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

Abstract

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. × C. × paradisi Macfad.] rootstock grown in a sandy soil under low volume irrigation, and (ii) to estimate partitioning of 15 N 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 15 NH 4 15 NO 3 (AN) and 15 N‐urea (UR) (10 atom % 15 N) 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 15 N by trees were 25.5% for UR and 39.5% for AN at fruit harvest, 280 d after fertilization. Mean accumulation of applied 15 N in recent leaf flush was 4.2% and that of older leaves was 2.5%. Accumulation of 15 N was low in woody tissue. Since fruit represented a large sink for N (10.2 and 18.4% recovery of 15 N applied as UR and AN, respectively), we confirmed the importance of N fertilization before fruit development.