Release of NH4+ in Soils by Oven Drying

There is often a pronounced increase in exchangeable NH₄⁺ content when soils are dried, particularly when oven-dried. The origin of the NH₄⁺ is unknown. The objectives of this study were (i) to determine the effects of oven drying on the release of NH₄⁺ in soils with different properties, and (ii) to evaluate possible sources that may account for the release. Samples form the Congaree, Davidson, Huntington, and Maury soils were oven-dried at 110°C for 24 hours and analyzed for 2N KCl-extractable NH₄⁺ (exchangeable NH₄⁺) and NO⁻₃. Values for moist samples of soils were subtracted from values for oven-dried soils to determine the amounts of NH₄⁺ released by drying. The amounts ranged from 5 to 20 ppm N, with the greatest amounts released from the top 30 or 45 cm of the Davidson and Maury soils. Generally, the amount of NH₄⁺ released by oven drying decreased with depth in the soils. Monochloracetic acid (CH₂ClCOOH) extracted more NH₄⁺ than did KCl from oven-dried surface samples of the Congaree and Davidson soils, but there was little or no difference in the amounts of NH₄⁺ extracted by the two solutions from moist samples of Ap horizons or from samples below the Ap horizons, whether moist or oven-dry. These results suggest an organic source of NH₄⁺ in the Ap horizons. The relatively large amount released from deep in subsoils suggests the presence of an inorganic source also. We conclude that NH₄⁺ is released from both organic and inorganic compounds during oven drying. Nonexchangeable NH₄⁺ in interlayers of clay minerals usually increased with oven drying. We therefore conclude that nonexchangeable NH₄⁺ is not a source of released NH₄⁺. In fact, some of the NH₄⁺ released by oven drying appears to be simultaneously fixed as nonexchangeable NH₄⁺ in certain cases. The organic source of NH₄⁺ is probably several constituents of rather highly decomposed soil organic matter. The inorganic source is thought to be NH₄⁺-phosphate complexes, perhaps taranakite-like compounds. Further research is needed to identify specific sources.