Nitrogen and carbon mineralization in soil of vermi-composted and unprocessed dry olive cake ("orujo seco") produced from two-stage centrifugation for olive oil extraction

A laboratory-based incubation study was conducted to examine N mineralizati on/immobilization and C mineralization of both vermi-composted and unproces sed dry olive cake when added to soil. Cattle manure and a soil-only treatm ent were also examined. The organic amendments were mixed with soil at rate s to add 1.5 times the soil C (soil C: 6.1 g/kg). The incubations were cond ucted at 28 degrees C, and were sampled at intervals of 1, 7, 14, 28, 49, 7 0 and 91 days for the analysis of soil NH4+-N and NO3--N, and soil respirat ion. The C to N ratios of the unprocessed olive cake, vermi-composted olive cake and manure were 42, 29 and 11, respectively. Both the unprocessed oli ve cake and vermi-composted olive cake immobilized soil N throughout the 91 days of the study. The cattle manure mineralized an appreciable amount of N during the study. The prolonged Immobilization of soil N by the vermi-com posted olive cake was attributed to the C to N ratio of 29 and to the recal citrant nature of its C and N composition. The three organic amendments enh anced soil respiration, with manure having a much larger effect. Respiratio n was generally moderately higher from the vermi-composted olive cake than from unprocessed olive cake, which was attributed to the use of manure as a co-compost. These data suggested that for vermi-composted dry olive cake t o be used as an organic soil amendment, the management of the vermi-compost ing process should be adjusted to ensure more favorable N mineralisation/im mobilization behavior in soil.