Chemical decomposition and fixation of nitrite in acidic pasture soils and implications for measurement of nitrification

The fate of added 15N-labelled nitrite (15NO2−-N) was investigated in a laboratory experiment with two acidic pasture soils collected from northeast Victoria (Maindample and Ruffy) and an alkaline soil collected from Waurn Ponds, Victoria, Australia. Two and a half hours after mixing and extraction, the 15NO2−-N recovered in 2 M KCl extracts was 22% and 33% of the applied NO2− in Maindample and Ruffy soils, respectively, and 100% in the Waurn Ponds soil. There was no difference in NO3− recovered with and without NaClO3 addition during this procedure, suggesting that biological oxidation of the applied NO2− was not the cause of the low recovery. Of the applied 15NO2−-N, 21% and 20% in the acidic Maindample and Ruffy soils, respectively, were recovered from the organic pool where it is believed to have been chemically fixed, leaving the total loss of 15NO2− as 57% and 47% from these two soils, most likely due to chemical self-decomposition to NO and NO2. When extracted with 0.005 M KCl, the salt concentration used in the short-term nitrification assay (SNA) 51% and 42% of applied 15NO2− were recovered in the extract from Maindample and Ruffy soils, respectively, but the total losses were only 9% and 10% of the applied 15NO2−-N, respectively. The chemical fixation and self-decomposition of NO2− in acidic soils are likely to cause an underestimate of nitrification rates by SNA.