Changes in soil carbon flux and carbon stock over a rotation of poplar plantations in northwest China

Forest soil is a major component of terrestrial ecosystems for carbon sequestration and plays an important role in the global carbon cycle. Soil carbon flux and soil carbon pools were investigated in a poplar plantation chronosequence over a rotation in northwest China. Based on continuous field observation in 2007, the results showed that mean soil CO(2) efflux rate was 5.54, 4.81, and 3.93 mu mol CO(2) m(-2) s(-1) for stands of 2-, 8-, and 15-year-old, respectively, during the growing season. Significant differences in soil respiration of three age classes were mainly because soil temperature, carbon allocation, and fine root growth changed greatly with stand age. Multiple regression analysis suggested that soil temperature and fine root biomass in the upper layer could explain 78-85% of the variation in soil respiration. Mineral soil C stock at 0-40 cm depth was 55.77, 55.09, and 58.14 t ha(-1) in the 2-, 8-, and 15-year-old stands, respectively. The average rate of soil C sequestration was 0.13 t ha(-1) year(-1) following afforestation on former crop lands. Although the plantations had similar management practices and soil types since their establishment, many biotic and abiotic factors such as root biomass and turnover rate, soil condition of the plantations had undergone marked changes at different development stages, which could result in the remarkable differences in soil carbon flux and storage over a rotation. Our results highlight the importance of the development stage within a rotation of poplar plantation in assessment of soil carbon budget.