In response to a very high increase of groundwater NO3- pollution, largely connected with intensive agricultural practices, a long-term experimentation has been set up close to Grenoble, France, with the following aims: first io characterize the response of maize (Zea mays L.), a predominant crop in the area, to fertilization, and second to quantify the N balance during and after the crop cycle. This study relates to results concerning the second issue: experiments were conducted on irrigated maize in 1991, 1992, and 1993 on the Experimental Farm at La Cote Saint-Andre, France, in the heart of one of the most important agricultural zones between the French Alps and the Rhone Valley. The dynamics of soil and fertilizer N (NO3- transport and N balance during cropping acid intercropping periods) were eoafinuousiv monitored using N-15 isotopic tracing and the tensio-neutronic method (i.e., continual measurement of soil water balance using a neutron moisture meter and tensiometers), together with porous suction cups installed at 0.3-, 0.5-, and 0.8-m depths. Water drainage and leaching of NO3--N, with a partition between that derived from fertilizer and that produced by soil mineralization, were thus obtained during and after the crop cycle. The balance of the labeled fertilizer at harvest was also determined by conventional soil coring and plant sampling. The two methods were used successfully during the 3 yr in which there were different climatic conditions and different fertilizer application rates. The results show that the traditional fertilizer input in the area (260 kg N ha(-1)) could be reduced nearly 30% without any substantial loss in grain yield but with a considerable reduction of nonpoint source pollution due to NO3- leaching. It is also shown that with the combined use of the two methods, it is possible to characterize separately fertilizer-N uptake, fertilizer-N leaching, and N immobilization and to estimate N-gaseous losses.