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Drainage and cultivation of peat soils stimulates soil organic matter (SOM) mineralization, which substantially increases CO2 emissions from soils. Large uncertainties are associated with this CO2 flux, and little data are available, especially in Norway. The objective of the present research was to estimate C losses from cultivated peatlands in West Norway by three independent methods: (1) long-term monitoring of subsidence rates, (2) changes in ash contents, and (3) soil CO2 flux measurements. Subsidence of cultivated peat soils averaged about 2.5 cm year−1. We estimated that peat loss and compaction were respectively responsible for 38% and 62% of the total subsidence during a 25-year period after drainage. Based on this estimate the corresponding C loss equals 0.80 kg C m−2 year−1. The observed increase in mineral concentration of the topsoil of cultivated peat is proportional to their C loss, providing no mineral particles other than lime and fertilizers are added to the soil. Using this novel approach across 11 sites, we estimated a mean C loss of 0.86 kg C m−2 year−1. Soil CO2 flux measurements, corrected for autotrophic respiration, yielded a C loss estimate from cultivated peat soils of 0.60 kg C m−2 year−1. The three methods yielded fairly similar estimates of C losses from Norwegian cultivated peatlands. Cultivated peatlands in Norway cover an estimated 63,000 ha. Total annual C losses from peat degradation were estimated to range between 1.8 and 2 million tons CO2 year−1, which equals about 3–4% of total anthropogenic greenhouse gas emissions from Norway.