Quantifying and isolating stable soil organic carbon using long-term bare fallow experiments

摘要

The stability of soil organic matter (SOM) is amajor source of uncertainty in predicting atmospheric CO₂ concentrationduring the 21st century. Isolating the stable soil carbon (C) from other, morelabile, C fractions in soil is of prime importance for calibrating soil Csimulation models, and gaining insights into the mechanisms that lead tosoil C stability. Long-term experiments with continuous bare fallow(vegetation-free) treatments in which the decay of soil C is monitored fordecades after all inputs of C have stopped, provide a unique opportunity toassess the quantity of stable soil C. We analyzed data from six bare fallowexperiments of long-duration (>30 yrs), covering a range of soil types andclimate conditions, and sited at Askov (Denmark), Grignon and Versailles(France), Kursk (Russia), Rothamsted (UK), and Ultuna (Sweden). A conceptualthree pool model dividing soil C into a labile pool (turnover time of aseveral years), an intermediate pool (turnover time of a several decades)and a stable pool (turnover time of a several centuries or more) fits wellwith the long term C decline observed in the bare fallow soils. The estimateof stable C ranged from 2.7 g C kg?1 at Rothamsted to 6.8 g C kg?1at Grignon. The uncertainty associated with estimates of the stable pool waslarge due to the short duration of the fallow treatments relative to theturnover time of stable soil C. At Versailles, where there is leastuncertainty associated with the determination of a stable pool, the soilcontains predominantly stable C after 80 years of continuous bare fallow.Such a site represents a unique research platform for characterization ofthe nature of stable SOM and its vulnerability to global change.