Quantifying and isolating stable soil organic carbon using long-term bare fallow experiments
- 1BIOEMCO Laboratory, UMR ParisVI-ParisXII-AgroParisTech-CNRS-IRD-ENS, Campus Grignon, 78850 Thiverval-Grignon, France
- 2LSCE, UMR CEA-CNRS-UVSQ, CE L'Orme des Merisiers, 91191 Gif-sur-Yvette, France
- 3University of Aarhus, Faculty of Agricultural Sciences, Department of Agroecology and Environment, 8830 Tjele, Denmark
- 4EGC laboratory, UMR INRA-AgroParisTech, Campus Grignon, 78850 Thiverval-Grignon, France
- 5Swedish University of Agricultural Sciences, Department of Soil and Environment, 75007 Uppsala, Sweden
- 6PESSAC laboratory, INRA, 78026 Versailles, France
- 7Rothamsted Research, Department of Soil Science, Harpenden, Herts, AL5 2JQ, UK
- 8Pryanishnikov All Russian Institute for Agrochemistry VNIIA, Moscow 127550, Russia
- *now at: Geology laboratory, UMR CNRS-ENS, Ecole normale supérieure, 75005 Paris, France
- **These authors contributed equally to this work.
Abstract. The stability of soil organic matter (SOM) is a major source of uncertainty in predicting atmospheric CO2 concentration during the 21st century. Isolating the stable soil carbon (C) from other, more labile, C fractions in soil is of prime importance for calibrating soil C simulation models, and gaining insights into the mechanisms that lead to soil C stability. Long-term experiments with continuous bare fallow (vegetation-free) treatments in which the decay of soil C is monitored for decades after all inputs of C have stopped, provide a unique opportunity to assess the quantity of stable soil C. We analyzed data from six bare fallow experiments of long-duration (>30 yrs), covering a range of soil types and climate conditions, and sited at Askov (Denmark), Grignon and Versailles (France), Kursk (Russia), Rothamsted (UK), and Ultuna (Sweden). A conceptual three pool model dividing soil C into a labile pool (turnover time of a several years), an intermediate pool (turnover time of a several decades) and a stable pool (turnover time of a several centuries or more) fits well with the long term C decline observed in the bare fallow soils. The estimate of stable C ranged from 2.7 g C kg−1 at Rothamsted to 6.8 g C kg−1 at Grignon. The uncertainty associated with estimates of the stable pool was large due to the short duration of the fallow treatments relative to the turnover time of stable soil C. At Versailles, where there is least uncertainty associated with the determination of a stable pool, the soil contains predominantly stable C after 80 years of continuous bare fallow. Such a site represents a unique research platform for characterization of the nature of stable SOM and its vulnerability to global change.