Final full-text version (definitively accepted version) of the paper published by Elsevier Sci. Ltd.Land application of city refuse compost (CRC), produced from aerobic-thermophilic composting of the organic fraction of municipal solid wastes, is an attractive alternative for the disposal of these wastes, currently land-filled or incinerated. Knowledge of the availability of N in compost is particularly important, given the current concern about groundwater contamination by NO3--N. In this work we assessed the capacity of a CRC with a high degree of maturity to supply N to a barley crop over 2 months in a controlled-phytotron experiment. The CRC was applied at a rate equivalent to 60 t ha-1, after incubation of the material (fraction < 2mm) for 3 months at 24 ºC (40-45 % moisture). The soil (Eutric Cambisol) was labelled with 15N as (15NH4)2SO4 with 9.614 atom % 15N excess. Available-N from CRC dilutes the isotopic 15N/14N ratio of the labelled soil in an important way; atom % 15N excess in the plant material (aerial part) after 2 months was approximately 50% in the compost treatment with respect to the non-amended soil (0.625 and 1.201 atom % 15N excess, respectively). Accordingly, CRC should not be considered as a poor-release N material when it has a high degree of maturity, i.e. it is highly biologically stabilised and "humified". On the contrary, a high N-fertiliser value to crops can be attained if a very high maturity degree is reached in commercial composting plants. This type of compost may be defined as a "postmature compost", a slow-release N material and a rich N material to plants at the same time. For this reason, the concept "postmature compost" is of great interest from an agronomic point of view and opens a wide portal for investigation of the agronomic reuse of composted organic materials.Peer reviewed