Abstract: Studies on soil greenhouse gas emission and its mechanism in the process of plantation development can provide theoretical basis for estimating forest greenhouse gas flux. We examined the soil greenhouse gas emission and absorption rate of Korean pine (Pinus koraiensis) and larch (Larix gmelinii) plantations with different stand ages (15, 30 and 50 years old) in Maoer Mountain of Heilongjiang Province, using laboratory incubation method. The results showed that soil CO₂ emission rates (Korean pine: 1724.18±98.57 μg C·kg^-1·h^-1; larch: 1306.37±142.27 μg C·kg^-1·h^-1) and CH₄ absorption rates (Korean pine: 5.12±0.68 μg C·kg^-1·h^-1; larch: 1.91±0.85 μg C·kg^-1·h^-1) were significantly higher in 30-year-old plantations than that in 15 and 50-year-old plantations. Soil N₂O emission rate of 30-year-old Korean pine plantation was significantly higher than that of 15 and 50-year-old Korean pine plantation (P<0.05). Soil N₂O emission rate in the larch plantations did not change across stand ages. The maximum soil N₂O emission rates were (0.139±0.016) μg N·kg^-1·h^-1 in Korean pine plantation and (0.137±0.056) μg N·kg^-1·h^-1 in larch plantation, respectively. Soil CO₂ emission rates were higher in Korean pine plantations of all stand ages than in larch plantations in 15 and 30-year-old plantations. For all stand ages, soil CH₄ absorption rate was significantly higher (P<0.05) in Korean pine plantations than in larch plantations. Soil N₂O emission rates in Korean pine plantations of all stand ages were not significantly different from those in larch plantations. The main factors driving the CO₂ emission rate were total soil carbon content and microbial biomass nitrogen in Korean pine and larch plantations. Microbial biomass nitrogen was affected by the tree species and stand age. The CH₄ absorption rate was affected by microbial biomass carbon, dissolved organic carbon, and dissolved organic nitrogen contents. Microbial biomass carbon  was regulated by tree species and stand age. The emission rate of N₂O was affected by the contents of dissolved organic nitrogen, ammonium and nitrate. Dissolved organic nitrogen content was affected by stand age. In conclusion, different soil physical and chemical properties and microbial biomass carbon and nitrogen  can explain the variation in soil greenhouse gas emission rates among forests with different tree species and stand ages.

Key words: forest development, conifer species, greenhouse gases, soil physicochemical properties, microbial biomass carbon and nitrogen.