Interdependence of ammonia volatilization and nitrification in arid soils

The effect of applied-N (urea) on interdependence of ammonia volatilization and nitrification was studied in twelve arid soils varying mainly in soil texture and CaCO3. Ammonia volatilization from applied urea was observed only above a threshold N concentration in soil (V-i). Values of V-i ranged from 50 in sandy soils to 250 mu g N g(-1) in loamy and clay loam soils. Soils with higher CaCO3 showed lower values of V-i. Concentration of applied-N in soils, in relation to V-i determined its transformation pathway(s). Below V-i, all of the applied-N was nitrified in all soils with a delayed nitrification period ranging from 0 to 5 days. Above V-i, ammonia volatilization was first to start. This reduced NH4+ concentration in soil and nitrification started later after the delay period was over. Duration of delay period increased with applied-N, sand content and CaCo3. Often 80% or more of the total ammonia, volatilized during the delay period. NH4+ concentration in soil at which volatilization ended was generally higher than V-i. It was concluded that both ammonia volatilization and nitrification were interdependent only if concentration of applied N was more than V-i. Above V-i only one process predominated at a time as volatilization stopped soon after the start of nitrification even if NH4 concentration in soil was sufficient to sustain both processes.