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 tree1) 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% 15N excess) was applied in order to accurately quantify N uptake, its partitioning among plantsoil 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 15N content in the majority of the tree storage organs available for next spring growth resumption. The amount of 15N remaining in the soil of ENS trees at the end of the trial, which represented 16% of the supplied 15N, 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 15N recovery in the soilplant 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.