Sensitivity of Soil Manganese Oxides

Abstract

Effects of sample treatment must be well understood to avoid artifacts during analysis. The effect of drying and storage was examined on nine medium‐ to high‐Mn aerobic soils using extractable Mn(II), the Cr Oxidation Test, estimated soluble C, and XANES spectroscopy. Long‐term storage (430 d) at 3°C had little effect on the Cr test. Air drying at room temperature (25°C ± 3°C) caused a drop in the Cr test within 24 h, with a further decline to as low as 45% of the original after 72 h, and less than 2% after 264 d. Extractability of Mn with pH 4.8 NH4OAc increased nearly linearly over the same time period from 0.2 mmol kg−1 to as high as 2.3 mmol kg−1 Increases in the absorbance of the extract at 360 nm, an estimate of soluble C, were well correlated within each soil with the increase in Mn(II). Pre‐treatments to remove soluble organics did not cause any increases in the Cr test of dried samples. Therefore, the loss of Cr oxidizing ability appears to be due to reduction of the oxides, not because of increased reduction of the Cr(VI) formed. No changes in XANES spectra were found after short‐term air drying at room temperature, but in the three samples examined after 428 d of drying, the main‐edge position had a downward shift of about 1.5 eV, indicating reduction. These results confirm previous findings that studies on the reactivity of soil Mn oxides need to avoid sample drying.