Controls of the partitioning of nitrate between DNRA and denitrifiers

Sammanfattning: In arable soils, the importance of denitrification, a respiratory process where nitrate (NO3-) is stepwisely reduced to nitrogen gas and nitrous oxide (N2O) is well established. More recently has dissimilatory nitrate reduction to ammonium (DNRA) gained interest as its importance in agricultural soils might have been largely overlooked. Since these two functional microbial groups both utilize NO3- as an electron acceptor for anaerobic respiration they are competing in the soil. One pathway can produce a potent greenhouse gas (N2O) while the other is producing plant available nitrogen, for this reason it is of importance to try to understand what factors can promote DNRA over denitrification This experiment aims to study what drivers control the partitioning of NO3- between denitrifiers and DNRA in an agricultural soil. It also serves as a pilot for a bigger study aimed to compare the effect of these drivers on soil with different management history. Soil was incubated under anoxic conditions and resources in form of organic carbon (C) and NO3- were added in different rations (C:NO3- ratios) and amounts. We hypothesised that there would be differences related to the either C:NO3- ratio and/or amounts and that low ratios and/or high NO3- amounts will promote denitrification whereas high NO3- ratios and/or NO3- nitrogen amounts will promote DNRA. The microcosms were destructively sampled at seven different timepoints. Gas samples from the headspace in the microcosms were analysed by gas chromatography for N2O. Soil samples were used for soil nitrogen and DNA extractions. Quantitative real-time PCR was performed on the denitrification marker gene nir, the DNRA marker gene nrfA and the 16S rRNA gene for assessing overall bacterial and archaeal community size. This experiment showed that DNRA was more abundant than denitrifiers, whereas most studies show the opposite. Further, the genetic potential for DNRA was continually high across the treatments. It was only somewhat lower for a treatment with a C:NO3- ratio of 4, with C amount equivalent to 8000 kg/ha and NO3- amount equal to 200 kg/ha, also for which genetic potential for denitrification was the highest. This experiment suggests that not C:NO3- ratio but a high amount of NO3- and carbon promotes denitrification on the expense of the otherwise continually high DNRA in the studied soil.