Most land use changes (LUC) significantly affect the amount of carbon (C) sequestered in vegetation and soil, thereby, shifting the C balance in ecosystems. Disintegration of the USSR and the followed collapse of collective farming system have led to abandonment of more than 58 million ha (Mha) of former croplands in Russia and Kazakhstan that comprise together about 90% of land area in the former USSR. This was the most widespread and abrupt LUC in the 20th century in the northern hemisphere. The spontaneous withdrawal of croplands in 1990s caused several benefits for environment including substantial C sequestration in post-agrogenic ecosystems. The new estimations of net ecosystem production (NEP) and changes in soil organic carbon stocks (Delta SOC) in post-agrogenic ecosystems presented here are based on the uniform bio-climatic approach, and hereby, allow to update C balance of the former USSR. The total extra C sink in abandoned croplands in Russia (45.5 Mha) and Kazakhstan (12.9 Mha) is estimated to be 15527 Mt C yr -1 and 312 Mt C yr -1, respectively. This additional C sink could cover about 18% of the global CO 2 release due to deforestation and other land use changes or compensate annually about 36% and 49% of the current fossil fuel emissions in Russia and Kazakhstan, respectively. The extra C sink to the post-agrogenic ecosystems in Russia and Kazakhstan contributes possibly about 1/3 part to the total current C balance of the former USSR. Hence, the disintegration of the former USSR significantly affected national and global C budget over few decades after LUC.
