Aluminum mobilization as influenced by soil organic matter during soil and mineral acidification: A constant pH study

Abstract

Soil acidification is an increasing challenge to food security due to the production of free aluminum ion (Al3+) that is toxic to crops. The production of exchangeable Al3+ in soils during acidification is a complex process and its specific mechanism is not clear. In this study, kaolinite, montmorillonite, and three acidic soils (two Ultisols and one Alfisol) were used to investigate the changes in exchangeable and soluble Al with pH using a constant pH automatic potentiometric titrator. The effect of soil organic matter (SOM) on soil pH and Al change was also investigated. The results showed that montmorillonite consumed more acid than kaolinite under the same conditions, which were consistent with the larger cation exchange capacity (CEC) of montmorillonite than kaolinite. When the pH was adjusted from 4.8 to 4.3, the exchangeable Al of montmorillonite was significantly higher than that of kaolinite. This was consistent with their CEC and indicated that montmorillonite adsorbed more H+ than kaolinite at the same pH, resulting in more exchangeable Al on its surface. Besides CEC, SOM shows a significant effect on acid buffering capacity and Al activation of acid soils. When acidified to the same pH, the content of exchangeable Al in Alfisol was larger than that in Ultisols. This observation was consistent with the larger CEC and smaller organic matter content of Alfisol. The presence of more organic matter in the Ultisols not only improved their acid buffering performance, but also inhibited the production of exchangeable and soluble Al. In conclusion, CEC of soils and minerals play an important role in soil acid buffering performance. While, SOM not only improves soil acid buffering performance, but also inhibits the mobilization of soil Al.