Grape marc is a common waste product of the wine production industry. When partially composted and applied to soil it may contain enough N to affect vine growth and hence wine quality. Yet little is known about the quantity and timing of N release from composted grape marc. A laboratory incubation was conducted where composted grape marc amended and non-amended soils were periodically sampled over 148 days at 15 &DEG; C for gross N mineralization rates, C mineralization and microbial biomass-C. Gross N mineralization rates were determined by N-15 pool dilution using both analytical equations and the numerical model FLUAZ (Mary, B., Recous, S., Robin, D., 1998. A model for calculating nitrogen fluxes in soil using N-15 tracing. Soil Biology & Biochemistry 30, 1963-1979.). Both analytical and FLUAZ determined gross N mineralization rates were in close agreement in the control soil. However, in composted grape mare amended soils there was a discrepancy between the two solutions. Findings indicate that composted grape marc caused a net immobilization of N for the first 50-days of incubation, after which enough N was released to require consideration in fertilizer-N strategies. © 2005 Elsevier Ltd. All rights reserved.