Nitrogen‐use efficiency of young citrus trees as influenced by the timing of fertilizer application

The aim of this study was to improve the fertilizer‐nitrogen‐use efficiency (NUE) for a sustainable citrus production where yield, fruit quality, and environment are reconciled. A soil pot experiment was conducted using 5‐year‐old orange trees ( Citrus sinensis cv. Navel Lane Late) to determine the response of NUE to timing of N‐fertilizer supply. An equal N rate (50 g tree –1 ) following two seasonal supply distributions was tested: (1) ENS (early supply from March to June of 75% of the N rate, with the remaining 25% administered between July and October) and (2) LNS (late supply of the bulk of the N rate with 25% between March and June and 75% between July and October). Labeled fertilizer (5 atom% 15 N excess) was applied in order to accurately quantify N uptake, its partitioning among plant–soil compartments and NUE at the end of fruit drop and at fruit maturity. LNS resulted in a significant increase in NUE in both stages (up to 19% at the end of fruit drop and 9% at fruit maturity), while also increasing summer/autumn flush development. ENS showed lower fruit abscission, an enhanced final fruit load (up to 45%), and a more profuse development of spring flush. It is worth mentioning that LNS led to higher 15 N content in the majority of the tree storage organs available for next spring growth resumption. The amount of 15 N remaining in the soil of ENS trees at the end of the trial, which represented 16% of the supplied 15 N, was 1.5‐fold higher than that of LNS trees as a result of the lower NUE of the former. Irrespective of the seasonal distribution of the fertilizer, mean 15 N recovery in the soil–plant system at the end of the trial was about 71%. The results clearly confirm that the N seasonal distribution curve affects NUE in young citrus trees and NUE increases when the bulk of supply of the N rate is delayed to the summer months.