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 mineralization/immobilization and C mineralization of both vermi‐composted and unprocessed dry olive cake when added to soil. Cattle manure and a soil‐only treatment were also examined. The organic amendments were mixed with soil at rates to add 1.5 times the soil C (soil C: 6.1 g/kg). The incubations were conducted at 28°C, and were sampled at intervals of 1, 7, 14, 28, 49, 70 and 91 days for the analysis of soil NH4 +‐N and NO3 ‐‐N, and soil respiration. 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 olive 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‐composted olive cake was attributed to the C to N ratio of 29 and to the recalcitrant nature of its C and N composition. The three organic amendments enhanced soil respiration, with manure having a much larger effect. Respiration 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 to be used as an organic soil amendment, the management of the vermi‐composting process should be adjusted to ensure more favorable N mineralisation/immobilization behavior in soil.