Susceptibility of soil organic carbon to priming after long-term CO2 fumigation is mediated by soil texture

© 2018 Elsevier B.V. Elevated CO2 (eCO2) may enhance soil organic carbon (SOC) sequestration via greater input of photosynthetic carbon (C). However, greater rhizodeposits under eCO2 may stimulate microbial decomposition of native SOC. This study aimed to examine the status and stability of SOC in three Australian cropping soils after long-term CO2 enrichment. Samples (0–5 cm) of Chromosol, Vertosol and Calcarosol soils were collected from an 8-year Free-air CO2 Enrichment (SoilFACE) experiment and were used to examine SOC dynamics by physical fractionation and incubation with 13C-glucose. Compared to the ambient CO2 (aCO2) (390–400 μmol mol−1), 8 years of elevated CO2 (eCO2) (550 μmol mol−1) did not increase SOC concentration of all soils, but changed SOC distribution with 12% more C in coarse soil fractions and 5% less C in fine fractions. Elevated CO2 also enhanced the susceptibility of SOC to 13C-glucose-induced priming, but this effect was only significant in the coarse-textured Calcarosol topsoil. The eCO2 history increased labile C (coarse C fraction, +13%) and soil pH (+0.25 units), and decreased available N (−30%) in the Calcarosol, which stimulated microbial biomass C by 28%, leading to an enhanced priming effect. Despite with greater total primed C, the Chromosol that had the highest amount of native C, had lower primed C per unit of SOC when compared to the low-C Calcarosol. In conclusion, the effect of long-term eCO2 enrichment on soil C and N availability in cropping soils depended on soil type with the coarse-textured Calcarosol soil being more susceptible to substrate-induced decomposition of its SOC.