Organic soils, particularly peatlands, are important carbon sinks. In Europe, almost half of its area has been drained and is primarily used for agriculture as grasslands. In Poland, 86% of the drained peatland area causes problems with high carbon dioxide emissions due to organic matter oxidation. The aim of the study was to analyze changes in organic soils between 1975 and 2017 using data from 476 reference soil profiles. The results showed a significant decrease in organic soils (≥12% SOC) from 62% to 35%, and an increase in organo-mineral (6.0-12.0% SOC) and non-humose mineral soils (≤1.75% SOC) by a similar percentage. The depth of SOC rich layers also decreased significantly, with deep soils (>100 cm) dropping from 20% to 5% and shallow soils (21-50 cm) increasing from 24% to 58%. The average SOC stock loss was almost 273 Mg C·ha ^-1 (from 500.42 Mg C·ha ^-1 to 228.04 Mg C·ha ^-1), with the median being four times lower. A decrease in SOC stock was observed in 64% of sites, while an increase was seen in 36%. The most significant decrease in SOC stock, by 64.67% compared to 1975, occurred in organic soils (from 762.19 Mg C·ha ^-1 to 269.26 Mg C·ha ^-1). There was an increase in SOC stock in organo-mineral, humose, and non-humose mineral soils, although it was not statistically significant. Moreover, the results indicated that continuous grassland use on organic soils does not offset carbon losses from organic matter oxidation due to drainage.