Nitrification activity in New Zealand grassland soils

Abstract

Initial Nitrification Activity (INA) was measured in 68 New Zealand grassland soil sampies (0–75 mm) using a 17-h laboratory perfusion technique to provide a measure of the nitrifying activity. Considerable variation (< 0.02–5.70 μg N oxidised/g soil/h)was found between soils, yellow-brown loams having higher INA values than other soil groups. Variation of pH, % total N, % organic C, and C:N ratio explained only 12–44% of the variation of INA. Four characteristic patterns of nitrification were found when soils were perfused for 300 h. Significant populations of ammonium oxidisers(1.6 × 10⁴–2.2 × 10⁶/g soil) and nitrite oxiders (5.4 × 10⁴-1.9 × 10⁷/g soil) were found in the top 75 mm of 13 soils, even those oflow INA. Field measurements of nitrification activity in 2 soils of low and medium INA values supported the predicted difference in INA. The mean rate of nitrification under field conditions was 0.68 × 10^-2 μg N oxidised/g soil/h in the soil of low INA (Wharekohe silt loam) and 5.3 × 10^-2 μg N oxidised/g soil/h the soil of medium INA (Kiripaka silt loam). This difference resulted in an average NH ₄ ^- N: NO ₃ ^- N ratio of 16:1 in the former and 1:1 in the latter soil.