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Al stress and ammonium–nitrogen nutrition often coexist in acidic soils due to their low pH and weak nitrification ability. Rice is the most Al-resistant species among small grain cereal crops and prefers NH4 + as its major inorganic nitrogen source. This study investigates the effects of NH4 + and NO3 − on Al toxicity and Al accumulation in rice, and thereby associates rice Al resistance with its NH4 + preference. Two rice subspecies, indica cv. Yangdao6 and japonica cv. Wuyunjing7, were used in this study. After treatment with or without Al under conditions of varying NH4 + and NO3 − supply, rice seedlings were harvested for the determination of root elongation, callose content, biomass, Al concentration and medium pH. The results indicated that Wuyunjing7 was more Al-resistant and NH4 +-preferring than Yangdao6. NH4 + alleviated Al toxicity in two cultivars compared with NO3 −. Both NH4 +-Al supply and pretreatment with NH4 + reduced Al accumulation in roots and root tips compared with NO3 −. NH4 + decreased but NO3 − increased the medium pH, and root tips accumulated more Al with a pH increase from 3.5 to 5.5. Increasing the NO3 − concentration enhanced Al accumulation in root tips but increasing the NH4 + concentration had the opposite effect. These results show NH4 + alleviates Al toxicity for rice and reduces Al accumulation in roots compared with NO3 −, possibly through medium pH changes and ionic competitive effects. Making use of the protective effect of NH4 +, in which the Al resistance increases, is advised for acidic soils, and the hypothesis that rice Al resistance is associated with the preferred utilization of NH4 + is suggested. [ABSTRACT FROM AUTHOR]