Nitrification of Urea and Assimilation of Nitrate in Saturated Soils Under Aerobic Conditions

Abstract

The aim of this study was to investigate nitrification activity of urea and the assimilation of nitrate in a well aerated soil using perfusion technique with addition of glucose as an energy and carbon source. In this study, urea was rapidly nitrified by the bacteria in the saturated soil but its course of transformation to NO₃ was not linear. There was an initial increase in the concentration of nitrite during the nitrification experiment which indicated that the conversion of nitrite to nitrate was appreciably slower than the rate of conversion of urea to nitrite. The rate of conversion of NH₄⁺ to NO₂^- was faster than the rate of conversion of NO₂^- to NO₃^- in the first 12 days and as a result the nitrate concentration reached 2.72 µg/ml on the 12^th day. After day 12, the concentration of NH₄⁺ in solution declined significantly and the rate of conversion of NO₂^- to NO₃^- became faster than the rate of conversion of NH₄⁺ to NO₂^-. The concentration of NO₂-N in the solution reached zero on the 23^rd day. The nitrification curve has the character of a sigmoid curve whose midpoint, which representing the most rapid rate of nitrification, fell at the point of half conversion of urea to nitrite. The curve asymptotically approaches a nitrate value that represents 98% conversion of urea into nitrate. The rest of the urea (NH₄) has presumably been synthesized into bacterial cells. The initial pH of the soil was 7.7 due to the presence of NH₄ which decreased gradually due to the production of NO₃ reaching 6.9 by day 23. A nitrate reduction was observed under aerobic conditions. Denitrification did not proceed according to the known fact that O₂ prevents the denitrifying organisms from producing the enzyme responsible for the process. The alternative pathway for nitrate reduction could be by assimilatory reduction where nitrate was converted to ammonium and then to cells. The removal of nitrate and production of ammonium caused a rise in the pH. The initial pH of the solution was 6.9 which increased with time reaching 7.3 by the 7^th day. The expected nitrate reduction was 50% according to the assumption, but the 59% nitrate reduction observed in the experiment suggests that more than 25% of glucose C was metabolized and less than 75% was oxidized, otherwise.