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a Center for Marine, Environmental Sciences and Biotechnology Research, Drew Griffith Hall, Savannah State Univ., Savannah, GA 31404
b USDA-ARS-PWA, 24106 North Bunn Road, Prosser, WA 99350
c Sentek Pty Ltd, 77 Magill Road, Stepney, South Australia, 5069 Australia
d CMESBR, Savannah State University, Savannah, GA 31404
Corresponding author (siva{at}savstate.edu)
Leaching of fertilizer nutrients and widespread NO3-N contamination of drinking water wells in proximity to citrus growing regions of central Florida are a serious concern. We evaluated NO3-N distribution in soil solution at various depths in the vadose zone, and N leaching below the root zone for two cropping seasons under the canopy of 21-yr-old Hamlin orange [Citrus sinensis (L.) Osbeck] trees on Cleopatra mandarin (Citrus reticulata Blanco) rootstock, on an entisol of central Florida. The treatments included 112, 168, 224, and 280 kg N ha-1 yr-1 as either dry granular fertilizer (DGF; broadcast, in 4 equal doses) or fertigation (FRT; 15 applications yr-1), and 56, 112, and 168 N kg ha-1 yr-1 as controlled-release fertilizer (CRF; single application yr-1). Irrigation was scheduled using recommended tensiometer set points as guidelines, with a target wetting depth of 90 cm. The NO3-N was measured in soil solutions bi-weekly at 60-, 120-, and 240-cm depths using suction lysimeters (SLs) installed under the tree canopy. The 240-cm depth sample represented soil solution below the rooting depth of the trees, and the NO3-N at this depth could contaminate groundwater. At the 60- or 120-cm depths, the NO3-N concentrations occasionally peaked at 12 to 100 mg L-1, but at 240 cm NO3-N concentrations mostly remained below 10 mg L-1. The careful irrigation management, split fertilizer application, and timing of application contributed to the low leaching of NO3-N below the root zone. Calculated NO3-N leaching losses below the rooting depth increased with increasing rate of N application and the amount of water drained, and accounted for 1 to 16% of applied fertilizer N.
Abbreviations: CRF, controlled-release fertilizer • DGF, dry granular fertilizer • FRT, fertigation • FLDEP, Florida Department of Environmental Protection • IBDU, isobutylidene diurea • NBMP, N best management practices • PRCU, polyolefin resin-coated Urea • SL, suction lysimeter
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