Nitrogen uptake, nitrogen-use efficiency and nitrogen leaching losses of citrus

The effects of varying nitrogen (N) supply on the uptake, N use-efficiency (NUE = dry weight per unit N) and subsequent leaching losses of N below the rootzone of citrus species were investigated in a series of greenhouse and field studies using the {dollar}sp{lcub}15{rcub}{dollar}N isotope.; In the greenhouse, saline irrigation water reduced plant N-uptake by reducing the growth and whole-plant transpiration of both salt-sensitive and tolerant citrus rootstock species. Residual soil-N and N leaching potential was, therefore, increased by salinity. Plant NUE decreased with salinity since plant growth was reduced more than N-uptake. Seedling growth rate was the primary determinant of N uptake when rootstock species were fertilized at different rates. Nitrogen uptake of both slow- and fast-growing species did not increase at N application rates above 53 and 105 mg N week{dollar}sp{lcub}-1{rcub}{dollar}, respectively, whereas residual soil N and the leaching potential did increase. Fertilizer-N uptake efficiency and plant NUE decreased with increasing N-supply, and N was allocated to new growth from existing tissues at N rates below 53 mg N week{dollar}sp{lcub}-1{rcub}{dollar}. Unaccounted-for-{dollar}sp{lcub}15{rcub}{dollar}N losses in Candler fine sand without plants increased above 4 C and in the presence of soil microbes.; Citrus tree growth and N-demand were strongly influenced by rootstock species. Four-year-old grapefruit trees on sour orange (SO) and Volkamer lemon (VL) rootstocks required 140 and 336 g N year{dollar}sp{lcub}-1{rcub}{dollar}, respectively, to maintain high growth rates. Total {dollar}sp{lcub}15{rcub}{dollar}N-uptake did not increase at N application rates above 140 g N year{dollar}sp{lcub}-1{rcub}{dollar} for SO, with 39% of the SO-336 g {dollar}sp{lcub}15{rcub}{dollar}N still present in the soil after 29 days, thus increasing N leaching potential in Candler fine sand, in comparison to VL. {dollar}sp{lcub}15{rcub}{dollar}Nitrogen-uptake and allocation were primarily determined by the sink demand of tree fruit load and vegetative growth. Tree NUE on SO decreased until growth was no longer limited by N-supply.; A single foliar application of 5%-N urea increased the total leaf N content by 7% after 48 h, compared to less than 2% from a KNO{dollar}sb3{dollar} solution (1.5%-N) of equivalent osmotic potential. Foliar N applications, therefore, provide an effective method of N fertilization in citrus.