Nitrification activity in acid soils of north-eastern Victoria, Australia, as affected by liming and phosphorus fertilisation

A short-term nitrification assay (SNA) was used to measure the activity of soil nitrifiers and their response to pH change in acid pasture soils (pH 4.8–5.3 in water) at the sites of Maindample and Ruffy in north-eastern Victoria, Australia. Changes in soil pH associated with lime applications in the field resulted in a change in the optimum pH (pHopt) of the nitrifying organisms in the range 4.93–6.94. Nitrification in these soils was predominantly autotrophic, and rates increased from 0.18 to 0.93 μg NO3–-N/g.h with increasing pH. The strong positive correlation between field soil pH and the respective pHopt values suggested that the indigenous nitrifier population had adapted to the change in soil pH. SNA measurements within 6 months of lime application to Maindample soil showed that the soil nitrifying organisms had rapidly adapted to the pH change. However, the residual effect of lime on nitrifier activity was long-lasting (up to 8 years) and may involve more than a simple effect on soil pH. Repeat application of lime further enhanced nitrification activity on an already elevated activity, but only if sufficient time was allowed (>3 years) after the earlier application. Phosphate applications to these soils did not affect the general pH response in nitrifier activity. Both soils had considerable capacity for nitrification, even at pHs much lower than the commonly accepted range for autotrophic nitrifiers.