Abstract: Soil light fraction is a critical soil quality improvement factor as it regulates soil fertility and minimizes negative environmental impacts. An experiment was established at Southwest University Farm (30°26′N, 106°26′E) in Chongqing. The farm belongs to the Key Field Station for Purple Soil Eco-Environment Monitoring of the Ministry of Agriculture, China. The 0~30 cm soil of the farm under rice/rape rotation system was sampled. The experiment aimed to quantify short-term dynamics of light fraction organic carbon (LFOC) and nitrogen (LFN) during rape growing season. Light fraction was extracted by NaI at a density of 1.8 g·cm^-3. The results showed that soil light fraction content in the 0~30 cm soil was 2.95%~5.51% during rape growing season, with an average value of 4.38%. The ranges of LFOC and LFN were 1.44~3.72 g·kg^-1 and 0.08~0.17 g·kg^-1, with average values of 2.79 g·kg^-1 and 0.14 g·kg^-1, respectively. Dry weight of LFOC showed distinct seasonal variations, with maxima occurring in mid-growing season, followed by late-growing season. Minimal dry weight of LFOC appeared at the start of the growing season. However, no significant seasonal variations (P > 0.05) were noted in LFN contents in the same period. The observed seasonal variation trend for LFN was similar to that of LFOC. The ratio of LFOC to SOC determined by content (LFOC/SOC) was 9.21%~24.47%, with marked seasonal variations (P< 0.05). This variation trend was similar to that of LFOC. There was no marked seasonal variation in the ratio of LFN to TN (LFN/TN), with a range of 4.55%~12.58% and average of 9.63%. This was higher than those for other areas, suggesting that organic nitrogen in purple paddy soils of Southwest China more easily mineralized with stronger N supply than in other areas. Average C/N ratio of light fraction organic matter was 20.66, with a range of 18.52~25.04. This was higher than that of the total soil (11.66), indicating that bio-availability of light fraction organic matter was higher than soil organic matter. Correlation analysis showed that both LFOC and LFN were positively correlated with root biomass, root carbon and nitrogen (P< 0.01 or P < 0.05). This indicated that root was the main regulator of seasonal dynamics of LFOC and LFN content. Also regression analysis showed that root biomass, root carbon and nitrogen content accounted for 40%~60% of soil LFOC and LFN content. Therefore plant root was concluded to be the key control factor of seasonal variations of LFOC and LFN in the study area.