Soil respiration in relation to photosynthesis of Quercus mongolica trees at elevated CO2

Knowledge of soil respiration and photosynthesis under elevated CO₂ is crucial for exactly understanding and predicting the carbon balance in forest ecosystems in a rapid CO₂-enriched world. Quercus mongolica Fischer ex Ledebour seedlings were planted in open-top chambers exposed to elevated CO₂ (EC = 500 µmol mol⁻¹) and ambient CO₂ (AC = 370 µmol mol⁻¹) from 2005 to 2008. Daily, seasonal and inter-annual variations in soil respiration and photosynthetic assimilation were measured during 2007 and 2008 growing seasons. EC significantly stimulated the daytime soil respiration by 24.5% (322.4 at EC vs. 259.0 mg CO₂ m⁻² hr⁻¹ at AC) in 2007 and 21.0% (281.2 at EC vs. 232.6 mg CO₂ m⁻² hr⁻¹ at AC) in 2008, and increased the daytime CO₂ assimilation by 28.8% (624.1 at EC vs. 484.6 mg CO₂ m⁻² hr⁻¹ at AC) across the two growing seasons. The temporal variation in soil respiration was positively correlated with the aboveground photosynthesis, soil temperature, and soil water content at both EC and AC. EC did not affect the temperature sensitivity of soil respiration. The increased daytime soil respiration at EC resulted mainly from the increased aboveground photosynthesis. The present study indicates that increases in CO₂ fixation of plants in a CO₂-rich world will rapidly return to the atmosphere by increased soil respiration.