Reduced tillage and cover cropping are often considered as measures to increase carbon sequestration in cropland soils. We hypothesized that these management practices could result in an increase in carbon leaching. To examine this possibility we assessed carbon leaching from an Irish arable soil under spring barley, either with conventional management or non-inversion tillage plus cover cropping. Concentrations of biogenic dissolved inorganic carbon (DIC) were considerably higher under reduced tillage probably due to a higher supersaturation of soil solution with respect to partial pressures of CO2 in soil air resulting from a reduced abundance of tillage-induced macropores. Leaching losses of DOC equalled 3 ± 0.3 g m−2 yr−1 for conventional management as well as for non-inversion tillage plus cover cropping. Losses of biogenic DIC were 14.5 ± 4.4 g m−2 yr−1 for conventional tillage and 34.0 ± 4.7 g m−2 yr−1 for non-inversion tillage plus cover crop. Higher leaching losses from the non-inversion tillage plus cover crop plot thereby reduced potential soil carbon gains by 20 g m−2 yr−1 compared to the conventionally managed treatment highlighting the need to consider leaching losses in estimates of carbon sequestration, especially if these are deduced from balancing carbon inputs and outputs.