One of the uses of spent coffee grounds (SCG) is its incorporation into agricultural field soils. However, their effect on N₂O emission and microbial N₂O production processes are not well understood. We conducted outdoor pot and soil incubation experiments to quantify the effect of SCG application on N₂O emission from andosol and to elucidate the relationship between N₂O emission and microbial N₂O production processes after SCG application. We cultivated spinach in the pot experiment and applied fertilizer in both the experiments at a rate in accordance with the local practice.

In the pot experiment, we observed a large decrease in N₂O emission from with SCG (WC) treatment as compared with those without SCG (NC) treatment between 5 and 21 days after SCG application. Cumulative N₂O emission over 70 days of the pot experiment was significantly lower in the WC treatment than in the NC treatment. Furthermore, soil ammonium oxidation potential was lower in the WC treatment compared with the NC treatment throughout the experimental period. The results of acetylene inhibition experiment suggested that the reduction of N₂O to N₂ was larger in the WC treatment than in the NC treatment. Moreover, the effect of SCG application on N₂O emission in the soil incubation experiment was not consistent among different WFPS (water-filled pore space). In the WFPS60％, the WC treatment tended to have higher N₂O emission compared with the NC treatment. However, in the WFPS90％, N₂O emission from the WC treatment was lower than that from the NC treatment. Our results suggest that SCG application decreases N₂O emission from andosol by changing both nitrification and denitrification in comparison with that of the NC treatment, although its effectiveness could vary with WFPS condition. This study also showed that SCG application tends to decrease crop yield.