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Title: NITROGEN BEST MANAGMENT PRACTICE FOR CITRUS TREES: II. NITROGEN FATE, TRANSPORT, AND COMPONENTS OF N BUDGET

Author
item Alva, Ashok
item PARAMASIVAM, S - SAVANNAH STATE UNIV
item FARES, A - UNIVERSITY OF HAWAII
item OBREZA, T - UNIVERSITY OF FLORIDA
item SCHUMANN, A - UNIVERSITY OF FLORIDA

Submitted to: Scientia Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2006
Publication Date: 6/1/2006
Citation: Alva, A.K., Paramasivam, S., Fares, A., Obreza, T.A., Schumann, A.W. 2006. Nitrogen best managment practice for citrus trees: ii. nitrogen fate, transport, and components of n budget. Scientia Horticultureae. 109:223-233.

Interpretive Summary: Drinking water quality monitoring studies conducted in Florida, in the early 1990s, have shown a pattern of increasing percent of sampled wells with nitrate concentrations above the maximum contaminant level (10 ppm of nitrate-nitrogen). This has prompted the need to develop crop specific nitrogen best management practices (BMP). This paper reports part of the results of 6 years field experiments conducted in Highlands County, Florida, using 20+ year-old 'Hamlin' orange trees on 'Cleopatra mandarin' rootstock grown on a Tavares fine sand with under-the-tree microirrigation. Different sources of N: (i) dry soluble granular fertilizer broadcast (4 appl/yr); (ii) fertigation (18 appl/yr); (iii) controlled release fertilizer (1 appl/yr); or (iv) combination of dry soluble granular (2 appl/yr) and fertigation (18 appl/yr) were applied at four N rates in the range of 112 to 280 kg/ha/yr. Evaluation of N fate and transport as well as partitioning of N into various N budget components is important in the process of N-BMP development. Suction lysimeters were used to sample soil solution at 60, 120, and 240 cm depths underneath the tree canopy. The concentration of nitrate in the suction lysimeter samples at 240 cm depth and estimated quantity of water leached at this depth were used to estimate the total N leached on an annual basis. Estimation of citrus tree N budget components at 280 kg N/ha/yr revealed that 15% of N input was unaccounted for, suggesting that this N was subject to leaching losses. Despite careful management of N and irrigation in this experiment, some degree of N leaching is unavoidable under the production conditions characterized by sandy soils (> 95% sand) and high rainfall (1250 to 1650 mm annual rainfall with up to 60% of rain during June through September). This study also demonstrated that the improved N and irrigation management practices contributed to an increase in N uptake efficiency and decreased N leaching losses.

Technical Abstract: Elevated levels of nitrate-N (NO3-N) in the surficial aquifer above the drinking water quality standard, i.e. maximum contaminant limit (MCL; i.e. 10 mg/L) have been reported in some parts of central Florida citrus production region. Soils in this region are very sandy, hence are vulnerable to leaching of soluble nutrients and chemicals. Objective of this study was to develop N and irrigation best management practices for citrus in sandy soils to maintain optimal crop yield and quality, and to minimize N leaching below the rootzone. A replicated plot experiment was conducted in a highly productive 20+ year-old Hamlin orange [Citrus sinensis (L.) Osbeck on Cleopatra mandarin (Citrus reticulata Blanco)] grove located on a well drained Tavares fine sand (hyperthermic, uncoated, Typic Quartzipsamments) in Highland County, FL. Nitrogen rates (112 to 280 kg/ha/yr) were applied as fertigation (FRT), water soluble granular (WSG), a combination of 50% FRT and 50% WSG, and controlled-release fertilizer (CRF). Tensiometers were used to monitor the soil moisture content at various depths in the soil profile as basis to optimize irrigation scheduling. Fruit yield and quality and nutritional status of the trees were reported in a companion paper. Soil solution was sampled at 60, 120, and 240 cm depth under the tree canopy using suction lysimeters. The mean concentrations of NO3-N in 240 cm depth soil solution and the estimated quantity of drainage water were used to calculate the quantity of NO3-N leached below the rootzone of the trees. The NO3-N concentrations in the 240 cm depth soil solution were used as an indicator of NO3-N leaching below the tree root zone, which generally remained below the MCL in most samples across all N sources and rates, but for few exceptions. The components of citrus tree N budget estimated at 280 kg N/ha/yr rate showed 15% of total N input was unaccounted, which could be subject to leaching loss. This estimate of potential leaching was very close to that predicted by LEACHM simulation model. The improved N and irrigation management practices developed in this study contributed to an improved N uptake efficiency, and a reduction in N losses.