Figure 1: Instability of acid-extracted soil H₂O₂ and generation of ^·OH.

Soil H₂O₂ results from the dismutation/hydrolysis of metal superoxides/peroxides, respectively. (a) Decreasing exponential solid line curves depict the change of 10 mM H₃PO₄-extracted soil H₂O₂ concentration (initially adjusted to 2 μM, and in the absence of the extracted soil) from the Mojave and Atacama +CIMA and Yungay sites (filled rhombs and squares), respectively, versus time, against a 2.0 μM H₂O₂ (in 10 mM H₃PO₄) control solution (filled circles). Increasing exponential line curves depict the generation (versus time) of ^·OH in the H₂O₂ extracts from the same Mojave and Atacama sites (open rhombs and squares, respectively) in the absence and presence of soils (solid and dotted lines, respectively). (b) ^·OH was detected from its reaction with TPA(added to the H₂O₂ extracts at 2 mM) and the equimolar production of the specific fluorescent product HTPA⁷⁰. HTPA was quantified and identified fluorometrically by HPLC (I, using as controls the ^·OH scavenger DMSO, II, and pure HTPA, III). Similar data (not shown) were obtained for H₂O₂ extracted from total peroxides from the other tested Atacama and Mojave Desert soil sites. Error bars designate s.d.