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

摘要

In pot experiments with two typical paddy soils, a study was made of 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 (Eh) on the fixation and release of NH4+, since Eh has been shown to affect the charge conditions of certain expandable clay minerals and may alter their cation exchange capacity. The results demonstrated 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 NH4+ 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 NH4+ was highest in the rhizosphere of rice plants (where Eh was greatly increased by the O2 emission of the roots)and decreased with increasing distance from the roots. Correlations between Eh and the concentration of non-exchangeable NH4+ indicate that fixation and mobilization of ammonium are strongly influenced by the redox potential in paddy soils.