitrification genes in a reliable way. In the following, the research question was addressed on the DNA level and it was studied how potential denitrification activities and sizes of denitrifying communities develop over time when microcosms filled with slurries made of two different agricultural soils receive regular spikes of nitrate and different carbon sources. It was found that spikes of low carbon concentration (166 mg C per g dry weight soil) were associated with an increase in potential denitrification activity over time while spikes of a tenfold higher carbon concentration brought along a decrease in potential denitrification activity. Addition of the complex C substrate hydroxyethylcellulose enhanced the potential denitrification activity in both soils independent of its concentration. For three of the denitrification genes, napA, nirS and nosZ clade I, positive correlations were found between copy numbers and potential denitrification activities. Principal component analysis revealed the identity of the carbon source as the main factor determining the copy numbers of the denitrification genes. Unexpectedly, both potential denitrification activities and gene copy numbers varied strongly between the three parallel samples taken at one timepoint in many cases, rendering the interpretation of the results more complex than anticipated. In general, previous findings that concentration and identity of a carbon source that is added to soil microcosms has a strong effect on composition and activity of the denitrifying communities could be confirmed. However, more sophisticated analyses including sequencing would be necessary in order to examine the full extent of this effect.
Publications
- Influence of genetically modified organisms on agro-ecosystem processes. Agriculture, Ecosystems & Environment
Volume 214, 27 December 2015, Pages 96-106
Kolseth, A-K, D’Hertefeldt, T., Emmerich, M., Forabosco, F., Marklund, S., Cheeke, T. E., Hallin, S. and Weih, M.
(See online at https://doi.org/10.1016/j.agee.2015.08.021)
