Analysis of the consequences of land-use changes and soil types on organic carbon storage in the Tarim River Basin from 2000 to 2020

With fast development of industry and agriculture in last two decades, land use changed greatly on specific soil types of intrinsic low quality with C loss potential and necessitates avoiding. Although researchers studied the effect of land-use changes on soil carbon storage, that on some soil types had rarely been reported, which led to relatively more C loss. We explored the impact of land-use changes and soil types on soil organic carbon (SOC) sequestration in the top 30 cm in the Tarim River Basin (the second largest inland river in the world) from 2000 to 2020. The land-use images with a one-kilometer spatial resolution and soil data from the Harmonized World Soil Database were analysed. The groundwater levels were monitored by 11 wells along the river from 2012 to 2017. Land-use changes were dominated by increases in the areas of cultivated land (1843–4099 km2) and woodland (5055–5433 km2) and decreases in grassland area ( 19076–12634 km2). The increase in cultivated land area was acquired from grassland (54%), woodland (32%) and unused land (14%), which was dominated by Phaeozems, Solonchaks and Fluvisols (84%). The converted land use to woodland was dominated by Fluvisols, Phaeozems, Arenosols and Solonchaks (98%). The converted land use from grassland to other land-use types was dominated by Arenosols, Fluvisols and Phaeozems (85%). The SOC was reduced by 9.83 Tg (+8.04 in cultivated land, +0.06 in woodland, −17.93 in grassland). The SOC stock change efficiency (SOCE kg C m−2) was SOC change divided by area change. The increase in SOC (Tg) of cultivated land was 5.04 from grassland (SOCE 3.76) and 2.9 from woodland (SOCE 3.68). The SOC of woodland increased by 8.66 Tg at cost of losing grassland (SOCE 3.35). Moreover, land-use changes affected local ecological environment. The cultivated land along the river increased 298 km2, and the desertification advanced towards oasis by decreasing 950 km2 of grassland in transition zone. The average groundwater table increased in the upstream (−3.12 to −2.33 m) and midstream regions (−1.84 to −1.71 m) monthly from 2012 to 2017 and in the downstream region (−7 to −2.84 m) annually from 2009 to 2017. This was attributed to expanded cultivated land and salt-washing cultivation on Solonchaks. In conclusion, the land-use conversion from grassland to woodland and cultivated land, mainly for Fluvisols, Phaeozems and Solonchaks, caused negative SOC storage, especially in riverbanks and oasis-desert transition zones facing climate change.