Suitability of Potential Nitrification Rate Determined by Shaken Slurry Method to Capture the Contingent Effect of Liming of Two Tropical Soils

Potential nitrification rate (PNR) provides maximum soil nitrification rate under optimal incubation conditions, but optimum conditions may vary for different nitrifier species. This study investigated whether the estimates from PNR and nitrifying potential (NP) assays reflect the effect of sudden changes in soil pH as with liming on nitrifying activity. Soil samples were collected from intensively cultivated and uncultivated lands representing soil orders Alfisol and Ultisol. A sub-sample of each cultivated soil was treated with CaCO3 (1.8 g/kg). Cultivated soil with and without liming and uncultivated soil from each soil types were incubated at 30 °C maintaining moisture content at 0.25 (v/v) for 2 weeks. PNR of soil was measured with shaken slurry method using P-buffer at pH 7 and modified P-buffer with adjusted pH to match soil pH, separately. NP of each soil was determined in a 21 days static soil incubation. Soil type and land use had significant effects (P<0.001) on PNR. Ultisol had significantly higher (P<0.05) PNR than Alfisol, and uncultivated soils had significantly higher (P<0.05) PNR than cultivated soils, irrespective of the P-buffer used. Liming yielded significant differences (P<0.05) in PNR only when pH adjusted P-buffer was used. Treatment effects on NP was significant only in Ultisol. Results suggest that PNRs may be biased towards certain groups of nitrifiers and PNR measured using modified P-buffer and percent increase in NO3- production due to external application of NH4+ are better indicators of potential nitrifying activity than traditional PNR measured using P-buffer at pH 7 and NP.