Photosynthetic limits on carbon sequestration in croplands

•Estimates for maximum soil C sequestration rates often take soil as starting point.•Our “Fermi approach” suggests instead to take photosynthesis as the starting point.•Our results suggest maximum sequestration rates in global croplands of 0.2 Pg yr−1.•This is an order of magnitude lower than most previous estimates.•Uncertainty analysis suggests that our results, while relatively rough, are robust.