Nitrogen (N) isotope ratio mass spectrometry (IRMS) by Dumas combustion and continuous flow mass spectrometry has become a wide‐spread tool for the studies of N turnover. The speed and labor efficiency of15N determinations from aqueous solutions such as soil solutions or soil extracts are often limited by sample preparation. Several procedures for the conversion of dissolved ammonium (NH4+) or nitrate NO3‐to gaseous ammonia and its subsequent trapping in acidified traps have been elaborated in the last decades. They are based on the use of acidified filters kept either above the respective solution or in floating PTFE envelopes. In this paper, we present an improved diffusion method with a fixed PTFE trap. The diffusion containers are continuously kept in a vertical rotary shaker. Quantitative diffusion can thus be achieved in only three days. For solutions with NH4+levels of only 1 mg N kg‐1and NO3‐concentrations of 12 mg N kg‐1, recovery rates of 98.8–102% were obtained. By addition of15N labeled and non‐labeled NH4+and NO3‐it was shown that no cross‐contamination from NH4+to NO3‐or vice versa takes place even when one form is labeled to more than 1 at %15N while the other form has natural15N content. The method requires no intermediate step of ammonia volatilization before NO3‐conversion.