Arsenic Reactions and Plant Uptake In Hawaiian Soils.

Arsenic (As) is toxic to most living organisms. In Hawaii, although agricultural uses of arsenical pesticides had been stopped many decades ago, elevated levels of arsenic (As) remain in some soils. Given the volcanic ash origin and fast weathering conditions of Hawaii, Hawaiian soils often contain high amounts of amorphous aluminosilicates and iron oxides, which can sorb and retain As strongly. More specifically, As sorption isotherms that were performed on eight soils of different soil orders showed that Andisols, particularly Hydrudands, sorb most As, followed by Oxisols and Ultisols, whereas Mollisols and Vertisols , consisting mostly of 2:1 layer silicates, sorb least. In an attempt to remove As by plants (phytoremediation), greenhouse experiments were established on an As-contaminated Andisol, which had 315 mg kg^-1 total As and was amended with 0, 5 g kg^-1 compost or 250 mg kg^-1 P as treble superphosphate, and on a low-As Ultisol, which was spiked with 0, 150 or 300 mg kg^-1 As as Na₂HAsO₄. Chinese brake fern (Pteris vittata L.), an As hyperaccumulator, was used as the test plant. Arsenic concentrations in the fern fronds varied on average from 380 mg kg^-1 in the Andisol to 2600 mg kg^-1 in the Ultisol, suggesting different chemical reactions and availability of soil As with respect to plant uptake. In fact, bioaccessible As (as extracted with HCl, pH 1.5) and Mehlich-3 extractable As were increased with P amendments and were higher in the Ultisol than in the Andisol having similar total As.