Nitrogen Transformations in an Incubated Soil as Affected by Combinations of Moisture Content and Temperature and Adsorption-Fixation of Ammonium

An incubation experiment was conducted on a clay loam soil using a factorial design to study the effects of temperature, water content, and applied NH 4 + -N on the change of NO 3 − -N and NH 4 + -N concentrations over time. Each of the factors and their interactions were highly significant, indicating the importance of the often neglected moisture–temperature interaction. Due to this interaction, optimum moisture contents for the activity of the nitrifying population appeared to be dependent upon temperature. Approximately one half of the added NH 4 + was fixed within 2 days after (NH 4 ) 2 SO 4 was added to the soil. A concurrent study showed that soluble NH 4 + , exchangeable NH 4 + and clay-fixed NH 4 + existed in equilibrium upon adding NH 4 + . A Langmuir kinetic model might describe the relationship between exchangeable and fixed NH 4 + whereas a nonlinear Freundlich equilibrium model yielded the best agreement between the soluble and exchangeable NH 4 + .