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Determination and Isotope-Ratio Analysis of Different Forms of Nitrogen in Soils: 7. Urea¹

ABSTRACT

Two methods of determining urea in soils are described. In one (extraction-distillation method), the soil sample is extracted with 2M KCl (10 ml/g of soil) for 5 min, and an aliquot of the extract is treated with potassium phosphate buffer (pH 8.0) and urease for 2 hours at 30C to convert urea N to ammonium N. In the other (direct-distillation method), the soil sample is treated directly with pH 8.0 potassium phosphate buffer and urease for 1 hour at 30C. In both, the ammonium released by the urease treatment is separated by steam distillation of the treated sample for 3.3 min and determined by titration of the distillate with standard acid. The extraction-distillation method is useful when analyses for nitrate and nitrite as well as urea are required, but it does not give completely quantitative recovery of urea added to some soils. The methods are rapid, specific and precise, and they permit isotope-ratio analysis of urea N in N¹⁵-tracer studies of urea transformations in soils.

¹ Journal Paper No. J-5533 of the Iowa Agr. and Home Econ. Exp. Sta., Ames. Projects 1070 and 1450. This work was supported in part by TVA.

² Formerly Research Associate and Professor, respectively, Department of Agronomy, Iowa State Univ., Ames. Senior author is now Assistant Professor of Soils, Univ. of Wisconsin, Madison.

Received for publication October 17, 1966. Accepted for publication December 27, 1966.