Storage and remobilisation of nitrogen by pear (Pyrus communis L.) trees as affected by timing of N supply

Orchard nitrogen (N) management should aim at reconciling productivity, fruit quality and environmental concerns. Fertilisation strategies should, therefore, maximise the efficiency of N fertilisers, including the choice of the optimal timing of N supply. In the present study, with the aid of labelled [Ca(15NO3)2] fertiliser, we assessed: (1) the uptake; (2) the partitioning of nitrogen in 1 year; and (3) its remobilisation the following year, as affected by the timing of N supply. Two-year old Abbe Fetel trees on quince C were grown in pots filled with sand and divided into three groups: one group (A) received 3 g (in total) of labelled N from mid March to mid June, while trees of group B received 3 g of labelled N from late June to fruit harvest (August 20). Trees of group A and B received unlabelled N (3 g/tree) from late June to fruit harvest and from mid March to mid June, respectively. A third set of trees (C) received labelled N (6 g/tree) throughout the season. At fruit harvest, fruits and leaves contained similar amounts of N derived from remobilisation of stored N and from spring uptake (March-June, treatment A) and only a small fraction (around 10%) of N derived from N taken up after June (treatment B). Although abscised leaves contained 10 times higher amounts of N taken up early (A) than late (B) treatments, similar amounts of labelled N were recovered in the tree framework in winter in trees of groups A and B. Remobilisation of N in the following spring accounted for 23-24% of the labelled N in the tree, regardless the timing of N uptake. Trees remobilised with preference N taken up during the previous year than N absorbed earlier. Results indicate that a limited supply of N before fruit harvest does not increase the fruit N content significantly, while it increases N storage in roots during winter to be remobilised the following spring.