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DIVISION S-4—SOIL FERTILITY & PLANT NUTRITION

Biomass Distribution and Nitrogen-15 Partitioning in Citrus Trees on a Sandy Entisol

^a Centro de Citricultura Sylvio Moreira- IAC, Via Anhanguera, km 158, 13490-970 Cordeirópolis-SP, Brazil
^b Univ. of Florida, Soil and Water Science Dep., 106 Newell Hall, P.O. Box 110510, Gainesville, FL 32611-0510
^c ARS-USDA, 24106 N Bunn Rd., Prosser, WA 99350-0000

^* Corresponding author (ddm{at}centrodecitricultura.br)

The area under citrus production in Florida is 332 000 ha, with a production of 10 to 12 million metric tonnes of fruit annually. Nutrient management recommendations are needed to increase N uptake efficiency and to minimize nitrate leaching below the root zone. The objectives of this study were (i) to evaluate biomass distribution of 6-yr-old ‘Hamlin’ orange trees [Citrus sinensis (L.) Osbeck] on ‘Swingle citrumelo’ [Poncirus trifoliata (L.) Raf. x C. x paradisi Macfad.] rootstock grown in a sandy soil under low volume irrigation, and (ii) to estimate partitioning of ¹⁵N fertilizer applied to the soil during early spring into different tree components. We evaluated biomass of tree components (leaves, twigs, trunk, taproot, roots, and fruit), and N recovery and distribution of ¹⁵NH₄¹⁵NO₃ (AN) and ¹⁵N-urea (UR) (10 atom % ¹⁵N) applied to the soil surface. About 70% of dry matter biomass of trees was aboveground (AG). Length density of feeder roots was concentrated at a depth of 0 to 15 cm below the soil surface and varied from 1.87 to 0.88 cm cm^-3 at 0.5- and 1.5-m distance from the trunk, respectively. Total recoveries of ¹⁵N by trees were 25.5% for UR and 39.5% for AN at fruit harvest, 280 d after fertilization. Mean accumulation of applied ¹⁵N in recent leaf flush was 4.2% and that of older leaves was 2.5%. Accumulation of ¹⁵N was low in woody tissue. Since fruit represented a large sink for N (10.2 and 18.4% recovery of ¹⁵N applied as UR and AN, respectively), we confirmed the importance of N fertilization before fruit development.

Abbreviations: AG, aboveground • AN, ammonium nitrate • BG, belowground • BMP, best management practice • DAF, days after fertilization • Ndff, N in plant components derived from the labeled fertilizer • PVC, polyvinyl chloride • UR, urea

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