One purpose of new land use concepts for degraded fens (organic soils with high N content) is the reduction of the mineralization process due to very high groundwater levels. However, knowledge of nitrogen mineralization process (net and gross) in degraded fen soils affected by reflooding is very small. Therefore, the objectives of our study were (a) to evaluate the suitability of15N pool dilution method for measurements of gross mineralization rates in degraded fen soils and (b) to investigate how the reflooding of a degraded fen affects the net and gross nitrogen mineralization in a short-term incubation experiment. The usability of the15N pool dilution method was diminished by the low recovery of the applied15NH4−at time zero. The recovery of the added15NH4−in the extractable soil NH4−pool was only 13.5% for the drained soil and 59.6% for the reflooded soil. However, the gross mineralization rates were similar for both soils and exceeded always the net rates substantially. The cumulative net mineralization rate was higher for the reflooded soil (1.58 μg N*cm−3*d−1) than for the drained soil (-0.67 μg N*cm−3*d−1). Differences between the two soils were also found in the nitrification intensity and the loss of15N. This was probably one reason for the higher net mineralization rate in the reflooded soil.