Abstract： The relationship between differential acid aluminum tolerance and rhizosphere pH change amongbarley cultivars was studied in small pot. It was showed that the rhizosphere pH change related to the Nsources in the media and Al-tolerance of cultivars. When plant grooved in the medium containing onlyNOa as N source，the pH increased rapidly. Al-tolerant cultivars B (83一122 ) and H (Humai No. 8 )maintained a higher rhizosphere pH than the Al-sensitive cultivar S (Shengxian awnless six-rowed).While the medium contained NHa and NOa (radio 1=12) as N sources,its pH change was biphasic. Dur-ing phase I，rhizosphere pH declined. Al-sensitive cultivar S maintained a lower rhizosphere pH and alonger period of decline than Al-tolerant cultivar B and H. During phase，,rhizosphere pH increased.The rhizosphere pH of cultivar B and H increased more rapidly than cultivar S. After the emergence ofthe phase 1< ,renewing the medium caused the different parttern of the rhizosphere pH change. The rhi-zosphere pH of Al-tolerant cultivar H only appear increase phase，Whereas Al-sensitive cultivar S stillremained biphasic一declined and increased. Finaly, one of Al-tolerance mechanisms was discussed onthe basis of results obtained in the present study.
