Carbon mineralisation and plant growth in soil amended with compost samples at different degrees of maturity.

The carbon and nitrogen mineralisation of a composting mixture of brewing yeast and lemon tree prunings was studied, at different degrees of stabilisation of this matrix, within an incubation experiment in soil. Meanwhile, a growth test in pots with ryegrass (Lolium perenne L.) was carried out using the selected soil and equal amounts of the composting mixture taken at different maturation steps, in order to evaluate the additions of these organic amendments in terms of fertilising value. Samples of the composting mixture, when poorly transformed through the biostabilisation process, showed high CO 2 -C releases in the soil, due to the microbial attack on easily degradable organic fractions still present in the mixture, with 24.7% mineralisation of the initial total organic carbon (TOC) after a 70 day incubation. On the other hand, mature compost was the most stable matrix, with only 5.4% of TOC mineralised after 70 days. Furthermore, amendments with the initial composting mixture led to negative net N-mineralisation during 56 days of incubation with soil. Only slight negative values of the net N-mineralisation were detected with fully stabilised compost. Nevertheless, pot experiments with ryegrass revealed that mature compost may promote N mineralisation to certain extents. Moreover, mature compost did not produce any phytotoxic effect, behaving as a slow-action organic fertiliser with N made available through a progressive mineralisation. Thus, the results gained through this study are a confirmation that the fertilising quality of a compost destined for agricultural uses is heavily affected by the complete exhaustion of the maturation reactions.