Heterotrophic and autotrophic nitrification in two acid pasture soils

Laboratory incubation experiments, using 15N-labeling techniques and simple analytical models, were conducted to measure heterotrophic and autotrophic nitrification rates in two acid soils (pH 4.8–5.3; 1/5 in H2O) with high organic carbon contents (6.2–6.8% in top 5 cm soil). The soils were from pastures located near Maindample and Ruffy in the Northeast Victoria, Australia. Gross rates of N mineralization, nitrification and immobilization were measured. The gross rates of autotrophic nitrification were 0.157 and 0.119 μg N g−1 h−1 and heterotrophic nitrification rates were 0.036 and 0.009 μg N g−1 h−1 for the Maindample and Ruffy soils, respectively. Heterotrophic nitrification accounted for 19% and 7% of the total nitrification in the Maindample and Ruffy soils, respectively. The heterotrophic nitrifiers used organic N compounds and no NH 4 + ‐ N as the substrate for nitrification.