Knowledge about nitrate transformation processes and how they are affected by different plants is essential in order to reduce the loss of valuable N fertiliser as well as to prevent environmental pollution due to nitrate leaching or N 2O emission after fertilisation or the reflooding of degraded fens with nitrate-containing municipal sewage. Therefore four microcosm 15N tracer experiments were performed to evaluate the effect of common wetland plants ( Phalaris arundinacea, Phragmites australis) combined with different soil moisture conditions (from dry to reflooded) on nitrate turnover processes. At the end of experiment, the total formation of gaseous N compounds was calculated using the 15N balance method. In two experiments (wet and reflooded soil conditions) the N 2O and N 2 emissions were also directly determined. Our results show that in degraded fen soils, which process mainly takes place—denitrification or transformation into organic N compounds—is determined by the soil moisture conditions. Under dry soil moisture conditions (water filled pore space: 31%) up to 80% of the 15N nitrate added was transformed into organic N compounds. This transformation process is not affected by plant growth. Under reflooded conditions (water filled pore space: 100%), the total gaseous N losses were highest (77–95% of the 15N-nitrate added) and the transformation into organic N compounds was very low (1.8% of 15N nitrate added). Under almost all soil conditions plant growth reduced the N losses by 20–25% of the 15N nitrate added due to plant uptake. The N 2 emissions exceeded the N 2O emissions by a factor of 10–20 in planted soil, and as much as 30 in unplanted soil. In the treatments planted with Phragmites australis, N 2O emission was about two times higher than in the corresponding unplanted treatment. 15% of the N 2O and N 2 formed was transported via the Phragmites shoots from the soil into the atmosphere. By contrast, Phalaris arundinacea did not affect N 2O emissions and no emission via the shoots was observed.