This study aims to test the effects of forest age on soil respiration in poplar ecosystems in northern China and to separate the contributions of root respiration (Rr) and soil microbes to the total soil respiration (Rs). Rs in three poplar forests (5, 10, and 15 years old) were measured using an LI-6400-09 soil chamber connected to an LI-6400 portable infrared gas analyzer during the growing seasons in 2007 and 2008. Root respiration was measured using the root excision method. The soil micro-organisms were quantified using the dilution-plate method. The results show that Rs was the highest in the 5-year-old forest and lowest in 15-year-old forest. The contribution of Rr to Rs ranged from 29.4 to 81.0%. Rr/Rs tended to be significantly higher in the 15-year-old forest than that in the younger forests; but Rr was the highest in the 5-year-old forest. Temporal variation in Rs can be largely accounted by fine-root biomass (R = 0.718), while soil N was significantly negatively correlated with Rs (R = -0.646). Rs, Rr and Rr/Rs vary significantly with the forest age. The lower Rs in the older forests increased their carbon use efficiency. Underground factors, dominated by fine-root biomass, affect Rs, Rr and Rr/Rs substantially. Soil microbial community structure is a particularly important underground factor.
