Estimation of nitrate leaching in an entisol under optimum citrus production

Leaching of fertilizer nutrients and widespread NO3-N c contamination of dr inking water wells in proximity to citrus growing regions of central Florid a are a serious concern, We evaluated NO3-N distribution in soil solution a t 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 I Citrus sinensis (L,) Osbeck I trees on Cleopatra mandarin (Citrus reticulata Blanc o) 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 (D GF; 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 fertilize r (CRF; single application yr(-1). irrigation was scheduled using recommend ed 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 24 0-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 th e root zone. Calculated NO3-N leaching losses below the rooting depth incre ased with increasing rate of N application and the amount of water drained, and accounted for 1 to 16% of applied fertilizer N.