Changes in Soil Organic Carbon Stocks between 1980s­–2010s in the Northwest Arid Zone of China

Soil is the largest carbon reservoir in terrestrial ecosystems, and minor changes in this reservoir can dramatically affect atmospheric CO 2 concentrations. The fragile ecological environment in the arid zone of northwest China is extremely susceptible to natural and anthropogenic factors, leading to variations in ecosystem structure, function, and carbon source-sink relationships. In this study, digital soil mapping of soil organic carbon density (SOCD) was performed at a 90-m resolution for two periods (1980s and 2010s) and five depth intervals (0–5, 5–15, 15–30, 30–60, and 60–100 cm) based on historical soil profile data and the random forest model. The results showed that the prediction accuracy of SOCD in the topsoil (0–30 cm) was superior than that of the subsoil (30–100 cm). In the past 30 years, the soil organic carbon stocks (SOCS) of the northwest arid zone decreased by 585.50 Tg (1Tg = 10 12 g), with an average decrease of 19.52 Tg C yr -1 . Moreover, we quantified the amount of SOCS change caused by land-use conversion, and SOCS reduction was revealed to be mainly attributed to grassland desertification and reclamation for agriculture. Although large-scale national ecological restoration projects have increased SOCS through afforestation, sand planting, and fallowing, these projects were unable to compensate for SOCS losses in the northwest arid zone. Variance decomposition revealed that land-use conversion was the dominant factor impacting the SOCS in the northwest arid zone, followed by changes in vegetation characteristics and climate. Our results are valuable for future studies the carbon cycle processes in terrestrial ecosystems in the context of climate change and promoting the achievement of China's "carbon neutrality" goal