The carbon cycle in forest ecosystems is affected by forest age, but little is known about carbon cycle in a young forest (＜10 years old).  This study aimed to clarify the characteristics of carbon cycle in young forest (6-9 years old) compared with an old-growth forest (92-95 years old).  Carbon cycle was estimated through net primary production (NPP) and soil respiration (SR) from 2014 to 2018.  NPP was quantified from tree growth (ΔB), understory growth (ΔBu) and litter fall (LF).
     ΔB and LF increased with forest age in the young forest.  In addition, ΔBu played an important role in the young forest because it contributed 15% of NPP.  Consequently, NPP in the young forest increased drastically from 2015 to 2018 (2.08－9.25 tC ha^-1 yr^-1).  In contrast, NPP in the old-growth forest fluctuated with small inter-annual variations (4.57±0.69 tC ha^-1 yr^-1).  On the other hand, SR decreased from 11.32 to 4.98 tC ha^-1 yr^-1 in the young forest.  SR in the old-growth forest showed small inter-annual variations (7.58±0.57 tC ha^-1 yr^-1) as well as NPP.  These results suggested that carbon budget in the young forest increased from year to year as the young forest shifted from carbon source to carbon sink.