Abstract: Most studies on straw decomposition in soil have been conducted with straw additions in the range of 0 100 g of straw per kilogram of soil. Actually, straw could be unevenly incorporated into soil resulting in very high local straw addition amounts, higher than 100 g per kilogram of soil under field conditions. There have been contradictory research results about straw decomposition in soil with straw addition of less than 100 g per kilogram of soil. In this study, the effects of straw addition amounts on organic carbon decomposition, composition and liability of yellow fluvo-aquic soils and lime concretion black soils were studied to develop a theoretical basis for straw decomposition in soils, and improvements in soil carbon cycle and soil fertility for high-harvest cultivation. After one year of indoor soil incubation with straw addition amounts of 0 400 g per kilogram of soil at (35±1) ℃ and 80% 95% field water capacity, the decomposition rates and humification coefficients of soil organic matter, soil C/N ratio, active soil organic carbon (AOC), inactive organic carbon (IOC), total organic carbon (TOC), AOC/TOC ratio and carbon lability (soil AOC/IOC ratio, L%) were determined. The results showed that humification coefficients of total soil organic matter were 20% 35%. With increasing straw dose, humification coefficient decreased while organic matter decomposition rate, AOC, IOC, TOC and L increased. These variables were significantly (P < 0.01) and positively correlated with straw addition amount. Also while a significant (P < 0.01) and positive correlation was noted between AOC and TOC, the correlation between the humification coefficient and L was significant (P < 0.01) but negative. The contents of clay particles smaller than 0.002 mm in yellow fluvo-aquic soils and lime concretion black soils were 33% and 41%, respectively. Straw was more likely to break down in yellow fluvo-aquic soils than in lime concretion black soils. Higher clay content in lime concretion black soils highly favored soil carbon storage. After one year of indoor incubation, soil AOC/TOC ratio and L in yellow fluvo-aquic soils were on average 7.6 and 12.3 percentage points higher than those in lime concre-tion black soils, respectively. Humification coefficient and soil C/N ratio in lime concretion black soils were on average 3.2 and 2.3 percentage points higher than those in yellow fluvo-aquic soils, respectively. In conclusion, the higher the soil carbon lability, the more beneficial was to increasing decomposition rate and reducing humification coefficient of soil organic matter. Then the higher the soil clay content, the lower the decomposition rate and the higher the humificatin coefficient of soil organic matter. This was in accordance with the results of previous studies conducted with straw addition amount less than 100 g per kilogram of soil. There was the need for further studies under field conditions under different soil types and different straw addition amounts.