In microplot experiments on two minerotrophic mire sites of northeastern Germany (a: highly degraded, shallow—Paulinenaue, Brandenburg; b: slightly degraded, deep-layer—Heinrichswalde, Mecklenburg-Vorpommern) the fate of15N-labelled fertilizer-N (120kg N ha−1as K15NO3, 46.94 at-%15Nexc) was investigated over one growing season. Only 53 to 69% of the applied fertilizer-N (63 to 82kg N ha−1) could be recovered. The plants had taken up about 70 to 79% of this portion, that is about 16% of their total N demand. At the end of the first year of experiment, about 21 to 30% of the fertilizer-N recovered were in the soil, more than 90% of which in 0–20 cm depth. It is assumed that the low rates of retrieval are due to gaseous losses of N-compounds (as N2; N2O).