Release of NH4+ in Soils by Oven Drying

Abstract

AbstractThere 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.