Changes in topsoil organic carbon of croplands in China's mainland over the last two decades

By searching literature databases, we obtained more than 200 articles published since 1993 that related to the measurements of topsoil organic carbon (SOC) in different regions. To objectively evaluate the changes in the SOC over the last two decades, we selected 132 representative articles from these documented articles. More than sixty thousand soil samples and/or sampling sites were included in the selected articles. Results from ana- lyzing these data sets indicated that the concentra- tion of SOC increased in 53%-59%, decreased in 30%-31% and stabilized in 4%-6% of the national croplands, respectively. A further investigation showed that the total increment of SOC in Chinese croplands ranged from 311.3 Tg to 401.4 Tg. In terms of administrative region, significant increase occurred in eastern and northern China and decrease in northeastern China, respectively. When evaluated by soil great groups, the SOC increased considerably in paddy soils and fluvo-aquic soils and reduced con- spicuously in black soils. The increase of SOC is attributed to the amendments of crop residues and organic manure, the augment of synthetic fertilizer application and the optimal combination of nutrients, and the development of no-tillage and reduced-tillage practice. Water loss and soil erosion and low input induced a great decrease of the SOC in black soils. In order to effectively enhance soil C sequestrations and to greatly control the SOC reduction in north- eastern China, future efforts should be made in de- veloping new techniques, training farmers and con- summating the policy of governmental compensation,by which the application of crop straw, the improve- ment of fertilization, the practice of no-tillage and reduced-tillage, and the control of water loss and soil erosion could be further realized. To respond to the increasing pressure from the Kyoto Protocol thence- forward, four aspects were further addressed for fu- ture research needs, including the quantification of SOC storage in the Second State Soil Survey and at present, the understanding of control mechanisms in both anthropogenic and non-anthropogenic causes that determine SOC dynamics, the investigation of options that can effectively enhance SOC sequestra- tion and/or reduce SOC loss, and the assessments of potentials and the likely SOC dynamics in the future on a national scale.