Fate and transformations of N-15 labelled nitrogen applied in spring to citrus trees
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The fate of N derived from labelled ammonium sulphate (8.5% N-15 excess) was determined in three year old 'Valencia' orange trees (Citrus sinensis (L.) Osbeck) on Troyer citrange rootstock (C. sinensis X P. trifoliata). The plants were fed with 30 g fertilizer N plus N supplied by the irrigation water from the beginning of the spring flush until the end of different phenological stages of the grown cycle. The trees were grown in containers filled with two different textured soils (sandy and loamy). Most absorbed N was accumulated in the new organs (mainly in the new flush leaves) and the values were higher in the loamy soil than in the sandy soil. At the end of flowering (May), about 26% of total N content in the new organs came from N applied in the sandy soil, whereas in the loamy soil the values were much lower. In both soils, the N derived from N applied was about one third of total N content in the whole tree at the end of the growth cycle. The 15 N taken up from the fertilizer in the whole tree plus fallen organs represented 40% of the applied N in the two types of soil. The percentages of mineral-N retained in the sandy soil profile were significantly lower (0.4 %) than those found in loamy soil (4.8 %) in November. The organic-N retained was also lower in the sandy soil (6.8 %) than in the loamy soil (18.9 %). The total recovery in the plant-soil system was 46.4 and 60.1 % in the sandy and loamy soils, respectively.