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Manganese (Mn) toxicity is a potential limitation to plant growth on acidic and poorly drained soils. Five laboratory experiments using such soils were conducted to examine the influence of soil temperature, pH and water potential on the redox reactions of Mn and the potential for Mn toxicity. The microbial inhibitor sodium azide was used in some experiments to assess the role of microorganisms in these reactions. The reduction of Mn oxides (MnOx) during waterlogging was faster at 20°C and 30°C than at 10°C or 4°C. Sodium azide slowed the reduction of Mn oxides at 20°C and 30°C during waterlogging but had little effect at 4°C and 10°C, suggesting that microbial MnOx reduction during waterlogging was minimal at the lower temperatures. Re-oxidation of Mn2+ in soil drained after severe waterlogging was only observed in soil not treated with sodium azide, indicating that even when very high concentrations of Mn2+ were present, Mn2+ oxidation was still microbial. Prior liming of aerobic soil established lower starting concentrations of water-soluble plus exchangeable (WS+E) Mn2+ and slowed the reduction of Mn oxides during subsequent waterlogging. After drainage, rapid re-oxidation of Mn2+ was observed in all lime treatments but was fastest at the two highest lime rates. In the fourth and fifth experiments, interactions between temperature and water potential were observed. When waterlogged soils were drained to –5 and –10 kPa, re-oxidation of Mn2+ occurred at both 10°C and 20°C. At –1 kPa, there was no net change in WS+E Mn2+ at 10°C, whereas at 20°C, the concentration of WS+E Mn2+ increased, possibly due to the lower concentration of O2 in the soil water at the higher temperature. In the fifth experiment, at 4°C and 10°C there was little or no effect on Mn reactions of varying water potential from –1 to –1500 kPa, but at 20°C and especially at 30°C, both Mn2+ oxidation and Mn oxide reduction were slowed at –1500 kPa compared with the higher water potentials. Overall, the experiments show that a delicate balance between the microbial oxidation of Mn2+ and the reduction of Mn oxides can exist, and that it can be shifted by small changes in soil water potential along with changes in temperature and pH.

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