Effect of controlled redox conditions on metal solubility in acid sulfate soils

Abstract

Rice (Oryza sativa L.) yields are constrained by Fe and Al toxicity and P deficiency on acid sulfate soils. In order to delineate the effects of pH and redox potential on metal availability in these soils, one or both of these parameters must be held constant. The objective of this study was to investigate metal behavior in acid sulfate soils in redox controlled suspensions. Three acid sulfate soils, Rangsit Very Acid (Rsa), Rangsit (Rs), and Mahaphot (Ma); a potential acid sulfate soil, Bang Pakong (Bg); and a non-acid marine soil, Bangkok (Bk) from Thailand were utilized. After pre-incubating the soils under anaerobic conditions, the soils were oxidized in 100 mV increments in a stepwise fashion (oxidation cycle). Afterwards, the oxidized soils were reduced in the same manner (reduction cycle). The pH's of all the soils decreased during the oxidation cycle and increased upon re-reduction. Water-soluble Fe decreased in all the soils (except Bg) as the Eh was increased in the oxidation cycle, whereas Fe increased in the reduction cycle when the Eh was decreased until -50 mV, at which time Fe sulfide precipitation was believed to occur. In the Bg soil, pyrite oxidation (which evidently started at +50 mV) brought about large increases in soluble Fe under oxidizing conditions, and soil pH decreased to 2.0. The influence of the redox status on Mn varied. Soluble Al increased with increases in Eh (due to decreases in pH) and vice versa in most of the soils. Water-soluble P decreased under oxidizing conditions and increased under reducing conditions. Ammonium acetate-extractable Fe and P were highly correlated (r=0.88), indicating that Fe plays an important role in P availability in acid sulfate soils.