Net carbon flux from cropland changes in the Central Asian Aral Sea Basin

The transboundary Aral Sea Basin (ASB) covers parts of the former Union of Soviet Socialist Republics (USSR), the central Asian portion of Kazakhstan (KAZ), Uzbekistan (UZB), Turkmenistan (TUK), Kyrgyzstan (KGZ), and Tajikistan (TAJ). During recent decades, the region has experienced widespread cropland changes and has therefore attracted attention. However, carbon flux caused by these changes, which is critical to understand the carbon cycle in the region and to develop strategies for carbon sequestration, has not been quantified. We applied the Bookkeeping Model to analyze carbon flux caused by cropland changes. We found that the cropland area expanded from 1975 to 2019. Prior to 1990, the net increment in cropland area was 64.47 kha yr^-1; this decreased to 11.02-18.69 kha yr^-1 after the collapse of the USSR (1991). Grasslands and bare lands with low carbon density were the main types of land that were converted to cropland, accounting for approximately 70.3-99.29% of the land cleared for cropland. In terms of cropland loss, grassland accounted for more than 40.69% of the cropland converted to other land cover types, followed by artificial surfaces, which accounted for 25.84-45.16%. The expansion of cropland served as a carbon sink owing to the increase in irrigation and fertilization. The overall potential carbon emission was -89.38 ± 35.34 Tg C, and contributions of the five countries were as follows: TUR (-33.65 ± 6.30 Tg C), UZB (-29.23 ± 25.35 Tg C), KAZ (-12.76 ± 12.16 Tg C), TAJ (-11.11 ± 5.47 Tg C), KGZ (-2.63 ± 7.83 Tg C). The net carbon flux was -2.04 ± 0.23 Tg C yr^-1 before 1990, and it decreased to -1.87 ± 1.03 Tg C yr^-1 during the post-Soviet era. Finally, we state sustainable agricultural management is urgently needed to deal with the environmental problems of the Aral Sea, which have been primarily caused by cropland expansion.