We used the inhibitor acetylene (C2H2) at partial pressures of 10 Pa and 10 kPa to inhibit autotrophic nitrification and the reduction of nitrous oxide (N2O) to N2, respectively. Soils (Andosol) from a Coffea arabica plantation shaded by Inga densiflora in Costa Rica were adjusted to 39, 58, 76 and 87% water-filled pore space (WFPS) and incubated for 6 days in the absence or presence of C2H2. Soil respiration, nitrification rates and N2O emissions by both processes were measured in relation to soil moisture conditions. At all WFPS studied, rates of N2O and N2 productions were small (4.8; 14.7; 23 and 239.6 ng N-N2O g-1 d.w. d-1 at 39, 58, 76 and 87% WFPS, respectively), and despite a low soil pH (4.7), N2O was mainly produced by nitrification, which was responsible for 85, 91, 84 and 87% of the total N2O emissions at 39, 58, 76 and 87% WFPS, respectively. At the three smaller values of WFPS, a linear relationship was established between WFPS, soil respiration, nitrification and N2O released by nitrification; no N2 was produced by denitrification. At more anaerobic conditions achieved by a WFPS of 87%, a large rate of N2O production was measured during nitrification, and N2 production accounted for 84% of the gaseous N fluxes caused by denitrification.