Regional monitoring, reporting and verification of soil organic carbon change occurring in managed cropland are indispensable to support carbon-related policies. Rapidly evolving gridded agronomic models can facilitate these efforts throughout Europe. However, their performance in modelling soil carbon dynamics at regional scale is yet unexplored. Importantly, as such models are often driven by large-scale inputs, they need to be benchmarked against field experiments. We elucidate the level of detail that needs to be incorporated in gridded models to robustly estimate regional soil carbon dynamics in managed cropland, testing the approach for regions in the Czech Republic. We first calibrated the biogeochemical Environmental Policy Integrated Climate (EPIC) model against long-term experiments. Subsequently, we examined the EPIC model within a top-down gridded modelling framework constructed for European agricultural soils from Europe-wide datasets and regional land-use statistics. We explored the top-down, as opposed to a bottom-up, modelling approach for reporting agronomically relevant and verifiable soil carbon dynamics. In comparison with a no-input baseline, the regional EPIC model suggested soil carbon changes (~0.1-0.5 Mg C ha-1 y-1) consistent with empirical-based studies for all studied agricultural practices. However, inaccurate soil information, crop management inputs, or inappropriate model calibration may undermine regional modelling of cropland management effect on carbon since each of the three components carry uncertainty (~0.5-1.5 Mg C ha-1 y-1) that is substantially larger than the actual effect of agricultural practices relative to the no-input baseline. Besides, inaccurate soil data obtained from the background datasets biased the simulated carbon trends compared to observations, thus hampering the model's verifiability at the locations of field experiments. Encouragingly, the top-down agricultural management derived from regional land-use statistics proved suitable for the estimation of soil carbon dynamics consistently with actual field practices. Despite sensitivity to biophysical parameters, we found a robust scalability of the soil organic carbon routine for various climatic regions and soil types represented in the Czech experiments. The model performed better than the tier 1 methodology of the Intergovernmental Panel on Climate Change, which indicates a great potential for improved carbon change modelling over larger political regions.

Central Institute for Supervising and Testing in Agriculture Hroznová 63 2 656 06 Brno Czech Republic

Crop Research Institute Division of Crop Management Systems Drnovská 507 73 161 06 Praha 6 Ruzyně Czech Republic

Environmental Change Institute University of Oxford South Parks Road Oxford OX1 3QY United Kingdom

European Commission Joint Research Centre 21027 Ispra Italy

Faculty of Natural Sciences Comenius University in Bratislava Ilkovičova 6 842 15 Bratislava Slovak Republic

Institute for Sustainable Economic Development University of Natural Resources and Life Sciences Vienna Feistmantelstrasse 4 1180 Vienna Austria

International Institute for Applied Systems Analysis Ecosystems Services and Management Program Schlossplatz 1 A 2361 Laxenburg Austria

National Agricultural and Food Centre Soil Science and Conservation Research Institute Trenčianska 55 821 09 Bratislava Slovak Republic

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