Soil respiration and carbon balance in a subtropical native forest and two managed plantations

From 1999 to 2003, a range of carbon fluxes was measured and integrated to establish a carbon balance for a natural evergreen forest of Castanopsis kawakamii (NF) and adjacent monoculture evergreen plantations of C. kawakamii (CK) and Chinese fir (Cunninghamia lanceolata, CF) in Sanming Nature Reserve, Fujian, China. Biomass carbon increment of aboveground parts and coarse roots were measured by the allometric method. Above- and belowground litter C inputs were assessed by litter traps and sequential cores, respectively. Soil respiration (SR) was determined by the alkaline absorbance method, and the contribution from roots, above- and belowground litters was separated by the DIRT plots. Annual SR averaged 13.742 t C ha(-1) a(-1) in the NF, 9.439 t C ha(-1) a(-1) in the CK, and 4.543 t C ha-1 a-1 in the CF. For all forests, SR generally peaked in later spring or early summer (May or June). The contribution of root respiration ranged from 47.8% in the NF to 40.3% in the CF. On average, soil heterotrophic respiration (HR) was evenly distributed between below- (47.3 similar to 54.5%) and aboveground litter (45.5%-52.7%). Annual C inputs (t C ha-1 a-1) from litterfall and root turnover averaged 4.452 and 4.295, 4.548 and 2.313, and 2.220 and 1.265, respectively, in the NF, CK, and CF. As compared to HR, annual net primary production (NPP) of 11.228, 13.264, and 6.491 t C ha(-1) a(-1) in the NF, CK, and CF brought a positive net ecosystem production (NEP) of 4.144, 7.514, and 3.677 t C ha(-1) a(-1), respectively. It suggests that native forest in subtropical China currently acts as an important carbon sink just as the timber plantation does, and converting native forest to tree plantations locally during last decades might have caused a high landscape carbon loss to the atmosphere.