title: Increase in soil stable carbon isotope ratio relates to loss of organic carbon: results from five long-term bare fallow experiments.
abstract: Changes in the (12)C/(13)C ratio (expressed as δ(13)C) of soil organic C (SOC) has been observed over long time scales and with depth in soil profiles. The changes are ascribed to the different reaction kinetics of (12)C and (13)C isotopes and the different isotopic composition of various SOC pool components. However, experimental verification of the subtle isotopic shifts associated with SOC turnover under field conditions is scarce. We determined δ(13)C and SOC in soil sampled during 1929-2009 in the Ap-horizon of five European long-term bare fallow experiments kept without C inputs for 27-80 years and covering a latitudinal range of 11°. The bare fallow soils lost 33-65% of their initial SOC content and showed a mean annual δ(13)C increase of 0.008-0.024‰. The (13)C enrichment could be related empirically to SOC losses by a Rayleigh distillation equation. A more complex mechanistic relationship was also examined. The overall estimate of the fractionation coefficient (ε) was -1.2 ± 0.3‰. This coefficient represents an important input to studies of long-term SOC dynamics in agricultural soils that are based on variations in (13)C natural abundance. The variance of ε may be ascribed to site characteristics not disclosed in our study, but the very similar kinetics measured across our five experimental sites suggest that overall site-specific factors (including climate) had a marginal influence and that it may be possible to isolate a general mechanism causing the enrichment, although pre-fallow land use may have some impact on isotope abundance and fractionation.
authors.affiliation: Department of Soil and Environment, Swedish University of Agricultural Sciences, P.O. Box 7014, 75007, Uppsala, Sweden. lorenzo.menichetti@slu.se.
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Menichetti
authors.affiliation: INRA, UMR Environnement et Grandes Cultures, 78850, Thiverval-Grignon, France.
authors.forename: Sabine
authors.initials: S
authors.lastname: Houot
authors.affiliation: INRA, Unité Pessac, 78026, Versailles, France.
authors.forename: Folkert
authors.initials: F
authors.lastname: van Oort
authors.affiliation: Department of Ecology, Swedish University of Agricultural Sciences, P.O. Box 7044, 75007, Uppsala, Sweden.
authors.forename: Thomas
authors.initials: T
authors.lastname: Kätterer
authors.affiliation: Department of Agroecology, Aarhus University, AU-Foulum, P.O. Box 50, 8830, Tjele, Denmark.
authors.forename: Bent T
authors.initials: BT
authors.lastname: Christensen
authors.affiliation: AgroParisTech, UMR 7618 BIOEMCO, Bâtiment EGER, 78850, Thiverval-Grignon, France.
authors.forename: Claire
authors.initials: C
authors.lastname: Chenu
authors.affiliation: Laboratoire de Geologie, UMR8538, Ecole Normale Supérieure, 75005, Paris, France.
authors.forename: Pierre
authors.initials: P
authors.lastname: Barré
authors.affiliation: AgroParisTech, UMR 7618 BIOEMCO, Bâtiment EGER, 78850, Thiverval-Grignon, France.|North-Eastern Federal University, Yakutsk, Russia.
authors.forename: Nadezda A
authors.initials: NA
authors.lastname: Vasilyeva
authors.affiliation: School of Science and Technology, Örebro University, 70182, Örebro, Sweden.
authors.forename: Alf
authors.initials: A
authors.lastname: Ekblad
mesh_terms: D000383:Agriculture; D002244:Carbon / Q000032:analysis*; D057486:Carbon Cycle; D002247:Carbon Isotopes / Q000032:analysis*; D002980:Climate; D018556:Crops, Agricultural; D007700:Kinetics; D012987:Soil / Q000737:chemistry*
chemical_list: D002247:Carbon Isotopes; D012987:Soil; D002244:Carbon