NITROGEN UPTAKE EFFICIENCY AND LEACHING LOSSES FROM LYSIMETER-GROWN CITRUS TREES FERTILIZED AT 3 NITROGEN RATES

Four-year-old 'Redblush' grapefruit (Citrus paradisi Macf.) trees on e ither the relatively fast-growing rootstock 'Volkamer' lemon (VL) (C. volkameriana Ten, & Pasq.) or on the slower-growing rootstock sour ora nge (SO) (C. aurantium L,) were transplanted into 7.9-m(3) drainage ly simeter tanks filled with native Candler sand, irrigated similarly, an d fertilized at three N rates during 2.5 years. After 6 months, effect s of N application rate and rootstock on tree growth, evapotranspirati on, fruit yield, N uptake, and leaching were measured during the follo wing 2 years, When trees were 5 years old, low, medium, and high N app lication rates averaged about 79, 180, or 543 g N/tree per year and ab out 126, 455, or 868 g N/tree during the following year, Recommended r ates average about 558 g N/tree per year, A lysimeter tank with no tre e and additional trees growing outside lysimeters received the medium N treatment. Nitrogen concentration in the drainage water increased wi th N rate and exceeded 10 mg liter(-1) for trees receiving the high ra tes and also for the no tree tank, Leachate N concentration and total N recovered was greater from trees on SO than from those on VL. Averag e N uptake efficiency df medium N rate trees on VL was 68% of the appl ied N and 61% for trees on SO. Nitrogen uptake efficiency decreased wi th increased N application rates. Trees outside lysimeters had lower l eaf N and fruit yield than lysimeter trees, Overall, canopy volume and leaf N concentration increased with N rate, but there was no effect o f N rate on fibrous root dry weight. Fruit yield of trees on SO was no t affected by N rate but higher N resulted in greater yield for trees on VL, Rootstock had no effect on leaf N concentration, but trees on V L developed larger canopies, had greater fibrous root dry weight, used more water, and yielded more fruit than trees on SO, Based on growth, fruit yield and N leaching losses, currently recommended N rates were appropriate for trees on the more vigorous VL rootstock but were 22% to 69% too high for trees on SO.