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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)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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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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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Perennial trees store large quantities of N within various plant components, which can be utilized for tree growth and fruit yield in subsequent seasons, as reported for almond trees {Prunus amygdalus Batsch [= P. dulcis (Mill.) D.A. Webb]} (Weinbaum et al., 1984), citrus (Legaz et al., 1995), kiwifruit [Actinidia deliciosa (A. Chev.) C.F. Liang & A.R. Ferguson] (Ledgard and Smith, 1992), and apple (Malus domestica Borkh.) (Millard and Neilsen, 1989; Khemira et al., 1998). The annual vegetative growth and the fruit yield of citrus trees contain a variable proportion of the fertilizer N applied during the growth period. A great amount of N in the new growth may be drawn from the tree biomass. Therefore, the N reserve in the leaves and structural components plays an important role in the development of new flushes of growth and flowers in the spring (Kato, 1986).
Nitrogen distribution in ‘Valencia’ orange trees, grown in sand culture and fertilized with a nutrient solution labeled with 15N, presented distribution of labeled N among tree parts ranked in the order: leaves (49.7%) > roots (19.2%) > twigs and stem (14.3%) > flowers (10.6%) > ovaries (6.1%) (Legaz et al., 1981). The mobilization of N reserves in oranges is a result of biochemical processes, in which total protein content of old leaves decreases progressively beginning in the spring. The mobilized protein comes from an aqueous protein that is the major fraction of the total leaf protein (Moreno and Garcia-Martínez, 1984).
The removal of N in harvested plant biomass has been used to estimate N requirements in some crops, particularly annual grain crops. In contrast, the large pool of N present in the structural components of citrus trees implies that distribution and remobilization of N within the tree play an important role in determining the N requirement on an annual basis which must be known to maximize N uptake efficiency and minimize its losses (Sanchez et al., 1995).
Efficient management of irrigation can minimize leaching losses of highly soluble nutrients (i.e., nitrate) through the soil profile below the rooting zone. Citrus production in deep sandy soils with high volume irrigation systems, as used in central Florida in the past, tends to cause the upper soil layers to dry between long irrigation intervals. This condition favors deep rooting, as reported by Cahon et al. (1962), Castle and Krezdorn (1977), Castle et al. (1993), and Boman et al. (1999). The same occurs in nonirrigated groves, where the majority of the root system can reach depths > 1.5 m in the soil (Pace and Araujo, 1986; Oliveira et al., 1998). However, using low volume irrigation systems to replenish the moisture deficit in the surface soil may lead to a shallow rootzone of the citrus trees. Since citrus growth and root distribution can be modified as a result of changes in the root environment, a clear understanding of the root system is also important to develop irrigation and nutrient BMPs in an effort to improve uptake efficiency and minimize losses below the rooting depth.
The objectives of this study were (i) to evaluate biomass distribution of 6-yr-old citrus trees grown in a sandy soil under low volume irrigation, and (ii) to estimate partitioning of soil-applied–15N during early spring in different plant components during fruit harvest.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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7.0, and CEC of 2.2 cmolc kg-1 within the 0- to 30-cm depth. This grove was managed with application of dry soluble fertilizer containing N, P, and K. The annual rate of N was equivalent to 230 g N tree-1 as AN. Trees were irrigated using one under-tree low volume emitter per tree covering 7 m2 with a delivery rate of 50 L h-1. Irrigation was initiated based on 33% depletion of the available soil moisture within the 40-cm soil depth determined by a multisensor capacitance probe (EnviroSCAN, Sentek PTY Ltd., South Australia) placed at the drip line (Alva and Fares, 1998; Fares and Alva, 1999). The experiment, a randomized complete block design with two treatments (N source) and three replications (with one tree each), was initiated in Feb. 1999 and continued until Dec. 1999. Fertilizer sources included: (i) UR and (ii) AN. The labeled fertilizers had an isotopic enrichment of 10 atom % 15N. Fertilizers were uniformly distributed as dry granules to the soil surface in a circular area (1.10-m radius) under the tree canopy. Twenty-five percent of the recommended annual N rate of 230 g tree-1 (Ferguson et al., 1995) was applied as labeled N on 15 Feb. 1999. Following fertilizer application, the area received 7 mm of irrigation water to promote fertilizer dissolution and shallow incorporation into the soil. The grove irrigation was then managed as described earlier.
