The objective of this study was to determine if competing sinks for added NH4 (i.e., immobilization and nitrification) affected NH4 fixation and release. Low- and high-fixing soils, N rate, K preaddition, and temperature were variables chosen for studying these processes. Labeled (NH4)-N-15)2SO4 was added to soils and they were aerobically incubated. At the end of each incubation, fixed NH4-N-15 and KCl-extractable NH4-N-15 and NO3-N-15 were determined. Ammonium fixation was mainly responsible for the disappearance of extractable NH4-N-15 in a high-fixing soil. The fixed NH4-N-15 fraction comprised 68% of the nonextractable N-15 fraction in the high-fixing soil, and 28% in the low-fixing soil. In both soils, about half of the NH4-N-15 fixation occurred within 6 h after application. The maximum quantity of fixed NH4-N-15 occurred after 30 d. An NH4-N-15-addition rate of 150 mg N kg-1 doubled the fixed (NH4)-N-15 concentration over that at 75 mg N kg-1. Low temperature did not affect the quantity of added (NH4)-N-15 fixed. Potassium preaddition reduced fixed (NH4)-N-15 concentration at 6 h, but did not appear to affect it subsequently. Immobilization of 5.7% of the added (NH4)-N-15 occurred in the high-fixing soil, and 3.9% in the low-fixing soil. Extractable- and fixed-NH4 fractions were interrelated pools. When fertilizer NH4 was added to the soil, a proportional amount was fixed by the clay minerals. When nitrification and immobilization depleted extractable NH4, fixed NH4 was released. The fixed-NH4 pool appeared to be a slow-release reservoir, with fixed-NH4 release being slower than the rate of fixation.
