Ammonium Fixation, Release, Nitrification, and Immobilization in High‐ and Low‐Fixing Soils

The objective of this study was to determine if competing sinks for added NH 4 (i.e., immobilization and nitrification) affected NH 4 fixation and release. Low‐ and high‐fixing soils, N rate, K preaddition, and temperature were variables chosen for studying these processes. Labeled ( 15 NH 4 ) 2 )SO 4 was added to soils and they were aerobically incubated. At the end of each incubation, fixed 15 NH 4 and KCl‐extractable 15 NH 4 and 15 NO 3 were determined. Ammonium fixation was mainly responsible for the disappearance of extractable 15 NH 4 in a high‐fixing soil. The fixed 15 NH 4 fraction comprised 68% of the nonextractable 15 N fraction in the high‐fixing soil, and 28% in the low‐fixing soil. In both soils, about half of the 15 NH 4 fixation occurred within 6 h after application. The maximum quantity of fixed 15 NH 4 occurred after 30 d. An 15 NH 4 ‐addition rate of 150 mg N kg −1 doubled the fixed 15 NH 4 concentration over that at 75 mg N kg −1 . Low temperature did not affect the quantity of added 15 NH 4 fixed. Potassium preaddition reduced fixed 15 NH 4 concentration at 6 h, but did not appear to affect it subsequently. Immobilization of 5.7% of the added 15 NH 4 occurred in the high‐fixing soil, and 3.9% in the low‐fixing soil. Extractable‐ and fixed‐NH 4 fractions were interrelated pools. When fertilizer NH 4 was added to the soil, a proportional amount was fixed by the clay minerals. When nitrification and immobilization depleted extractable NH 4 , fixed NH 4 was released. The fixed‐NH 4 pool appeared to be a slow‐release reservoir, with fixed‐NH 4 release being slower than the rate of fixation.