Nitrate removal from drained and reflooded fen soils affected by soil N transformation processes and plant uptake

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 N2O 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 N2O and N2 emissions were also directly determined. sults 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 N2 emissions exceeded the N2O 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, N2O emission was about two times higher than in the corresponding unplanted treatment. 15% of the N2O and N2 formed was transported via the Phragmites shoots from the soil into the atmosphere. By contrast, Phalaris arundinacea did not affect N2O emissions and no emission via the shoots was observed.