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Very little information is available for Mediterranean areas about the soil N dynamics and crop N use efficiency during the transition phase from conventional tillage (CT) to no-tillage (NT). Hence, a 2-yr experiment was conducted under semiarid Mediterranean conditions in three sites to study how soil N dynamics, crop N uptake, grain yield, and N use efficiency vary with N-fertilization rate and crop genotype in the switch year from CT to NT. Treatments consisted of two tillage systems (CT and NT), five N-fertilization rates (0, 40, 80, 120, and 160 kg N ha−1), and two durum wheat (Triticum durum Desf.) genotypes (one modern variety and one old landrace). Irrespective of the genotype, NT reduced compared to CT both wheat N uptake and grain yield under low soil N availability. The greater soil N supply under CT was the main reason for this outcome, so much so that the differences between CT and NT for these traits gradually decreased with the increase of N fertilization rate, practically disappearing at 80 kg N ha−1). The analysis of the N use efficiency components corroborated this hypothesis showing clearly that the yield advantage observed in CT depended mainly on the increase in N supply in soil under CT than NT condition. The two wheat genotypes responded similarly to varying soil tillage system. However, the adverse effects of NT practice were more evident in the modern variety than the old landrace. This study ultimately indicates that in the Mediterranean areas the switch year from CT to NT regime is rather delicate. Given that the lack of soil cultivation considerably reduces the soil N availability, hence, using NT technique alone as a substitute for CT is not agronomically feasible. Instead, an optimal application of NT is achievable by acting simultaneously on other factors of the cropping management, particularly the N-fertilization strategy, to maximize the crop N use efficiency and increase crop yield, which are essential requirements for a more sustainable agriculture.