Gross and net rates of nitrogen mineralisation in soil amended with composted olive mill pomace

Olive mill pomace is the major waste product in the olive oil industry and composting these by-products for the purpose of recycling nutrients and organic matter is a sound environmental strategy. Yet little is known about the quantity and timing of nitrogen (N) release from composted olive mill pomace. This paper assesses both gross (using the (15)N dilution technique) and net (aerobic incubation) nitrogen (N) mineralisation and N(2)O emissions of soil amended with seven commercially available composts of olive mill pomace (COMP). All are currently produced in Andalusia and differ in the proportions of raw materials co-composted with the pomace. The absence of significant differences in net N or gross mineralisation and nitrification in COMP-amended soil compared with a control, except for COMP combined with poultry manure, highlighted the recalcitrant nature of the COMP-N. Applications of COMP are hence unlikely to supply available N in available forms, at least in the short-term. Furthermore, N(2)O emissions from COMP-amended soil were negligible and, therefore, applications in the field should not result in increased N loss through denitrification.