For each root sampling, a cube of soil (25 cm in length × 16 cm inwidth × 20 cm in depth) around each individual hill was removedup using a sampling core. Such a cube contains approximately 95%of total root biomass (Kukal and Aggarwal, 2003; Yang et al., 2008).Plants of three hills from each plot formed a sample at each mea-surement. The roots in each cube of soil were carefully rinsed withhydropneumatic elutriation device (Gillison’s Variety Fabrications,Benzonia, MI, USA). After combining roots of three hills and record-ing fresh weight, portions of each root sample were used for themeasurement of ROA or root dry weight. The ROA was determinedby measuring oxidation of alpha-naphthylamine (-NA) according