Use of Diffusion for Enzymatic Determination of Urea-Nitrogen in Soil Extracts

Urea-N in soil extracts is normally determined by colorimetric procedures using diacetyl monoxime, due in part to the use of a urease (urea amidohydrolase, EC 3.5.1.5) inhibitor to prevent urea hydrolysis during extraction. Alternatively, this determination can be done enzymatically if no inhibitor was employed in the preparation of extracts, in which case N-isotope analyses can be performed on urea as well as extractable NH₄, NO ₃, and NO₂. An accelerated diffusion method for determining exchangeable NH₄ was employed to recover (urea+NH ₄)-N by heating 10-100 mL of soil extract at 45-50°C for 1.75-8. 5 h with jack bean (Canavalia ensiformis) urease and MgO (50 mg of each). Studies using labeled urea showed ≥ 98% recovery of ¹⁵N as urea, with virtually no recovery as NH₄, even when filtration was delayed up to 1 h after shaking 2M KCl soil suspensions. The procedure described provides a convenient alternative to colorimetry for research concerning transformations of urea-N in soils.