Abstract: Roots have two functions, “source” and “sink”, which are important for substance and energy exchange between plants and soil. Understanding these processes requires studies of the dynamics of fine root growth and mortality. This study reports on the growth, death and turnover of fine roots in cotton (Gossypium hirsutum) and alfalfa (Medicago sativa) using minirhizotron techniques over different seasons. The fine root growth rate [G, cm/(cm³·d)], death rate [M, cm/(cm³·d)] and the ratio of growth to death (R) were determined. Results showed that the average standing crop of cotton in the growing season is 8.0 mm/cm³, with that of alfalfa being 14.8 mm/cm³ in 2011 and 8.0 in 2012. The average G and M of cotton in the growing season are 0.056 and 0.011 mm/(cm³·d) respectively, with those of alfalfa being 0.026 and 0.007 mm/(cm³·d) in 2011 and 0.041 and 0.028 in 2012. The G of cotton decreased rapidly from the end of May to mid-July, increased from mid-July to early September and then decreased. The G of alfalfa showed a consistent trend in 2011 and 2012, peaking from mid-June to mid-July and from early August to early September. The M of both cotton and alfalfa decreased from mid-June to mid-July and then increased to reach the maximum by the end of the growing season. The R of cotton and alfalfa decreased gradually from May to October, with cotton R>1 and alfalfa R<1. The estimated turnover rate is different according to whether it is calculated in terms of maximum or mean biomass. Mean biomass calculations increased turnover rates by about 22% compared with maximum biomass. During the growing season, the standing crop and G of cotton and alfalfa showed unimodal and bimodal peaks respectively, but the M of both rose continuously. At same time, cotton had a higher turnover rate to meet water and nutrient requirements.