N-15 TRACERS COMBINED WITH TENSIO-NEUTRONIC METHOD TO ESTIMATE THE NITROGEN-BALANCE OF IRRIGATED MAIZE

In response to a very high increase of groundwater NO3- pollution, lar gely connected with intensive agricultural practices, a long-term expe rimentation has been set up close to Grenoble, France, with the follow ing aims: first io characterize the response of maize (Zea mays L.), a predominant crop in the area, to fertilization, and second to quantif y the N balance during and after the crop cycle. This study relates to results concerning the second issue: experiments were conducted on ir rigated maize in 1991, 1992, and 1993 on the Experimental Farm at La C ote Saint-Andre, France, in the heart of one of the most important agr icultural zones between the French Alps and the Rhone Valley. The dyna mics of soil and fertilizer N (NO3- transport and N balance during cro pping acid intercropping periods) were eoafinuousiv monitored using N- 15 isotopic tracing and the tensio-neutronic method (i.e., continual m easurement of soil water balance using a neutron moisture meter and te nsiometers), 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 part ition between that derived from fertilizer and that produced by soil m ineralization, were thus obtained during and after the crop cycle. The balance of the labeled fertilizer at harvest was also determined by c onventional soil coring and plant sampling. The two methods were used successfully during the 3 yr in which there were different climatic co nditions and different fertilizer application rates. The results show that the traditional fertilizer input in tile area (260 kg N ha(-1)) c ould 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 uptak e, fertilizer-N leaching, and N immobilization and to estimate N-gaseo us losses.