Mechanism of Native Manganese Release in Salt‐Treated Soils

Abstract

AbstractThe release of native soil Mn was evaluated under laboratory conditions in Aiken (fine, kaolinitic, thermic Abruptic Durixeralfs) and Redding (fine, kaolinitic, thermic Abruptic Durixeralfs) clay loam soils saturated with either deionized water, or 75 and 150 mmol L−1 NaCl‐CaCl2 solution. As the NaCl‐CaCl2 concentrations increased, soil solution Mn increased and exchangeable Mn decreased proportionately. The regression of the net increases (Δ) in soluble Mn vs. the corresponding decreases (Δ) in exchangeable Mn produced the relationship, Δ exchangeable Mn (mg kg−1) = 0.788 + 0.868 Δ soluble Mn (mg kg−1), R2 = 0.63** (P < 0.01). This suggested that in these salt‐treated soils Mn was released into solution by a cation exchange reaction involving Ca and Na. The increases (122 to 252%) in exchangeable Mn after 5 d incubation, as compared to untreated air‐dried soils, and the favorable pH (4.44 to 6.01) and Eh (redox potential) values (+202 to 254 mV) suggested that Mn(III,IV) was reduced to Mn(II). The Mn(II) primarily accumulated on the exchange phase and was subsequently displaced from the exchange sites by added Na+ and Ca2+. At equal equivalent concentrations, almost twice as much Mn entered the soil solution with Ca2+‐salts (Cl, ClO4, and NO‐3) as with Na+‐salts. The sum of soluble + exchangeable Mn did not increase (rather decreased significantly at the highest salt treatments). It was concluded that salinity had no effect on the extent to which Mn(III,IV) were reduced to Mn(II). These results indicated that management practices that increase soil solution salinity on soils high in native Mn could elevate soil solution Mn to concentrations potentially toxic to Mn‐sensitive crops.