Carbon mineralization of Chinese fir (Cunninghamia lanceolata) soils under different temperature and humidity conditions

Burned and unburned mineral soils (0–10 cm) from a 40-year-old Chinese fir (Cunninghamia lanceolata) forest in Nanping, Fujian, China were incubated for 90 days at different temperatures (25 °C and 35 °C) and humidity [25%, 50%, and 75% of water holding capacity (WHC)] conditions. Carbon (C) mineralization of all soils was determined using CO₂ respiration method. The results showed that CO₂ evolution rates of the burned and control soils exhibited similar temporal patterns, and similar responses to temperature and moisture. CO₂ evolution rates for all soil samples decreased with incubation time. At different humidity conditions, average rate of C mineralization and cumulative mineralized C from burned and control soils were significantly higher at 35 °C than at 25 °C. This implied that C mineralization was less sensitive to soil moisture than to temperature. In both soils at 25 °C or 35 °C, the amount of soil evolved CO₂ over the 90 days incubation increased with increasing moisture content from 25% to 75% WHC. A temperature coefficient (Q₁₀) varied with soil moisture contents. The maximum values recorded for Q₁₀ were 1.7 in control soil and 1.6 in burned soil both at 25% WHC. However, there were no significant differences in Q₁₀ values between the control and burned soils over all moisture ranges (P > 0.05). The data of cumulative C–CO₂ released from control and burned soils were fitted to two different kinetic models. The two simultaneous reactions model described mineralization better than the first-order exponential model, which reflected the heterogeneity of substrate quality. Based on these results, it is possible to conclude that temperature and moisture are important in the controls of C mineralization, and the combined effects of these variables need to be considered to understand and predict the response of CO₂ release in subtropical ecosystems to climate change.