Root samples were collected with cylinders (110mm in diameter and 250mm high) from five sites in each plot after the harvest of second rice in 2011, resulting in 15 cylinders in each field treatment. The roots were separated from the soil by soaking the cylinders with 0.6 mol L1 sodium hexametaphosphate and washing in water on a 0.053-mm sieve. The root morphological parameters such as total root length and numbers of root tips were measured with WinRHIZO (a commercial package to scan image of roots). Finally, all the root samples were oven dried to calculate root weight.