Microorganisms in soil can use nitrate in two important ways to generate energy when there is no oxygen available: denitrification and dissimilatory nitrate reduction to ammonium (DNRA). While denitrification leads to production and emission of the gases dinitrogen and nitrous oxide (N2O), DNRA retains mineral nitrogen (nutrients) in the ecosystem in the form of ammonium. In a joint project with Martina Putz from the Swedish University of Agricultural Sciences, we investigate the fate of nitrate at a long-term trial field site in Säby (Sweden). The different agricultural management practises include conventionally tilled crop and ley (grass) farming with no and high nitrogen fertilization levels, respectively. In laboratory experiments, we investigate the nitrogen fluxes in the soils by using stable nitrogen isotopes (15N), combined with investigations of the microbial community composition. Preliminary results show significantly higher 15N-N2O production from the crop soil than from the ley soil indicating higher denitrification activity in crop soil, whereas nitrogen compounds are better retained in ley soil. Crop soil showed a further increase in 15N-N2O production when long-term fertilized, whereas this was not observed for the ley soil. These first results show us that the soil management practise and soil organic matter content can be crucial for microorganisms to carry out DNRA. In our ongoing work, we will quantify the gross (actual) transformation rates of the two processes and will link this to the soil microbial community for better predictions of management effects on the emissions of the greenhouse gas N2O.
Published 22. May 2015 - 14:14 - Updated 2. June 2015 - 12:47