Nitrification Rates in Two Undisturbed Tropical Rain Forests and Three Slash-and-Burn Sites of the Venezuelan Amazon

Abstract

Nitrification was measured during the rainy season in two undisturbed tropical rain forests and three slash-and-burn sites in the Amazon region of southwestern Venezuela. Nitrification was higher in the soils of the slash-and-burn sites than in the undisturbed forests. Rates of nitrate production were 14.9 mg/kg/30 days in a forest on an Oxisol, 3.5 mg/kg/30 days in a forest on Ultisol, 23.2 mg/kg/30 days in a 1.5-yr-old slash-and-burn agricultural site on an Oxisol, 12.9 mg/kg/30 days in a 0.5-yr-old slashand-burn agricultural site on Ultisol, and 12.1 mg/kg/30 days in a 3.0-yr-old pasture on Ultisol. In the undisturbed forests, rates of nitrate production in the root mat averaged 93.5 mg/kg/30 days on Oxisols and 11.2 mg/kg/30 days on Ultisols. Nitrification rates dosely followed total nitrogen mineralization rates. Ammonium availability appears to regulate nitrification in both the undisturbed forests and in the slash-and-burn sites. Rates of nitrification in the undisturbed forests in this Amazon site on poor soils were lower than in other tropical forests on more nutrient rich soils.