Altered soil microbial community at elevated CO(2) leads to loss of soil carbon

Abstract

Increased carbon storage in ecosystems due to elevated CO(2) may help stabilize atmospheric CO(2) concentrations and slow global warming. Many field studies have found that elevated CO(2) 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 CO(2) doubling reduced soil carbon in a scrub-oak ecosystem despite higher plant growth, offsetting approximately 52% of the additional carbon that had accumulated at elevated CO(2) 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 CO(2) 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 CO(2). The isotopic composition of microbial fatty acids confirmed that elevated CO(2) increased microbial utilization of soil organic matter. These results show how elevated CO(2), by altering soil microbial communities, can cause a potential carbon sink to become a carbon source.

Fig. 1.

Declines in soil carbon content over time are driven by the acceleration of soil organic matter decomposition. (A) The difference between mean soil carbon content at elevated and ambient CO₂ over time. The dashed lines show upper and lower 95% confidence intervals of the regression line. (B) The relationship between the priming observed during the laboratory experiment and light fraction soil C content in April 2002. Here priming is defined as (C_l − C_c), where C_l is the amount of CO₂ generated from soil organic matter decomposition in soils to which litter was added and C_c is the amount of CO₂ generated from soil organic matter decomposition in control soils. The light fraction is the largest and most rapidly cycling soil carbon pool. We also found a negative relationship between priming in the laboratory and total soil C content (r² = 0.28, P = 0.034).

Fig. 2.

Soil microbial activity and composition are altered by elevated CO₂. (A) Phenol oxidase activity (μmol·h⁻¹·g⁻¹) before the beginning of the decomposition experiment (±1 SE). (B) Fungi:bacteria values derived from analyses of phospholipids fatty acid contents of soil microbial communities (±1 SE). During analysis, samples from two elevated CO₂ chambers were lost; therefore, for the elevated CO₂ treatment n = 6, and for ambient CO₂ n = 8. (C) The shift in the isotopic signatures of fungal and bacterial fatty acids in soils to which depleted ¹³C litter was added (±1 SE); this is expressed relative to the fatty acid signatures of control soils. Uppercase letters denote differences across treatments in fungal fatty acid shifts, and lowercase letters are used for comparisons of bacterial fatty acid shifts at P < 0.10.