FIELD-MEASUREMENTS OF WATER AND NITROGEN LOSSES UNDER IRRIGATED MAIZE

An intensive multidisciplinary experiment has been conducted over seve ral years at La Cote Saint-Andre, near Grenoble, France. The major obj ective is to determine an optimal fertilizer application scheme for an irrigated agricultural system. Such a scheme would not degrade the qu ality of the environment, and yet would maintain a profitable level of crop production. This study is explicitly related to the cultivation of irrigated maize, a major crop in the area. The various terms of the water balance (consumption, drainage, soil storage) and of the nitrog en cycle (mineralization, plant uptake, leaching) were obtained from i ntensive monitoring in the upper layer of the 0.8 m of soil which corr esponds to the root zone of the crop. This entailed the combined use o f a neutron moisture meter, tensiometers and soil suction cups. To det ermine the specific effects of fertilization and crop growth, there we re different treatments. These corresponded to a traditional fertilize r application of 260 kg N ha-1, no fertilization, and bare soil, carri ed out within an area of approximately 2 ha. Several sites were instru mented on each treatment, one of them being specifically for the appli cation and the monitoring of N-15-tagged fertilizer. The results have shown that, in terms of the water balance, irrigation water management is extremely efficient, as drainage losses under the maize culture ar e negligible during the crop cycle. The situation is totally different , however, during the intercrop period (October-April), owing to rainf all. Then the soil is left bare and evaporation is very small, and now the drainage corresponds to about 90% of total inputs from precipitat ion. In terms of the nitrogen cycle, the results showed clearly that u p to 150 kg N ha-1 was produced by mineralization in the soil. Nitroge n leaching beyond the root zone during the crop cycle is negligible, r egardless of the rate of fertilizer application, as a result of the ve ry small amount of drainage, despite irrigation. A very important cont rast was found, however, between the fertilized and unfertilized treat ments at harvest. There was a residue of 182 +/- 64 kg N ha-1 in the f ertilized sites, but none for the others. The whole quantity remaining in the root zone at harvest was then totally leached by winter rains. To decrease the risk of groundwater pollution, a reduction of about 1 00 kg N ha-1 from the traditional application rate has been recommende d. Finally, the method of estimation of N balance has been successfull y validated by a comparison between N uptake determined by direct anal ysis of the whole plant and the value estimated from the temporal vari ations of the N content in the soil.