Alternate Method for Characterizing Nitrifier Activity in Soil

The method for studying short-term nitrifier activity (SNA) of soils described by Schmidt and Belser (1982) was evaluated and an alternate method is proposed. Effects of different soil treatments such as drying, liming, enrichment with NH⁺₄, and cropping were studied using the alternate method for evaluating SNA. Several soils were selected varying in SNA values to study transformation of nested urea prills (0.1 g) or large urea granules (2.0 g) under controlled temperature conditions. Short-term nitrifier activity measurements made by the method described by Schmidt and Belser were quite variable, often negative, and filtration of slurries with high clay content was difficult. An alternate method is proposed which involves leaching 25-g soil in a Buchner funnel with 100 mL 0.01 M CaCl₂ solution initially and then with 100 mL of a buffered phosphate solution containing 100 mg NH⁺₄-N L⁻¹ as nitrifier substrate under 67-kPa suction. Following incubation (usually 16 h) at 25°C, the soil is leached with 100 mL 0.01 M CaCl₂ solution and (NO⁻₂ plus NO⁻₃)-N production determined as an estimate of SNA. This method gave larger and more consistent SNA values than the Schmidt and Belser method. Nitrifier population enrichment of alkaline soils with NH⁺₄ increased the SNA values, while it resulted in lower SNA values in acid soils. Both liming (3 g CaCO₃ kg⁻¹ oven-dry soil) and liming plus enrichment with NH⁺₄ increased SNA values in acid soils. Air drying resulted in decreased SNA values. The effect of different crops on SNA values was not readily apparent, as SNA was related to amounts of NO⁻₃ present in soil. However, there was some evidence of inhibition of the nitrifier population under alfalfa (Medicago sativa L.) and bluegrass (Poa pratensis) turf. The percent nitrification of a nested placement of 0.1 g of urea prills or 2.0 g urea granules gave highly significant relationships with SNA values. Percent nitrification was greater with soils varying in textural class from loamy sand to silt loam than for clayey soils at similar SNA levels. During the nitrification of large urea granules in soils with a relatively active nitrifier population, only small amounts of NO⁻₂ were measured.