Nitrogen implantations were performed into iron at a fluence of 1017 15N cm−2, an energy of 50 keV, a temperature of 150 °C, two vacuum pressures (2×10−3 and 6×10−5 Pa), and five current densities (6, 18, 24, 40, 60 μA cm−2). Experimental profiles display a preferential nitrogen migration towards the surface. In order to interpret the variation of the depth distribution shapes versus the pressure and the current density, a calculation model is elaborated. This model takes into account the presence of the contamination carbon at the sample surface. A comparison between experimental and simulated profiles is presented. An interpretation of the carbon role is proposed. The different hypothesis on the nitrogen migration mechanisms are reviewed.