The effects of rapeseed root exudates on the forms of aluminum in aluminum stressed rhizosphere soil

Aluminum stress is a serious problem in many rapeseed-growing regions of the word. In this study, we examined how four different aluminum (Al) stress levels affected two rapeseed varieties, Zhongshuang No. 7 and Huayou 2790, in terms of their soil pH and the amount of soluble Al, exchangeable Al (Al3+), monomer hydroxy Al [Al(OH)(2+), Al(OH)(2)(+)], acid-soluble inorganic Al [Al(OH)(3)] and humic acid Al (HA-Al) produced in the root exudates. From four treatment groups (control, rhizosphere, non-rhizosphere and exogenous root exudates), we found that under the same Al concentration level and exposure time, the Al forms of non-rhizosphere soil group had no significant changes when compared with the control group. The contents of soluble Al, exchangeable Al, monomer hydroxy Al and acid-soluble inorganic Al in both the rhizosphere and the exogenous exudate treatment groups decreased significantly. As the Al concentration increased, variety Zhongshuang No. 7 experienced significant increase in its humic acid Al contents, but had decrease in contents for the other various Al forms. More specifically, after 14 days of Al treatments, the monomer hydroxy Al content had a slightly increase, the humic acid Al content had a significantly increase, and the exchangeable Al, soluble Al and acid-soluble inorganic Al contents had a significantly decrease. The analysis of soil pH showed that under the influences of rapeseed root exudates and 0 g/kg Al treatment, the soil pH measurements of the treatment groups were lower than that of the control. As the Al treatment concentrations increased, the soil pH of the rhizosphere and the exogenous exudates treatment groups changed moderately and were overall higher than that of the control. The results showed that the root exudates had limited acidification effects on the rhizosphere soil and that while the soil pH had a direct effect on the transformations between Al forms, the root exudates have an indirect effect. Rapeseed root exudates could partially alleviate Al toxicity and its anti-toxicity effects on the Al-resistant variety were more apparent than on the Al-sensitive variety. (C) 2011 Elsevier Ltd. All rights reserved.