In situ, incubations of intact soil cores were carried out to identify factors controlling nitrogen (N) mineralisation and its spatial variability under field conditions. The analysed factors were soil moisture, temperature, and the content of light-fraction (density ≤ 2 Mg/m3) organic carbon (LC) contained in the soil. The error associated with the estimate of in situ N mineralisation was analysed using undisturbed samples in laboratory incubations. The coefficient of variation of in situ N mineralisation ranged from 58 to 234%. Nitrogen and LC mineralisation in the field showed a similar temporal pattern. The major factor affecting this pattern was soil temperature, soil moisture being near the optimum level throughout the experiment. The rate of N mineralisation during an incubation period was correlated with the content of LC at the beginning of the period; this factor explained 40–50% of the variation in N mineralisation. At a low rate of N mineralisation, a large proportion of the spatial variability was attributed to the error of estimation. From the relationship between N mineralisation and LC content, we estimated the rate constant k which could be expressed as a function of soil temperature. Within the observed temperature range (daily mean average 11–17°C), the Q10 (temperature coefficient) of in situ N mineralisation was 1.5. Negative values of N mineralisation were associated with the lower LC content of each period, indicating the presence of an immobilisation process, or that a proportion of LC was not involved in N mineralisation.