A proposed method for determining potentially environmentally available trace metals in paddy soil

The validity of the Asami-Kumada method (sodium hydrosulfite + EDTA extraction, AK method) for determining the potentially environmentally available trace metals (Cr, Mn, Co, Ni, Cu, Zn, Cd, Pb) in paddy field soil was evaluated. This was accomplished by comparing it with the results of the dilute HNO3 extraction method (ISO method), nitrate acid – perchloric acid degradation method, and dilute hydrochloric acid extraction method on 250 paddy field soil samples. The coefficient of determination for the AK method in metals, excluding Cr, ranged from 0.92 (Ni of non-Andisols) to 0.99 (Mn, Cu, Zn and Cd of non-Andisols) compared to the ISO method. The slope of the linear regression of the AK method’s measured values against those of the ISO method for metals, excluding Cr, ranged from 0.99 (Cd of non-Andisols) to 1.52 (Cu of Andisols). However, the measurements of Fe using the AK method were significantly higher than those obtained using the ISO method. The slope was measured to be 3.6 (for non-Andisols) or 5.7 (for Andisols). The increase in trace metal measurements using the AK method was probably due to the dissolution of Fe oxides and hydroxides, which release trace metals after reduction by sodium hydrosulfite. Thus, although the measured values using the AK method of trace metals, excluding Cr, tended to be slightly higher than those obtained using the ISO method as Fe oxides and hydroxides dissolved, the measured values of those metals by both methods were closely correlated. The AK method is a viable alternative to the ISO method for measuring the potentially environmentally available trace metal contents, excluding Cr, in paddy field soils. Additionally, the AK method conveniently measures free Fe oxides simultaneously.