Projections of 30-year soil carbon balances for a semi-natural grassland under elevated CO2 based on measured root decomposability

Long-term soil C projections were made for a semi-natural grassland under CO2 elevation in Central Sweden based on treatment-induced differences in input quantity, input quality and measured microclimate. Three treatments were applied to the grassland during 6 years: Ambient and Elevated (+350 μmol mol−1) CO2 levels in open-top chambers and Control. Roots grown during the fifth experimental year were incubated under controlled conditions for 160 days and showed significant treatment differences in decomposition rates. The fraction of C lost during incubation of Elevated was 57% while only 53% for Ambient and 52% for Control. The incubation results were fit to a decomposition model and a humification coefficient was used to account for treatment-induced differences in root quality. Annual soil C input was calculated considering both root and shoot input estimates and was 103, 74 and 92 g C m−2 year−1 for Elevated, Ambient and Control, respectively. A climate factor represented measured microclimate differences. Due to drier conditions this factor was somewhat lower for Ambient than for Control and Elevated. The ICBM model was used for 30-year soil C projections. The input, quality and climate parameters for Control resulted in projections indicating that the present measured soil C store, 5.5 kg C m−2 at 0–15 cm, is near steady-state. The soil C pool in Ambient was projected to lose 90 g C m−2 in 30 years primarily due to the decreased input. Although Elevated had the greatest input, this did not compensate for the increased root decomposability and Elevated was projected to lose 70 g C m−2. We discuss the validity of the projections and test other possible scenarios. A tentative conclusion is that the expected CO2 and temperature increase during the next 30 years will have only minor effects on the soil carbon content of this system, unless plant production is severely reduced by weather irregularities or even disasters.