Decadal exposure to elevated CO2 and elevated O3 decreases soil carbon content in northern temperate forests: Final results from Aspen FACE

Background/Question/Methods

Because soil carbon (C) is ultimately derived from plant C, it is often hypothesized that environmental factors that influence plant productivity will cause parallel changes in soil C content. However, while it has been widely observed that elevated atmospheric carbon dioxide (CO₂) increases plant growth and tropospheric ozone (O₃) pollution decreases plant growth, there is considerably less information about how these gases individually and interactively impact soil C. The Aspen free-air CO₂ enrichment (FACE) experiment exposed three northern temperate forest communities in the north-central United States to factorial combinations of CO₂ and/or O₃ for 11 years. At the conclusion of the experiment, an extensive sampling of the mineral soil and fine roots (< 2 mm diameter) was conducted to a depth of 1 m from the soil surface in 10 cm increments.  Here, we report changes in fine root C and soil C. In addition, we use the differences in plant δ¹³C caused by the use of fossil fuel –derived CO₂ that is depleted in ¹³C to quantify the amount of soil C formed since the beginning of the experiment under elevated CO₂.

Results/Conclusions

Neither CO₂ nor O₃ affected the overall amount of fine root C. However, elevated O₃ shifted the distribution of fine roots toward the soil surface (O₃ × depth: P = 0.041), with increased fine root C in the top 20 cm of soil (+7 g m^-2) and decreased fine root C deeper in the soil, particularly at 50 to 70  cm in depth (-3 g m^-2). As with fine root C, neither CO₂ nor O₃ affected the total amount of C in the top 1 m of mineral soil. However, near the soil surface, each gas significantly decreased mineral soil C content: elevated O₃ decreased soil C within the top 10 cm of soil (P = 0.003), whereas elevated CO₂ decreased soil C from 10 to 20 cm in depth (P < 0.001). Elevated CO₂ also decreased soil C at 40 to 50 cm in depth (P = 0.003). Differences in soil δ¹³C caused by elevated CO₂ decreased with depth, but were significant at all increments except 70-80 cm. Within the top 10 cm of soil, elevated O₃ increased the proportion of isotopically new soil C formed under elevated CO₂ and decreased the pool of older soil C.