Tree species play a key role in regulating soil CH₄ uptake and N₂O emission through altering soil physical, chemical and microbial properties in forest ecosystem. In northeast China, about 70% of forest areas are dominated by secondary forests, and the rest are larch and other tree species plantations which were established following secondary forests being harvested. But few studies have investigated the effects of the conversion of secondary forest to larch plantation on soil CH₄ and N₂O fluxes. In this study, by using static chamber/gas chromatograph techniques, we measured soil CH₄ and N₂O fluxes from secondary forest and adjacent larch (Larix olgensis) plantations. Four static chambers were set in each forest type. Gas samples were took every two weeks (growing season) or one month (dormant season) during June 2007 to June 2008. Soil temperature, soil moisture and soil available nitrogen content near each chamber were concurrently measured. The fluxes of soil CH₄ in secondary forest and larch plantation varied from -168.8 to 22.7μg CH₄ m^-2 h^-1 and -191.4 to -40.5μg CH₄ m^-2 h^-1 respectively, and the corresponding values of soil N₂O were -29.1 to 34.6 μg N₂O m^-2 h^-1 and -3 to 61.8 μg N₂O m^-2 h^-1. The cumulative soil CH₄ uptake and soil N₂O fluxes in larch plantation were respectively 20% and 2.6 fold greater than those in secondary forest. Similar seasonal dynamics of soil CH₄ uptake and N₂O emission fluxes were presented in both secondary forest and larch plantation, showing higher fluxes in growing season but lower ones in dormant season. The fluxes of soil CH₄ uptake positively correlated with soil temperatures, but negatively correlated with soil moisture. Through intercepting more rainfall in summer or melt water in spring, canopy and thicker litter can decrease soil moisture, and then enhance soil CH₄ uptake in larch plantation. The fluxes of N₂O emission were positively related to soil temperature and soil NH₄⁺-N contents, but had no significant correlation with soil-water content in both forest types. Nevertheless, thicker litter store more water during drought in spring and autumn in the larch plantation, which can increase N₂O production of soil organic horizon. In conclusion, the conversion of secondary forest to larch plantation increased soil CH₄ uptake and N₂O emission, mainly due to litter-induced changes in spatial and temporal distribution of soil water.