Mechanisms of fixation and release of ammonium in paddy soils after flooding II. Effect of transformation of nitrogen forms on ammonium fixation

Abstract

 An incubation experiment under aseptic and septic conditions using ¹⁵N-labelled NH₄ ⁺-N and NO₃ ^–-N, was carried out to study the effect of N transformations after flooding on NH₄ ⁺ fixation in a paddy soil from China. After flooding ammonification was favoured, providing NH₄ ⁺ for fixation by clay minerals. NH₄ ⁺ fixation was more pronounced under low redox potential (E_h) conditions. Close correlations existed between exchangeable NH₄ ⁺, E_h, and non-exchangeable NH₄ ⁺. Therefore, two major conditions for NH₄ ⁺ fixation induced by flooding in paddy soil were found, namely flooding promoted net production of NH₄ ⁺ due to the deamination of organic N and, in addition, decreased the E_h of the soil. A lower E_h was caused by reduction and dissolution of Fe oxide coating on the clay minerals' surfaces, eliminating the obstacles for NH₄ ⁺ diffusing into or out of the interlayers of clay minerals. A higher concentration of exchangeable NH₄ ⁺ from deamination of organic N would drive NH₄ ⁺ diffusing from the soil solution into the interlayers of clay minerals. ¹⁵N-labelled NO₃ ^– incorporated into the flooded soil was not reduced to NH₃. The addition of NO₃ ^– retarded the decrease in the soil E_h and, therefore, NH₄ ⁺ fixation.