Estimation of Nitrate Leaching in an Entisol under Optimum Citrus Production

Leaching of fertilizer nutrients and widespread NO 3 ‐N contamination of drinking water wells in proximity to citrus growing regions of central Florida are a serious concern. We evaluated NO 3 ‐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 NO 3 ‐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 NO 3 ‐N at this depth could contaminate groundwater. At the 60‐ or 120‐cm depths, the NO 3 ‐N concentrations occasionally peaked at 12 to 100 mg L −1 , but at 240 cm NO 3 ‐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 NO 3 ‐N below the root zone. Calculated NO 3 ‐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.