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