Soil respiration under elevated CO2 and its partitioning into recently assimilated and older carbon sources

Efflux of soil CO2 (soil respiration) plays a crucial role in the global carbon cycle and may be strongly altered by global change. In this study, we measured soil respiration in situ under elevated CO2 concentration (550 μmol mol−1), and estimated the importance of recent carbon (root/rhizosphere respiration) vs. older carbon to the total soil CO2 efflux. The measurements were carried out during 2001 in a sugar beet field at the Free-Air Carbon-dioxide Enrichment (FACE) facility at the Federal Agricultural Research Centre (Braunschweig, Germany). During a period of high plant activity, soil respiration was considerably higher (34%) in the rings with elevated CO2 concentration than in the control rings (fumigated with ambient air). During a day with low plant activity, however, no differences in soil CO2 effluxes were detected. Different levels of N fertilisation generally had no effect on soil respiration in ambient and elevated rings, and soil microbial biomass generally did not change in response to the CO2 enrichment. To determine the contribution of recently assimilated C to total soil respiration, we used the stable C isotope tracer provided by the CO2 used to increase the atmospheric CO2 concentration, which was depleted in 13C. Roots and rhizosphere contributed 70±4% to total soil respiration, independent of overall soil CO2 fluxes. The observed dominance of carbon losses from recently assimilated carbon and a minute depletion in the carbon isotope ratio of the top soil of 0.4‰ under elevated CO2 adds major uncertainties to the anticipated increase of soil carbon storage in the future.