It is well established that non-exchangeable NH₄⁺ is an important N source for plants. However, release from clay minerals only occurs when the NH₄⁺ concentration of the mineral environment declines below a certain level. This may be conditioned by the growing plant. Although root development differs depending on plant species, the influence of root architecture on the mobilization of non-exchangeable NH₄⁺ is still open to question. We found that mobilization of non-exchangeable NH₄⁺ was higher under oilseed rape as compared to barley; oilseed rape even has allorhize and fescue homorhize roots. This observation could be proved by labeling soil samples with ¹⁵NH₄⁺, which were incubated in soil holders under oilseed rape and barley. The higher mobilization of non-exchangeable NH₄⁺ by oilseed rape is therefore assumed to be caused by higher release of H⁺, displacing interlayer NH₄⁺. Although allorhize roots from pre-crops create more stable biopores with an increasing diameter and living roots of the subsequent crop enter these biopores, we could not find an influence of the pre-crop on the mobilization of non-exchangeable NH₄⁺.