Effect of Urea Application at Different Rates on the Microbial Activity and Mineralization of Urea in Egyptian Soils

Abstract

Summary The effect of urea application at various levels to different Egyptian soils on the microbial densities, urease activity, and mineralization of urea was studied in laboratory experiments. The investigated soils were representing loamy, saline, alkali, sandy, and calcareous regions. The levels of urea chosen, being within the farmer application rate, were 50, 100, 150, 200, and 250 kg/ fed. The results revealed the following conclusions: Application of urea at different rates generally increased the total microbial flora in different soils, especially in the early experimental period. The activity of urease enzyme, expressed as mg NH4-N/100g soil/hr, showed highest values in loamy soil at zero time, followed by saline and alkali soils, while calcareous and sandy soils showed the lowest values. After application of urea the enzyme activity increased, especially during the first stage of the experiment, then gradually decreased. The differences between treatments in urease activity were very low, especially in sandy and calcareous soils. Ammoniacal nitrogen in soils amended with urea increased markedly in loamy and alkali soils one day after application, followed by gradual decrease. The decrease in NH 3 -N was associated with concomitant increases in NO 3 -N in different treatments. The increase in NO 3 -N, attributed to urea amendment, falls in the following descending order: loamy, saline, alkaline, sandy, and calcareous soils. This could be attributed to the high volatilization of NH 3 in alkali, sandy, and calcareous soils.