Geochemistry of Manganese in Neutral to Alkaline Soils of Punjab, Northwest India and Its Availability to Wheat and Rice Plants

Abstract

ABSTRACTManganese (Mn) release in 18 soil–water suspensions after their equilibration for 24 and 240 h periods at 25°C was studied in a laboratory experiment. Total dissolved Mn released into the soil solution was observed to increase from a range of 0.03–0.41 mg L−1 (mean = 0.13 mg L−1) to a range of 0.45–44.44 mg L−1 (mean = 22.40 mg L−1) with the increase in incubation periods from 24 to 240 h, respectively. The increase in Mn released was observed to be related with the redox potential (pe) induced by incubation conditions. After 24 h of equilibration period, pe of soil–water suspension ranged from −1.75 to 0.77 (mean = −0.24). Increasing the incubation period to 240 h, pe of soil–water suspensions declined in the range of −4.49 to −2.74 (mean = −3.29). Laboratory results of redox pe and corresponding dissolved manganese concentrations of some soil–water equilibrated systems were compared with the leaf Mn content in wheat and rice plants grown in the fields, from where soil samples were collected for laboratory experiment. These results demonstrated that decline in pe due to longer equilibration period (240 h) of soil–water systems in the laboratory experiment or keeping standing water for a couple of weeks in the fields for cultivation of rice crop results in higher release of Mn and eventually its higher uptake in rice than in wheat plants. Leaf manganese content in rice ranged from 94 to 185 mg kg−1, which was markedly higher than its range from 25 to 62 mg kg−1 found in the wheat grown at 10 different sites. Pourbaix diagrams were drawn for different soil–water systems containing carbonate, phosphate, or sulfate along with manganese. The presence of carbonate and phosphate anions along with manganese oxides minerals in the soil–water systems of all soils results in its precipitation as MnCO3 and MnHPO4, respectively, in both oxidized and reduced soil field environment. In Punjab, wheat and rice crops are generally cultivated on soils heavily fertilized with P fertilizers. The presence of phosphate anion with manganese oxides minerals in the soil–water systems of all soils results in the precipitation MnHPO4 in both oxidized and reduced soil field environment. Thus, in P-fertilized soil, MnHPO4 compound is even more predominant than aqueous Mn2+ and its solubility actually controlled the availability of Mn2+ to plants.