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

The fate of added N-15-labelled nitrite ((NO2-)-N-15-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 (NO2-)-N-15-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 (NO2-)-N-15-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 (NO2-)-N-15 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 (NO2-)-N-15 were recovered in the extract from Maindample and Ruffy soils, respectively, but the total losses were only 9% and 10% of the applied (NO2-)-N-15-N, respectively. The chemical fixation and self-decomposition of NO2- in acidic soils are likely to cause an underestimate of nitrification rates by SNA. (c) 2007 Elsevier Ltd. All rights reserved.