Nitrification activities of some New Zealand soils and the effect of some clay types on nitrification

Short‐term nitrification activities (SNA) and the numbers of nitrifying bacteria in nine New Zealand soils were examined. All the soils contained significant numbers of nitrifying bacteria (in the range 10 4 –10 7 /g). Nitrification activities of different soils were widely different, but no relationship was observed between SNA and the numbers of nitrifying bacteria. Separation of a high nitrifying soil into various size fractions did not affect the short‐term nitrification activity. Although pure cultures of Nitrosomonas sp. failed to nitrify below pH 6.2, a colloid fraction separated from an allophanic soil was capable of nitrification at pH 5.5. Similarly Nitrosomonas sp. grown in the presence of laboratory synthesised allophane was able to oxidise ammonium to nitrate at pH 5.5.