Elevated [CO₂] has increased the production of fine roots in a long-term Free-Air CO₂ Enrichment (FACE) experiment in a sweetgum plantation in eastern Tennessee. Increased fine-root production has led to greater C and N inputs to the soil from root mortality, as fine roots in this forest turn over in less than 1 year. The degree to which root inputs are protected from decomposition will determine the longevity of C and N in soils under elevated atmospheric [CO₂]. Our objective in this study was to determine whether greater root inputs under elevated [CO₂] would alter the transfer of C and N into soil fractions ranging from particulate organic matter (POM) to mineral soil. We allowed roots sampled from the current and elevated [CO₂] treatments to decompose in root-free soil in a reciprocal design. Roots from both treatments were allowed to decompose in their soil of origin and soil from the opposite treatment for two years. To reflect true C inputs, root amounts varied within and among treatments. We physically fractionated the soil, and quantified root-derived C and N distribution among soil pools using the unique ¹³C-depleted isotopic signature of organic matter produced under elevated [CO₂], in addition to mass-based approaches.

Results/Conclusions

Sustained C and N inputs under elevated [CO₂] were reflected in greater initial C and N content in the POM pools of the elevated soil. During the decomposition experiment, identifiable root material in soil of both current and elevated [CO₂] origin declined by more than 50 %, while POM pools in both soils increased. The amount of root material moving into POM fractions was correlated with initial root inputs in both soils. Further, root inputs were rapidly transferred to mineral pools, appearing in a period of months rather than years. Some soil pools, such as the light coarse POM fraction (> 250 µm, density < 1.4 g cm^-3), which is composed of root-like organic matter, and the clay-sized fraction (< 2 µm), which has a large surface area for C exchange and adsorption, were more dynamic than others in C and N accrual. Interactions between root inputs and soil origin determined the amount of C and N in each pool over the course of the decomposition experiment. In conclusion, both the amount of root input and the soil origin affected the amount of C and N transferred among soil pools in a CO₂-enriched sweetgum plantation.