NITROGEN IN DRAINAGE WATER AS INFLUENCED BY SOIL DEPTH AND NITROGEN-FERTILIZATION - A STUDY IN LYSIMETERS

To evaluate the environmental risk associated with nitrogen fertilizat ion of arable crops, the concentration of (NO2 + NO3)-N (Nitr-N) and N H4-N (Amm-N) in leachate, as well as the water volumes drained from cu ltivated field lysimeters were monitored for three years in Udine (nor th east Italy). Three crops (barley, soybean and maize) were subjected to three levels of nitrogen fertilization: nil, intermediate (about 1 25 kg ha(-1) year(-1)) and high (about 250 kg ha(-1) year(-1)). Lysime ters collected leachates at two depths in the soil profile (0.5 and 1. 0 m), whose depth was limited by gravel. The concentration of Nitr-N i n leachate increased with soil depth and fertilization level from 7.0 mg l(-1) (unfertilized soil 0.5 m deep) to 25.3 mg l(-1) (highly-ferti lized soil 1.0 m deep). Even in the absence of fertilization, there wa s a significant probability (19 per cent in the 0.5 m deep soil and 57 per cent at 1.0 m deep) of exceeding the current threshold values for NO3-N content in waters (11.3 mg l(-1)), while the probability was ne gligible for Amm-N. At the intermediate and high fertilization levels, NO3-N limits were exceeded more frequently (34 and 55 per cent of occ asions in the shallow soils, respectively; 70 and 86 per cent in the d eep soils, respectively). The highest values of Nitr-N in leachate wer e observed between June and July; however, the peaks of Nitr-N concent ration were recorded later in the year in the deep soil than in the sh allow one. Nitrogen fertilization and soil depth increased total N los ses, but their effect on N concentration of leachate was balanced by a decrease of drainage volumes, which were affected by the evapotranspi ration of cultivated crops. Grain yield of unfertilized crops was econ omically inadequate, but the intermediate fertilization level appeared adequate for satisfactory economic returns. Considerations are also r eported on the relationship between efficiency and leaching of N ferti lization.