Soil carbon dioxide efflux (FCO₂) plays a critical role in the carbon cycling of forest ecosystems because it may contribute as much as 60–80% of total respiration in the ecosystems. We measured the soil FCO₂ of Chinese fir plantations with three age classes: 5–year-old (young), 12-year-old (middle age), and 20-year-old (mature age) for the entire year of 2006 in Hunan province, China. Soil temperature (T_soil,), moisture (W_g), pH, depth to bedrock, and organic matter contents were also measured. The specific objectives of the study were (1) to quantify daily and seasonal patterns of soil FCO₂ in Chinese fir plantation at various growth and development stages; (2) to determine if the different-aged Chinese fir stands affect soil FCO₂; and (3) to examine relationships between soil FCO₂ and relevant soil properties among forest age classes.

Results/Conclusions

The results indicated that the rate of soil FCO₂ was decreased with the increases of forest ages. Annual averaging soil FCO₂ was 1.20, 1.13 and 0.85 μmol m^-2 s^-1 in young, mid-age and mature stands, respectively. The measured CO₂ fluxes ranged from 0.20-0.35 µmol^.m^-2.s^-1 in January to 1.60-2.85 µmol m^-2 s^-1 in July for the three aged stands. A significant seasonal pattern of soil FCO₂ was found in the three ages of the plantations with a peak value occurring in July and the lowest values appearing in January. The daily patterns of soil FCO2 showed that the highest value was appeared around 2:00 pm while the lowest value occurred at the early morning about 4:00 am for three age classes. FCO₂ was strongly correlated with T_soil, but not for W_g. The significant nonlinear relationship between FCO₂ and T_soil was described using an exponential function (r² at 0.81 and Q₁₀ at 2.6) for all three aged forests. It was found that soil depth to bedrock had a strong effect was on soil FCO₂; however, W_g, soil pH and soil organic matter concentration had no detectable effects. Our results suggested that the influences of various aged stands on soil FCO₂ was mainly attributed to the changes in both forest canopy density and root systems in the Chinese fir plantations.