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We studied the effect of 15N labelling duration on the mineralisation and immobilisation of native and applied (residual) N in the humus layer of a Humo-Ferric Podzol. Ammonium sulphate, labelled with 15N, was applied to 1 m2 plots at a rate of 200 kg N ha−1. Fertiliser application was timed so that when samples were collected they had been labelled with 15N for 24 hours, 7 months and 31 months. In a 42-day aerobic incubation of the samples, net mineralisation of total and applied N was greatest in the 24-hr treatment followed by those from the 31-month treatment (p 24-hr > 31-month treatment. Net mineralisation of C and N was poorly correlated with each other (r=-0.02, p=0.89). Anaerobic incubation showed net mineralisation for the 7- and 31-month treatments but net immobilisation for the 24-hr treatment for both total and applied N, suggesting that immobilisation of inorganic N was encouraged when there was a large pool of mineral N in the soil. Both total and applied N in the extractable organic N fraction and in the N flushed after fumigation with chloroform had the following order: 24-hr > 7-month > 31-month treatment. The results confirmed that N fertiliser was being immobilised within hours after application by the humus material through the microbial population and that the immobilised N had a low mineralisation potential after one growing season.