Seasonal responses of soil respiration to elevated CO2 and N addition in young subtropical forest ecosystems in southern China

Atmospheric carbon dioxide (CO2) concentration and nitrogen (N) deposition may interactively impact soil respiration in terrestrial ecosystems. However, the effects of elevated CO2 and N deposition on soil respiration are not fully understood especially in subtropical forest ecosystems in southern China where ambient N deposition is high. In this study, we investigated the seasonal responses of soil respiration to elevated CO2 and N addition in young subtropical forest ecosystems in southern China using open-top chambers. Two CO2 treatment levels (i.e., ambient CO2 and 700 μmol mol−1) and two N levels (i.e., ambient and 100 kg N ha−1 yr−1) were considered. Forty-eight 1–2 years old seedlings (eight seedlings per each of 6 species) were planted in each chamber. In the wet season, soil respiration significantly enhanced under elevated CO2 and N addition with no interactive effect. There was not significant relationship between treatment-enhanced soil respiration and soil moisture in the wet season, suggesting that increases in soil respiration resulted in large part from treatment-induced increases in tree growth and C inputs. In the dry season, soil respiration significantly enhanced under elevated CO2, but did not significantly change under N addition. The N-induced change in soil respiration negatively corrected with soil moisture in the dry season, indicating that decrease in soil moisture under N addition, together with increase in tree growth interactively resulted in negligible net change in soil respiration. Our results highlighted the seasonal importance of environmental controls under elevated CO2 and N deposition in the assessment of soil C sequestration potential in subtropical forest ecosystems in southern China.