Regulation of N2O emissions from agricultural soil on the transcriptional level

• Applicant: Dr. Maren Emmerich
• Subject Area: Microbial Ecology and Applied Microbiology
• Term: from 2012 to 2014
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 229528027

Nitrous oxide (N2O) is an important greenhouse gas and the single most potent ozone-depleting substance of the 21st century. About 62 % of total global N2O emissions stem from soils, a great fraction of which is released as an intermediate of bacterial denitrification, the stepwise reduction of nitrate (NO3-) to di-nitrogen gas (N2). Besides the composition of the denitrifying community, the activities of the gene products that mediate the four steps of denitrification largely determine the efficiency and gaseous end product composition (N2O versus N2) of this process. Previous studies have focused on the role of community composition on denitrification activity in the field or identified detached factors such as pH and O2 partial pressure that have an influence on the expression of the different denitrification genes using laboratory experiments with isolated strains. However, studies elucidating the connection between environmental conditions and expression of denitrification genes in field samples are scarce, even though this kind of studies would provide us with the data we need in order to predict which ways of soil management will lead to an increase or decrease of N2O emissions from denitrification. Consequently, this project aims to identify correlations between different fertilization regimes, denitrification rates and expression of denitrification genes using a more than 50 year-old fertilization experiment as a field site. From this field site, samples from soils which have received two different mineral (ammonium sulfate and calcium nitrate) and two different organic (sewage sludge and cattle manure) fertilizers will be compared to samples from unfertilized soil. Gene and mRNA transcript numbers of two different nitrite (NO2-) reductase genes whose products are involved in N2O production will be quantified in these samples and compared to potential denitrification rates. The same will be done for the N2O reductase gene, the product of which is the only known mediator of N2O consumption. Given the unique role of the N2O reductase, the diversity among the gene and transcript sequences encoding this enzyme within the individual samples will be subject to further analysis, since the differences among the expressed variants of this gene are very likely to play a key role in the regulation of N2O degradation. Once fertilization regimes that lead to an increase or decrease of the expression of the denitrification genes have been identified, the speed with which this effect comes into play will be determined using microcosm experiments. These will further be used in order to elucidate which primary factors increase or decrease the transcription rate of NO2-- and N2O reductase genes. Thus, by sheding light on the regulation of the production and consumption of N2O under different environmental conditions on the genetic level, our study will help to predict more reliably how N2O emissions can be reduced by fertilizing arable soil accordingly.

• DFG Programme: Research Fellowships
• International Connection: Sweden
• Host: Professorin Sara Gates Hallin, Ph.D.