Fixation and release of ammonium in flooded rice soils as affected by redox potential

In pot experiments with two typical paddy soils, we studied the significance of ammonium fixation under waterlogged conditions and the availability of this N fraction for wetland rice. Special attention was given to the influence of redox potential (E_h) on the fixation and release of NH⁺₄, since the E_h has been shown to affect the charge conditions of certain expandable clay minerals and may alter their cation exchange capacity (CEC). The results demonstrate that ammonium formed by mineralization after flooding was converted, to a substantial degree, into a non-exchangeable form when sufficient amounts of expandable 2:1 minerals were present. The newly fixed NH⁺₄ was protected from N losses via nitrification-denitrification processes, which may occur, especially during the drying and rewetting of the soil, but was completely available to the following rice crop. The release of fixed NH⁺₄ was highest in the rhizosphere of rice plants (where the E_h was greatly increased by the O₂ secretion of the roots) and decreased with growing distance from the roots. Correlations between the E_h and the concentration of non-exchangeable NH⁺₄ indicate that fixation and mobilization of ammonium are strongly influenced by the redox potential in paddy soils.