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

Dep. of Land Resource Science, Univ. of Guelph, Guelph, Ontario, N1G 2W1

C. F. Drury

Agriculture Canada Res. Stn., Harrow, Ontario, N0R 1G0

* Corresponding author.

ABSTRACT

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 (¹⁵NH₄)₂)SO₄ was added to soils and they were aerobically incubated. At the end of each incubation, fixed ¹⁵NH₄ and KCl-extractable ¹⁵NH₄ and ¹⁵NO₃ were determined. Ammonium fixation was mainly responsible for the disappearance of extractable ¹⁵NH₄ in a high-fixing soil. The fixed ¹⁵NH₄ fraction comprised 68% of the nonextractable ¹⁵N fraction in the high-fixing soil, and 28% in the low-fixing soil. In both soils, about half of the ¹⁵NH₄ fixation occurred within 6 h after application. The maximum quantity of fixed ¹⁵NH₄ occurred after 30 d. An ¹⁵NH₄-addition rate of 150 mg N kg^–1 doubled the fixed ¹⁵NH₄ concentration over that at 75 mg N kg^–1. Low temperature did not affect the quantity of added ¹⁵NH₄ fixed. Potassium preaddition reduced fixed ¹⁵NH₄ concentration at 6 h, but did not appear to affect it subsequently. Immobilization of 5.7% of the added ¹⁵NH₄ 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.

Received for publication February 12, 1990.

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