INTERACTION OF N-15-LABELED AMMONIUM NITROGEN WITH NATIVE SOIL-NITROGEN DURING INCUBATION AND GROWTH OF MAIZE (ZEA-MAYS-L)

A laboratory incubation experiment and a pot experiment on maize (Zea mays L.) were made to study the interaction with native soil N of N-15-labelled ammonium applied at 25, 50, 100 or 200-mu-g N g-1 soil. Incubation of soil for 4 weeks showed a significant enhancement in mineralization of native N (added nitrogen interaction, ANI) due to applied N and the effect increased with increased rate of application. The ANI occurred only during the first week of incubation and its extent was not affected significantly during the subsequent 3 weeks of incubation. The ANI was not observed in the pot experiment at any concentration of applied NH4-N and was assumed to be due to the absence of root-driven immobilization and pool substitution in maize. Root growth showed a negative response to increased amounts of N addition. However, N uptake efficiency of plants (as revealed by total plant N) increased significantly at increasing rates of applied N but without the occurrence of ANI. Heavy losses of applied N occurred both in the incubation study (20-30%) as well as plant experiment (47-83%); the losses being more as the rate of applied N increased.