| abstract: | 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 sil 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 reticulate 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, slit fertilizer application, and timing of application contributed to 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. |
