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Abstract

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 mumol mol(-1)), and estimated the importance of recent carbon (root/rhizo sphere 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 C-13. 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 0.4parts per thousand under elevated CO₂ adds major uncertainties to the anticipated increase of soil carbon storage in the future. [References: 42]