Altered soil microbial community at elevated CO2 leads to loss of soil carbon

Increased carbon storage in ecosystems due to elevated CO₂ may help stabilize atmospheric CO₂ concentrations and slow global warming. Many field studies have found that elevated CO₂ leads to higher carbon assimilation by plants, and others suggest that this can lead to higher carbon storage in soils, the largest and most stable terrestrial carbon pool. Here we show that 6 years of experimental CO₂ doubling reduced soil carbon in a scrub-oak ecosystem despite higher plant growth, offsetting ≈52% of the additional carbon that had accumulated at elevated CO₂ in aboveground and coarse root biomass. The decline in soil carbon was driven by changes in soil microbial composition and activity. Soils exposed to elevated CO₂ had higher relative abundances of fungi and higher activities of a soil carbon-degrading enzyme, which led to more rapid rates of soil organic matter degradation than soils exposed to ambient CO₂. The isotopic composition of microbial fatty acids confirmed that elevated CO₂ increased microbial utilization of soil organic matter. These results show how elevated CO₂, by altering soil microbial communities, can cause a potential carbon sink to become a carbon source.