Forest soil carbon cycle under elevated CO2 - a case of increased throughput?

Abstract

Forest soils account for a large part of the stable carbon pool held in terrestrial ecosystems. Future levels of atmospheric CO₂ are likely to increase C input into the soils through increased above- and below-ground production of forests. This increased input will result in greater sequestration of C only if the additional C enters stable pools. In this review, we compare current observations from four large-scale Free Air FACE Enrichment (FACE) experiments on forest ecosystems (EuroFACE, Aspen-FACE, Duke FACE and ORNL-FACE) and consider their predictive power for long-term C sequestration. At all sites, FACE increased fine root biomass, and in most cases higher fine root turnover resulted in higher C input into soil via root necromass. However, at all sites, soil CO₂ efflux also increased in excess of the increased root necromass inputs. A mass balance calculation suggests that a large part of the stimulation of soil CO₂ efflux may be due to increased root respiration. Given the duration of these experiments compared with the life cycle of a forest and the complexity of processes involved, it is not yet possible to predict whether elevated CO₂ will result in increased C storage in forest soil.