The concept of potentially mineralizable N is discussed and data that support the identification of N0 as an active fraction of soil organic N are presented. Aerobic medium‐term incubations (13 weeks) were used to measure the potentially mineralizable nitrogen (N0) in four cropping systems on four different occasions from early spring to late autumn. The cropping systems consisted of barley (Hordeum distichum L.) with no N fertilizers (B0), barley receiving 120 kg N ha−1 yr−1 (B120), meadow fescue (Festuca pratensis Hudson) ley receiving 120 + 80 kg N ha−1 yr−1 (GL), and lucerne (Medicago sativa L.) ley with no N additions (LL). The amounts of N mineralized during the 13‐week incubations (37°C) ranged from 188 to 483 kg N ha−1, and the cropping systems ranked in the order GL > LL > B120 > B0 and showed a steady decline in all systems from spring to harvest and a subsequent increase from harvest to autumn. The seasonal differences were as large as the differences between systems. Three related models were employed to describe the kinetics of N mineralization during the incubations: (i) first‐order, (ii) two‐component (sum of two first‐order models), and (iii) a simplified special case of the two‐component model. In all cases, the special case of the two‐component model offered the best description of the curves of accumulated mineral N. Based on the mineralization course and an observed relationship between literature values of N0 and the rate constant (k), it is hypothesized that the mineralization is mediated by two distinct populations: an opportunistic, mainly immobile, population (bacteria) and a generalistic, mobile one (fungi), and that the k value is influenced both by the amount of substrate and its quality. The fact that the amount of mineralizable N decreases during the growing season and increases in autumn as a result of organic matter input provides evidence for the existence of an active fraction of soil organic matter.

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