Evaluation of ammoniacal volatilization and urea nitrogen transformation in two soils of the DR Congo

The objective of this study is to evaluate, by incubation in the laboratory, the hydrolysis and mineralization of urea in an Arenoferralsol and a Ferralsol and to estimate the quantity of CO2 released during 15 days. In Arenoferralsol, the sharp rise in pH observed until the 7thday of incubation indicates a strong hydrolysis of urea, inhibiting the activity of microorganisms and causing low nitrogen mineralization. The sharp rise in pH correlates with the massive volatilization of NH3 recorded at this level. Beyond the 7th day of incubation, the decrease in volatile NH3 began, the resumption of microbial activity and the increase in nitrification. In the Ferralsol, a slight rise in pH was recorded until the 7th day, indicating a weak release of NH3, a weak microbial respiration and a weak transformation of urea nitrogen. The pH, although raised to 5.55, remained in the zone of high acidity. The drop in pH observed from the 11th day of incubation slightly stimulated the formation of NH4+-N and CO2 still remaining low. Thus, the NO3--N content experienced some increase. Hence, the conclusion that nitrification in this type of soil is very slow due to its high acidity. From our observed results, nitrification and volatilization seem to be more active in Arenoferralsol than in Ferralsol. Because, the acid state of Ferralsol and the quality of the organic matter combine to inhibit nitrification by destroying microbial life, especially those responsible for nitrification.