We compared nitrous oxide (N2O) emissions over 1 year from soils of plantations growing acacia, which is a leguminous plant capable of symbiotic nitrogen fixation in root nodules, and secondary forests in Sumatra, Indonesia. N2O emissions from acacia plantation soils fluctuated seasonally, from high in the wetter season to low in the drier season, whereas N2O emissions from secondary forest soils were low throughout the year. Water-filled-pore-space data showed that denitrification contributed substantially to N2O emissions from soils at acacia sites. The average annual N2O flux in acacia plantations was 2.56 kg N ha−1 a−1, which was eight times higher than that from secondary forest soils (0.33 kg N ha−1 a−1). In secondary forests, NH4+ was the dominant form of inorganic nitrogen. However, in acacia plantations, the NH4+: NO3− ratio was relatively lower than that in secondary forests. These results suggest that secondary forests were nitrogen limited, but acacia plantations were less nitrogen limited. Leguminous tree plantations may increase nitrogen cycling, resulting in greater N2O emissions from the soil. However, on a global warming potential basis, N2O emissions from acacia plantation soils accounted for less than 10% of the carbon uptake by plants. Nevertheless, because of the spread of leguminous tree plantations in Asia, the importance of N2O emissions from leguminous tree stands will increase in the coming decades.