Visiting dark sides of model simulation of carbon stocks in European temperate agricultural soils: allometric function and model initialization

Aims Predictions of medium to long-term changes in soil organic carbon (SOC) stocks involve simulation models with plant carbon (C) inputs estimated from crop yields using crop harvest indices and allometric scaling involving fixed root-to-shoot ratios. The choice of allometric approach and initial distribution of SOC among model pools is crucial for model estimates. Methods We evaluated the impact of a modified allometric approach for grass leys and initial model set-up using the C-TOOL model. Four European agricultural long-term experiments were considered including three experiments with different frequency of grass leys in the crop rotation and one permanent grassland. Results The modified allometric function for ley relied on fixed below-ground C input regardless of mineral fertilizer inputs and modified pool initialization involved available site history. Including available, but insufficient, pre-experiment history to adjust the initial set-up of model SOC pools did not improve to the C-TOOL simulations. Changing the allometric approach for ley from fixed shoot-to-root ratios to fixed below-ground C input decreased the soil C input dramatically and improved the C-TOOL simulation of SOC stocks for fertilized treatments in all experiments when combined with standard model set-up. For unfertilized treatments, however, the efficiency of the standard allometric function was superior to the modified one. Conclusions Our results challenge the prevalent use of allometric approaches based on fixed root-to-shoot in SOC simulation models. The potential of modifying the initial distribution of SOC among model pools by accounting for historic land use remains in the dark.