Soil Respiration And Total Belowground Carbon Allocation Along A Chinese Fir Chronosequence

Age-related changes and controlling mechanisms of soil respiration (SR) and total belowground carbon allocation (TBCA), the two most important carbon fluxes of forest ecosystems, are of much significance in understanding the effects of stand age on forest carbon source/sink, regional forest carbon budget and forest carbon management.A Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) chronosequence, consisting of stands of 2-, 7-, 16-, 40- and 88-year-old that encompassed the longest age span ever selected, located on the same sites at the famous Chinese fir planting area in Ancaoxia, Wangtai, Nanping City, Fujian Province, was chosen for a research purpose on age-related changes and controlling factors of soil respiration and TBCA. This will help greatly for carbon cycle model construction, carbon sink/source account and carbon management of this most important timber species in southern China.Monthly soil respiration rate was related to seasonal fluctuation of litterfall in addition to changes of soil temperature and soil moisture content. When all ages exept the 2-year-old stand were pooled together, monthly rates of total soil respiration, heterotrophic respiration and root respiration were postively correlated with the 3-month-preceding litterfall, and negatively related to the following month litterfall. The Qio values of total soil respiration and heterotrophic respiration were not significantly different among stand ages, and those of root respiration were detected significantly different only between age 21 and age 88. Annual CO₂ flux from total soil respiration first increased and then decreased with stand age, with significantly higher in age 40 than in age 2 and 7. A similar trend occurred for annual CO₂ flux from root respiration, with significantly lower in age 2 and 88 than in age 7～40. Annual CO₂ flux from soil heterotrophic respiration showed a decrease and then an increase pattern with stand age, with significant differences between age 88 and age 7 or 16. The age-related changes of total soil respiration, heterotrophic respiration and root respiration can be fitted very well by a time curve model. Annual CO₂ flux of total soil respiration was highly correlated with annual litterfall, and that of root respiration positively correlated with fine root biomass and aboveground NPP and negatively correlated with SOC stock. The contributions of root respiration to total soil respiration, ranging from 25.0% to 40.7%, first increased and then decreased with stand age, and was highly positively correlated with fine root biomass, aboveground NPP and total NPP and negatively correlated with SOC stock. TBCA showed an increase trend followed by a decrease trend with stand age, with significantly higher in age 16 and 21 than in age 7 and 40, and with the lowest value in age 88. The root carbon use efficiency (RCUE) was close to 0.5 in age 7～21 and much lower than 0.5 in age 40 and 88, indicating the failure of the general RCUE hypothesis of 0.5 in the mature and old-growth stages.