In order to establish a rational nitrogen (N) fertilisation and reduce groundwater contamination, a clearer understanding of the N distribution through the growing season and its dynamics inside the plant is crucial. In two successive years, a melon crop (Cucumis melo L cv. Sancho) was grown under field conditions to determine the uptake of N fertiliser, applied by means of fertigation at different stages of plant growth, and to follow the translocation of N in the plant using N-15-labelled N. In 2006, two experiments were carried out. In the first experiment, labelled N-15 fertiliser was supplied at the female-bloom stage and in the second, at the end of fruit ripening. Labelled N-15 fertiliser was made from (NH4)-N-15 (NO3)-N-15 (10 at.% N-15) and 9.6 kg N ha(-1) were applied in each experiment over 6 days (1.6 kg N ha(-1) d(-1)). In 2007, the N-15 treatment consisted of applying 20.4 kg N ha(-1) as (NH4)-N-15 (NO3)-N-15 (10 at.% N-15) in the middle of fruit growth, over 6 days (3.4 kg N ha(-1) d(-1)). In addition, 93 and 95 kg N ha(-1) were supplied daily by fertigation as ammonium nitrate in 2006 and 2007, respectively. The results obtained in 2006 suggest that the uptake of N derived from labelled fertiliser by the above-ground parts of the plants was not affected by the time of fertiliser application. At the female-flowering and fruit-ripening stages, the N content derived from N-15-labelled fertiliser was close to 0.435 g m(-2) (about 45% of the N applied), while in the middle of fruit growth it was 1.45 gm(-2) (71% of the N applied). The N application time affected the amount of N derived from labelled fertiliser that was translocated to the fruits. When the N was supplied later, the N translocation was lower, ranging between 54% at female flowering and 32% at the end of fruit ripening. Approximately 85% of the N translocated came from the leaf when the N was applied at female flowering or in the middle of fruit growth. This value decreased to 72% when the N-15 application was at the end of fruit ripening. The ammonium nitrate became available to the plant between 2 and 2.5 weeks after its application. Although the leaf N uptake varied during the crop cycle, the N absorption rate in the whole plant was linear, suggesting that the melon crop could be fertilised with constant daily N amounts until 2-3 weeks before the last harvest. (C) 2011 Elsevier B.V. All rights reserved.
