ABSTRACT

A rational nitrogen fertilization should contemplate not only the input of a dose adjusted to crop requirements, but also its correct distribution along the fertilizing period. However, there is a lack of information about the effect of fertilizer seasonal distribution on the uptake and the mobilization of reserve N accumulated in citrus plants.
The aim of this study was to evaluate the effect of the seasonal distribution of nitrogen fertilization on N uptake and its partitioning between tree organs, the mobilization of N accumulated in the reserve old organs to developing organs, and their impact in fruiting in young citrus plants. This will deepen both the understanding of the dynamics of N in the plant-soil system in citrus fruits as well the possible factors involved in this process, in order to broaden nitrogen fertilization criteria and thus optimize the seasonal application of fertilizers.
In order to achieve these objectives, the stable isotope 15N was incorporated into plant-soil system to trace N-fertilizer (isotope dilution technique). The fertilizer was applied from the onset of vegetative activity (early march) until the full fruit development (late october), following three seasonal distributions. A symmetrical distribution, appliying the same fraction of the dose from march to july (end of fruit drop) and from that moment onwards, was compared with two distributions in which the maximum contribution of fertilizer, 75% of the dose was delayed (from July to October) or anticipated (from march to july) with respect to it. Destructively harvesting of labelled plants in different phenological stages of development (flowering, fruit set, end of fruit drop and fruit maturity), allowed to study the evolution of N absorbed and translocated from the reserves.
According to the information obtained, it was concluded that early supply of the bulk of the dose (from march to late june), increased N uptake during critical periods of flowering and fruit set and hence decreasing dependence on N reserves accumulated in the plants. In addition, delaying this supply to the period comprised between the end of fruit drop (early july) and fruit maturity, resulted in a greater accumulation of N in the plant at the end of the period, available for next growing cycle. Improved N use efficiency associated with delayed supply would result in the reduction of residual nitrate in the soil susceptible to leaching. The incidence of seasonal distribution on yield was not consistent, since lower uptake during critical periods of flowering and fruit set associated with the distribution in which N supply is delayed, was compensated with increased translocation of N from the reserves of the plants.