Nitrogen transformations in a soil of the Lower Burdekin, Queensland. 2. Laboratory incubation studies

Abstract

Two laboratory incubation experiments were carried out to investigate the relation between soil water, Eh 7, nitrification and denitrification and to examine the relation between pH and nitrification. Samples were incubated for 16 days under aerobic and waterlogged conditions with 50 mg N kg-1 of added nitrate or ammonium. Apparent denitrification on waterlogging was rapid with 50% of nitrate disappearing in three days, but the presence of more than 7 mg N kg-1 as nitrate prevented Eh, falling below 270 mV. An initial wetting to 0.1 bar tension after drying and grinding caused apparent denitrification but a subsequent wetting to the same tension did not. If high nitrate levels occurred in the field, denitrification losses would be important. Net mineralization and nitrification were independent of soil water in the range 2 to 30% gravimetric. Samples of a pasture soil of initial pH about 4.2 were incubated aerobically with added ammonium and increasing amounts of K2CO3 to increase pH. Nitrate production increased with increasing pH and the relation between pH and nitrate production changed between pH 5.0 and 5.1. Denitrification losses from ammonium-containing fertilizers on waterlogging will be reduced if low pH has inhibited nitrification.