Production of nitrite in soil by simultaneous nitrification and denitrification

Recent studies have shown relatively high nitrite concentrations occurring in soil. The contribution to nitrite accumulation of the two major source processes, nitrification and denitrification, was studied by differential 15N-labelling of nitrate and ammonium pools in soils incubated at a range of moisture contents. Nitrite concentrations and apparent sources varied greatly over 10 days. At the beginning and at the end of the incubations, when concentrations were low, denitrification appeared to be the major source process; nitrification appeared to be responsible for the bulk of the much larger concentrations accumulated at intermediate periods, during which an observed rise in pH suggested that high free ammonia concentrations might have inhibited nitrite-oxidising bacteria. Throughout most of the incubation nitrification and denitrification appeared to be producing nitrite simultaneously. The effect of moisture content was not clear, though there were some indications that nitrate reduction was favoured in wetter soil. A full interpretation of the complex interactions at work requires modelling.