Soil carbon and nitrogen cycling and storage throughout the soil profile in a sweetgum plantation after 11 years of CO 2 ‐enrichment

Increased partitioning of carbon (C) to fine roots under elevated [ CO 2 ], especially deep in the soil profile, could alter soil C and nitrogen (N) cycling in forests. After more than 11 years of free‐air CO 2 enrichment in a Liquidambar styraciflua L. (sweetgum) plantation in Oak Ridge, TN , USA , greater inputs of fine roots resulted in the incorporation of new C (i.e., C with a depleted δ 13 C) into root‐derived particulate organic matter ( POM ) pools to 90‐cm depth. Even though production in the sweetgum stand was limited by soil N availability, soil C and N contents were greater throughout the soil profile under elevated [ CO 2 ] at the conclusion of the experiment. Greater C inputs from fine‐root detritus under elevated [ CO 2 ] did not result in increased net N immobilization or C mineralization rates in long‐term laboratory incubations, possibly because microbial biomass was lower in the CO 2 ‐enriched plots. Furthermore, the δ 13 CO 2 of the C mineralized from the incubated soil closely tracked the δ 13 C of the labile POM pool in the elevated [ CO 2 ] treatment, especially in shallower soil, and did not indicate significant priming of the decomposition of pre‐experiment soil organic matter ( SOM ). Although potential C mineralization rates were positively and linearly related to total SOM C content in the top 30 cm of soil, this relationship did not hold in deeper soil. Taken together with an increased mean residence time of C in deeper soil pools, these findings indicate that C inputs from relatively deep roots under elevated [ CO 2 ] may increase the potential for long‐term soil C storage. However, C in deeper soil is likely to take many years to accrue to a significant fraction of total soil C given relatively smaller root inputs at depth. Expanded representation of biogeochemical cycling throughout the soil profile may improve model projections of future forest responses to rising atmospheric [ CO 2 ].