Nitrogen availability from a mature urban compost determined by the 15N isotope dilution method

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<2 mm) for 3 months at 24°C (40–45% moisture). The soil (Eutric Cambisol) was labelled using 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”.