Long-term free-air-CO₂-enrichment increases carbon distribution in the stable fraction in the deep layer of non-clay soils

Elevated CO₂ (eCO₂) in the atmosphere can increase plant C input into soils. However, in dryland cropping systems, it remains unclear how eCO₂ may alter soil organic C content and stability in relation to potential changes in microbial community composition and whether these changes may depend on soil type and depth. Using an eight-year free-air-CO2-enrichment (SoilFACE) system, this study addressed these questions in three farming soils including a sandy Calcarosol, a clay Vertosol and a silt loam Chromosol at depths of 0–40 cm. Long-term eCO₂ did not change soil C content or its distribution in different C fractions in the top 30-cm soil. The majority of the relatively abundant bacterial taxa significantly affected by eCO₂ in the 0–10 cm layer were copiotrophic; this also occurred to fungal community, except for the Calcarosol where some saprotrophs showed a decreasing trend. These changes in microbial taxa indicate that eCO₂ accelerated the decomposition of both new and pre-existing C pools in the topsoil. Although eCO₂ did not change soil C content in the 30–40 cm layer, it increased soil C content in the stable C fraction associated with particles < 50 μm in the Calcarosol (by 39%) and particles < 2 μm in the Chromosol (by 29%). In the 30–40 cm layer of the Calcarosol, many fungal saprotrophs were enriched, and the abundance of fungal community increased under eCO₂. Further investigation is warranted on whether the enhanced stability subsoil C under eCO₂ results from the leaching of stable organic molecules from the topsoil to the subsoil for buildup in the non-clay Calcarosol and Chromosol. Overall, these findings suggest that eCO₂ is likely to enhance soil C stability in the deeper parts of the profile of non-clay soils.