The robustness of the assumption of equilibrium between native and added N during N-15 isotope dilution has recently been questioned by Watson et al. (Soil Biol Biochem 32 (2000) 2019-2030). We re-analyzed their raw data using equations that consider the added and native NH4+ and NO3- pools as separate state variables. Gross mineralization rates and first-order rate constants for NH4+ and NO3- consumption were obtained by combining analytical integration of the differential equations with a non-linear fitting procedure. The first-order rate constants for NH4+ consumption and NO3- immobilization for the added NH4+ and NO3- pool were used to estimate gross mineralization rates and first-order rate constants for nitrification of native NH4+. The latter were 2-4 times lower than the first-order rate constants derived from the added N pool. This discrepancy between first-order rate constants for nitrification implies that one or more process rates estimated for the added N pools cannot be applied to the native N pools. Preferential use of the added N resulted in an overestimation of the gross mineralization by 1.5-2.5-fold, emphasizing the need for critical evaluation of the assumption of equilibrium before gross mineralization rates are calculated. (C) 2004 Elsevier Ltd. All rights reserved.