Initial Nitrification Activity (INA) was measured in 68 New Zealand grassland soil sampies (0#x2013;75 mm) using a 17-h laboratory perfusion technique to provide a measure of the nitrifying activity. Considerable variation ( 0.02#x2013;5.70 #x3BC;g 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#x2013;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 #xD7; 104#x2013;2.2 #xD7; 106/g soil) and nitrite oxiders (5.4 #xD7; 104-1.9 #xD7; 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 #xD7; 10-2#x3BC;g N oxidised/g soil/h in the soil of low INA (Wharekohe silt loam) and 5.3 #xD7; 10-2#x3BC;g N oxidised/g soil/h the soil of medium INA (Kiripaka silt loam). This difference resulted in an average NH4-N: NO3-N ratio of 16:1 in the former and 1:1 in the latter soil.
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