Size and dynamics of soil organic carbon stock in cropland of the Eastern Qinghai-Tibetan Plateau

• We analyzed the size and dynamics of soil organic carbon (SOC) stock in cropland of the Qinghai-Tibetan Plateau over 20 years.
• We investigated SOC in different soil types.
• SOC per unit area increased, but carbon stock was affected by loss of arable land.
• SOC density increased by 0.22–0.85 Mg ha−1 yr−1 under combined chemical fertilization and straw return treatments.

Estimating the size and changes in cropland soil organic carbon (SOC) stock in the Qinghai-Tibetan Plateau is important for the improvement of soil quality and mitigation of greenhouse gas emissions in this region. In this study, we investigated the dynamics of SOC concentrations, SOC density (SOCD), and SOC stock in cropland topsoil (0–20 cm) in the eastern Qinghai Province from the 1980s (1980–1989) to recent years (2000–2014). We conducted a long-term fertilization field experiment initiated in 1992 in eastern agricultural region of the Plateau, and used additional data of Qinghai section collected in 1980s by the Second State Soil Survey of China (SSSSC), observational data obtained from field studies in the area from 2006 to 2007. The results showed that the average SOC concentration during 2006–2007 increased by 16.0% compared to that in the 1980s, and increased by 11.9–24.1% in the treatments of the long-term field experiment in 2014 compared to that in 1992. Up until the 1980s, 21.9 Tg SOC was stored in 711751.6 ha of topsoil throughout the entire cropland region. The average SOCD in Huangzhong County during 2006–2007 was 35.4 Mg ha−1, 9.7% higher than that during 1980–1989 (32.3 Mg ha−1). The annual rate of increase in SOCD ranged from 0.10 to 0.41 Mg ha−1 yr−1in the long-term chemical fertilization treatments and from 0.22 to 0.85 Mg ha−1 yr−1in the combined long-term straw return and chemical fertilization treatments. The order of the soil types in the region with respect to mean SOC concentration was Chernozems > Gleysols > Anthrosols > Luvisols > Cambisols > Kastanozems > Calcisols. The long-term field experiment verified the increase of SOC stock per unit area in the whole region. Organic fertilization, especially straw return and chemical fertilization and other management strategies such as conservation tillage, contributed to the increase of the SOC stock. This study provides an understanding of SOC size and dynamics in cropland of Qinghai-Tibetan Plateau and highlights the importance of protecting cropland and using straw return to improve management practices and increase the SOC stock in cropland of the plateau.