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Salt-induced Manganese Solubilization in California Soils

Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521

*Corresponding author.

ABSTRACT

The solubility of Mn in soil solution depends on soil pH, redox potential (Eh), amount and type of higher valency Mn oxides, and a variety of organic and inorganic soil constituents. Laboratory experiments were conducted to determine the extent of salt-induced Mn release from a variety of California soils. To investigate the effect of salinity and time of incubation on Mn release, Redding (fine, mixed, thermic Abruptic Durixeralf) clay loam and Hanford (coarse-loamy, mixed, nonacid, thermic Typic Xerorthent) sandy loam soils were saturated with 0, 75, and 150 mol m^–3 NaCl-CaCl₂ solution (prepared on an isoequivalent basis) and incubated in a glasshouse for 0, 2, 4, 7, and 10 d. The Mn concentration in the soil saturation extract increased 2- to 4-fold with time of incubation at all levels of salt solution and 7- to 16-fold with increasing salt concentrations (ionic strength) at a given incubation time. Increases in [Mn] with time and increases in pH in the nonamended soil suggest reduction of Mn oxides while increases in [Mn] with increasing salt levels implicates the release of Mn into soil solution through reduction of Mn oxides, displacement by Na⁺ and Ca²⁺ and/or ligand (Cl^–) complexation processes. In another experiment, Mn release by NaCl-CaCl₂ salinity was investigated in 20 California soils varying in pH, easily reducible Mn (hydroquinone + NH₄OAc), total Mn (HF-HNO₃-HClO₄), and in total C concentrations. These soils were saturated with 0, 37.5, 75.0, 112.5, and 150.0 mol m^–3 NaCl-CaCl₂ solution and extracted after 5 d of incubation in a glasshouse. Depending upon soil pH, easily reducible and total soil Mn levels, the [Mn] in soil saturation extracts increased from 10 to 4000 mmol m^–3 for 150 mol m^–3 NaCl-CaCl₂ levels over the nonsaline (deionized water) treatments. Although the order of magnitude of salt-induced Mn release was high in soils low in pH and high in easily reducible Mn, a significant salt effect was noted for even high pH (above 7.0) soils. Multiple regression analysis showed that Mn solubility in salt-amended soils was mainly related to soil pH, easily reducible and total Mn (R² = 0.68***, significant at p < 0.001). This indicates that factors related to geochemical environments in which soils exist might explain the variations in Mn solubility in salt-amended soils.

Received for publication January 29, 1988.