Dynamic relationship between soil active Mn and pH, Eh in acid soils and its biological response

The dynamic changes of soil pH, Eh, active manganese and their relationship were studied in three acid soils(brown-red soil, yellow-brown soil and red soil)by contrast with a calcareous alluvial soil collected from Hubei Province in pot experiment of rape plant. The results showed that pH and Eh of the soils would decrease sharply within 80 d of the growth of rape. In the brown-red soil and yellow-brown soil with acidification treatment the contents of the exchangeable Mn (EXC-Mn) rose, and it was positively correlated with carbonate bond Mn (CARB-Mn) during the first 70d in the growth of rape. The increase of EXC-Mn was later than that of CARB-Mn. During the 30-90d, CARB-Mn concentrations were correlated with easily reducible oxide Mn (ERO-Mn). For every tested soil, the values of pe + pH remained stable with changes of pH and Eh. The models of soil pH, Eh and EXC-Mn could better reflect the relationship of the three factors in the acid soils with acidification except the red soil. Electrical pair MnO2/Mn2+ in the brown-red soil would be a better description to dynamic change of Mn oxides to Mn2+ ion. And electrical pairs MnOOH/MnO would be main pattern in process of oxidation-reduction in the yellow-brown soil. The dissolution of MnO may be the reason of increase of Mn2+ ion in this soil at the acidification condition. The experiment also showed that the radio of Mn/Fe of the plant increased significantly with enhancing of Mn toxicity in rape, this change would be more obvious than that of the radio of Mn/Fe in corresponding soil acidification.