Bacteria, fungi and other microorganisms perform numerous functions in soils that are essential to plant growth. This includes the cycling of nitrogen. The group at University of Copenhagen has established itself as a world leader on quantification of soil microbial responses to environmental perturbations. We will investigate if nitrification inhibitors change the microbial cycling of inorganic nitrogen potentially leading to changes in leaching or gaseous loss of nitrogen from the investigated fields. We employ mRNA-based methods to establish quantitative data on the expression of all key genes involved in the microbial cycling of inorganic soil nitrogen. Specifically, we will use RT-qPCR on cDNA (reverse transcribed RNA), a strong and sensitive tool, which has found its way into microbial ecology when specific, well-described genes are the focus of interest. RT-qPCR provides solid quantitative data on the expression of the selected nitrogen-cycling genes, which we will link to, e.g., the flux of nitrous oxide from the field.