Effect of reduction associated with organic matter decomposition on magnetic properties of red soils

Five soils derived from different parent materials were sampled from red soil region of southern China and studied by magnetic methodology to understand to what extent iron reduction would affect soil magnetic properties and how iron reduction would affect the magnetic minerals in soils. Reduction associated with organic matter decomposition strongly affected soil magnetic parameters at low pH. The losses of original soil magnetic signals in terms of magnetic susceptibility (χ), 'Soft' isothermal remanent magnetization ('Soft' IRM), anhysteretic remanent magnetization (ARM) and saturation isothermal remanent magnetization (SIRM) at pH 4-6 during the two-month saturation treatment, were 66%-94%, 54%-90%, 64%-95% and 33%-83%, respectively. These changes were interpreted as a consequence of substantial dissolution of maghaemite and haematite in the soils. At pH 10, however, there was no significant magnetic change observed. Moreover, stable single domain soil maghaemite grains were also sensitive to reduction, which suggested that both pedogenic and detrital maghaemite were not stable in acid and reducing environments. Goethite, instead, was the most stable iron form under reducing conditions.