The recovery fractions of fertilizer nitrogen (N) by crops can be calculated by the difference or the isotope-dilution method. In the difference method, an apparent recovery fraction (ARF) is calculated from the N uptake by fertilized and unfertilized crops, whereas in the isotope-dilution method a 15N recovery fraction (15SNRF) is calculated from isotope-ratio analysis and N uptake by fertilized crops. The recovery fractions calculated by the two methods are compared on the basis of simplified models for the distribution of 15N-labeled N over different N pools in the soil-crop system, considering plant uptake and mineralization-immobilization in soil. In N-deficient soils, ARF-values are likely to be higher than those of 15SNRF, due to pool substitution. Plant uptake and immobilization do not affect the relationship between ARF and 15SNRF, as these processes do not discriminate between 15N and 15N. However, immobilization reduces the range of values of ARF and 15NRF. Mineralization is the main factor causing the discrepancy between ARF and 15NRF. It results in dilution of the soil mineral N pool with soil-derived N and thus affects the 15N/15N ratio in the soil mineral N pool. The combined action of mineralization-immobilization and plant uptake increases the effect of dilution of the soil mineral N pool by (re)miner-alized soil N. The effects of plant uptake and mineralization-immobilization on the relationship between 15NRF and ARF are discussed and analytical expressions are derived for the relevant quantities considered in the model. The difference and the isotope-dilution method measure essentially different quantities and complementary information can be derived from recovery fractions calculated by the two methods, thus contributing to the understanding of fertilizer N-dynamics and transformations in soil.