Soil organic matter dynamics in grassland soils under elevated CO 2: Insights from long-term incubations and stable isotopes

Abstract

Elevated atmospheric carbon dioxide (CO 2) levels generally stimulate carbon (C) uptake by plants, but the fate of this additional C largely remains unknown. This uncertainty is due in part to the difficulty in detecting small changes in soil carbon pools. We conducted a series of long-term (170–330 days) laboratory incubation experiments to examine changes in soil organic matter pool sizes and turnover rates in soil collected from an open-top chamber (OTC) elevated CO 2 study in Colorado shortgrass steppe. We measured concentration and isotopic composition of respired CO 2 and applied a two-pool exponential decay model to estimate pool sizes and turnover rates of active and slow C pools. The active and slow C pools of surface soils (5–10 cm depth) were increased by elevated CO 2, but turnover rates of these pools were not consistently altered. These findings indicate a potential for C accumulation in near-surface soil C pools under elevated CO 2. Stable isotopes provided evidence that elevated CO 2 did not alter the decomposition rate of new C inputs. Temporal variations in measured δ 13C of respired CO 2 during incubation probably resulted mainly from the decomposition of changing mixtures of fresh residue and older organic matter. Lignin decomposition may have contributed to declining δ 13C values late in the experiments. Isotopic dynamics during decomposition should be taken into account when interpreting δ 13C measurements of soil respiration. Our study provides new understanding of soil C dynamics under elevated CO 2 through the use of stable C isotope measurements during microbial organic matter mineralization.