Mechanisms of fixation and release of ammonium in paddy soils after flooding

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 (Eh) conditions. Close correlations existed between exchangeable NH4+, Eh, 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 Eh of the soil. A lower Eh 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 Eh and, therefore, NH4+ fixation.