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 ¹³C) 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 ¹³CO ₂ of the C mineralized from the incubated soil closely tracked the δ ¹³C 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]. Published 2012 This article is a U.S. Government work and is in the public domain in the USA.