AbstractA study to investigate the transport and the transformation of two N15‐tagged N‐fertilizers (Ca(NO3)2and (NH4)2SO4) was carried out in the laboratory at 4° and 23°C. Undisturbed soil columns from the upper 30 cm of neutral agricultural and acid forest loess soils were used. An unsaturated steady state water flow in the columns was established and maintained by a darcian flow of 5mm of water/day. The effluent collected from each column after two days, was analysed for NO3‐ and NH4+. The data collected were used to obtain the Breakthrough Curves (BTCs). The BTCs after NO3‐fertilizers from agricultural soil approached the ideal symmetrical curves from non‐reacting solutes. While those from acid forest soil were somewhat flat and ended with pronounced tails. ‐Transformation of the applied NH4‐fertilizer to NO3‐N was much higher in agricultural soil than in acid forest soil. Even at lower temperature, nitrification in agricultural soil was active though at a reduced rate while in forest soil, nitrification rate was extremely low and appeared to be independe