Abstract: 【Objective】 The coupling of soil C sequestration with productivity is a hotspot in soil C cycling; moreover the increase of saturation capacity is a key approach in agricultural C sequestration and mitigation. The objective of this study was to clarify the saturation capacity of soil C sequestration and its cooperation with C input (crop source and organic fertilizer source C) and N input. 【Method】Data from four sites of long-term soil agro-ecosystem experiments of fertilizations from South China was collected, with which the soil cover four types of paddy soils derived from sandstone (PP) in Sichuan Basin, from Quaternary red earth (RP) in red soil region of Jiangxi, from lucustrine deposit (TP) in Tai Lake region of Jiangsu and from lagoon deposit (SP) in costal area of Zhejiang respectively). Changes in crop yields and topsoil SOC content, and the relationship of topsoil SOC with C and N input was analyzed and further the saturation capacity of soil C sequestration was explored with different fertilization treatments. 【Result】A significant linear correlation was found of annual soil C sequestration with C input under different fertilization treatments. SOC accumulation resultant from C input was high in PP and RP compared to that in TP and SP. The saturation capacity of C sequestration under good fertilization managements was shown highest in RP, followed by SP over TP and PP. Statistic analysis exhibited a negative linear correlation of C sequestration efficiency (C sequestered per unit of C input) with the initial SOC content but with the clay content. However, the saturation capacity of C sequestration was in a positive linear correlation with free iron oxyhydrates and annual precipitation for the soils studied. In addition, a positive correlation of N use efficiency on crop yields with soil C sequestration rates was observed though it was less distinctive for SP probably due to the high N stock. 【Conclusion】Well designed fertilization could promote soil C sequestration from the increased C input through increasing crop productivity. N use efficiency of rice paddies could be coupled with soil C sequestration under good fertilization practices. The abundant iron oxyhydrates not clay in the paddy soils was likely to be a key factor for soil C stock and sequestration in the paddy soils. Climate characteristics could also have exert influences on the changes in soil C stock with different paddy soils from various region of South China, which deserves further research.

Key words: long-term experiment, rice productivity, fertilization management, paddy soil, soil C sequestration, N efficiency