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 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.