In this study,physical protection of soil organic carbon(SOC) in particle size fractions of paddy soils was investigated using cross-site study of top soil samples from three sites of long-term soil-agro-ecosystem experiments from South China.Undisturbed top soil samp les collected at 0～15 cm were used for size factions of micro-aggregates by low-energy ultra-sonic dispersion procedure.SOC content and its distribution in particle-size fractions under different fertilization and cultivation treatments were analyzed to address the relationship between SOC accumulation and physical protection of aggregates.The results were as follows:the distribution of particle-size fractions of three tested paddy soils was dominated by 200～20 μm and 20～2μm fractions(22%～43% and 27%～44%),showing prominent micro-aggregation in these paddy soils.SOC content was higher in fractions of 2 000～200μm and < 2μm than in the other fractions,while labile organic carbon(LOC) content and LOC/SOC ration higher in the coarse fraction of 2 000～200 μmonly.The newly accumulated young SOC found its fate mainly in the coarse fraction of 2 000～200 μmin active response to tillage and fertilization treatments.Of the three studied paddy soils,SOC and LOC in the red-earth derived paddy soil showed most intense response to fertilization treatments,exerting a most prominent trend of SOC accumulation under well management practices.Furthermore,statistic analysis indicated significant parabola relationship between SOC accumulation in bulk soil and in 2 000～200 μm coarse fraction.Therefore,it is inferred that accumulation of young SOC of top soil may reach a saturation controlled by the physical protection in the coarse particle-size fractions.However,as shown by the calculation using the curve obtained,the maximum protection capacity by the coarse micro-aggregates of the three tested paddy soils had not yet realized,still possessing obviously a potential of carbon sequestration.The higher potential in the red-earth derived paddy soil suggests that C sequestration had been mostly obvious in paddy soils from South China especially those in the Red soil region.Physical protection in soil aggregates and carbon sequestration of paddy soil may also involve the binding of physically protected SOC in aggregates and chemical stabilization of SOC in the paddy soils rich in iron oxy-hydrates.