The stable isotope technique and the difference method are common approaches for estimating fertiliser N uptake efficiency. Both methods, however, have limitations and their suitability may depend on N management and environmental conditions. A field experiment was conducted on a humus sandy soil in northern Germany to estimate fertiliser N uptake efficiency of silage maize in the year of application (Zea mays L.) by the stable isotope and the difference method as influenced by the type of N fertiliser (mineral vs. cattle slurry), the application mode (separate or combined application), and N rate. Seven N treatments were included (0, 50, 100 and 150 kg mineral N ha -1 ; 20, 40 m 3 cattle slurry ha -1 ; 50 kg mineral N ha -1 plus 40 m 3 slurry ha -1 ), where either mineral N or slurry N was labelled, and mineral N was split into two dressings. In addition, 4.1 kg ha -1 labelled mineral N was incorporated into otherwise unlabelled treatments (0, 20, 40 m 3 ha -1 , and 50 kg mineral N ha -1 plus 40 m 3 ha -1 ) to estimate N uptake from the upper soil layer. Uptake of 15 N was followed in leaves, stalk, ear, and the whole crop. Fertiliser N uptake efficiency (FNUE 15N ) of mineral fertiliser N obtained by the isotope technique ranged between 51 and 61%. Recovered fertiliser N was mainly found in the ear, while less labelled N remained in leaves and the stalk. The nitrogen rate tended to increase the amount of recovered N, but the effect was not consistent among plant parts and the whole crop. Plant N uptake from non-fertiliser N was found to increase N input up to 100 kg N ha -1 . Nitrogen recoveries of the two mineral N dressings were similar for the different plant parts as well as for the whole crop. Fertiliser N uptake efficiency (FNUE diff ) of mineral N estimated by the difference method resulted in substantially higher values compared to FNUE 15N , varying between 56 and 98%. More N was taken up from the upper soil layer with increasing N supply, which is regarded as a major error source of the difference method. Slurry N was taken up less efficient in the year of application than mineral fertiliser N as indicated by recovery rates of 21-22% (FNUE 15N ) and 39-62% (FNUE diff ), respectively. When mineral N and slurry were applied together, the difference method estimated significantly lower N uptake efficiencies for both mineral and slurry N compared to a single application, while values obtained by the isotope method were not affected.