Estimating nitrogen transformation rates in surface aerobic soil of a paddy field

Rates of nitrogen transformations in the surface aerobic soil of a paddy field (fine-textured strong-gley soils) were estimated by combination of a mathematical model and parallel incubation experiments using ¹⁵N as a tracer. The mineralization rate was nearly constant throughout the incubation. However, the ammonium assimilation rate during the initial phase of the incubation was two to three times higher than the mineralization rate, and rapidly decreased with time. This decrease did not follow first-order kinetics for the ammonium content in soil. The nitrification rate increased slightly with time. Reduction of nitrate to ammonium was detectable in all the experiments, but its rate was very low (<50μg N kg h^-1). The relative magnitudes of the rates of N transformations changed with the incubation temperature. The rates of denitrification plus nitrate assimilation were greatly enhanced at 30°C. The mineralization rate also increased with temperature, but the nitrification rate decreased slightly. The rates of N transformations in the surface layer, especially under aerobic condition, were from 10 to 100 times greater than those in the subsurface layer under anaerobic condition. This indicates that N transformations in the surface layers of paddy soils are quantitatively of great importance.