Two additional applications of nonlabeled N were made on 8 June and 9 Sept. 1999 (in equal amounts of 86 g tree-1) to supply the trees with the remaining recommended annual N rate. Phosphorus and K were applied at 18 g P tree-1 and 200 g K tree-1, also in June and September.
Tree Sampling, Biomass Estimation, and Isotopic Determination
Tree flowering started in late February 1999 and the spring flush had completely expanded leaves in April 1999. Leaves and fruit were collected in the experiment following fertilizer application in February 1999. The mature flush of leaves and the summer/fall 1998 leaf flush were sampled at 0 (day of fertilization), 7, 14, 21, 28, 35, 49, 63, and 77 d after fertilization (DAF). The summer/fall 1998 leaf component was also sampled at 113 and 206 DAF. The 1999 spring flush leaves were sampled at 49, 63, 77, 113, and 206 DAF. Leaf samples on each sampling date comprised 10 leaves per tree. Fruit samples were taken in April (63 DAF; 20 per tree), June (113 DAF; 10 per tree), and September 1999 (206 DAF; 10 per tree), when they had an average diameter of 1.5, 3.5, and 5.0 cm, respectively. Leaves and fruit were washed in detergent solution and thoroughly rinsed in tap water, followed by distilled water, and then dried at 65°C for 72 h (fruit were sliced in small pieces before drying). The dried tissue was ground to pass through a 0.635-mm screen using a ball mill. Grinding containers were washed with 0.2 mol L-1 H2SO4 solution and rinsed with deionized water between samples.
The concentration of N and the N isotopic ratio of tissue material were determined with an automated Roboprep C/N analyzer linked to a Tracer Mass Isotope Ratio Mass Spectrometer (Europa Scientific, Ltd., Cheshire, UK) at the Stable Isotope Research Unit, Oregon State University. The tracer mass system is capable of analyzing with a precision of 0.8 atom 
15N for levels above natural abundance.
Trees that received AN fertilizer (one per plot) were destructively harvested in Dec. 1999 for evaluation of dry mass distribution in different tree components and sampled for determinations of N concentration and isotopic ratio as described above. The AG portion was divided into (i) summer/fall 1999 leaf flush; (ii) spring 1999 plus older leaves (the latter component was most made up by summer/fall 1998 flushes); (iii) twigs >1.5 cm in diameter; (iv) twigs 
1.5 cm in diameter; (v) trunk; and (vi) fruit. Soil was excavated in two opposing quadrants (NW and SE) of 1.75 x1.75 m each, marked on the soil surface, and which had the tree trunk as a common vertex. Then, roots removed from 0- to 15-, 15- to 30-, and 30- to 45-cm depths were separated from soil with a 0.2-cm mesh sieve into the following size classes: (i) fibrous roots (<0.2-cm diam.), (ii) woody roots (0.2- to 1.0-cm diam.), and (iii) woody roots (>1.0-cm diam.). The taproot was also separated from the soil and together with the roots, comprised the belowground (BG) portion of the tree.
Samples from tree components were collected in the field and placed in sealed plastic bags to prevent water loss, then weighed in the field. Later, the same material was washed in the laboratory and dried in an oven (65°C; 72 h) for dry mass determination and further N analysis as described earlier. Woody tissue samples were cut in small pieces (<10-mm), ground to pass through a 100-mm screen using a rotary mill, and then to pass a 0.635-mm screen using a ball mill. Total dry mass of roots was estimated by multiplying the values obtained for both excavated soil quadrants by two. Juice quality of fruit subsamples was determined (total soluble solids, citric acid content, and soluble solids/acid ratio) according to standard procedures (Wardowski et al., 1995).
Trees that received the UR fertilizer also had the AG portion destructively harvested. Dry mass of components was determined based on the moisture content of samples collected in the field and dried in an oven (65°C; 72 h). Root excavation was not done from this treatment, since we assumed that root distribution for UR-treated trees would be similar to that of AN-treated trees. Nitrogen concentration and N isotopic ratios were determined for each tree part collected.
The percentage of N in the plant components derived from the labeled fertilizer (Ndff) and the total amount of N recovered (fertilizer N recovery) in different plant components were calculated using the isotopic dilution equations described by Hauck and Bremner (1976).
Root Distribution
Roots of AN-treated trees were also sampled in December 1999, before destructive harvest, using a 5-cm diam. polyvinyl chloride (PVC) corer for root density and average root diameter estimations. Samples were collected at 0- to 15-, 15- to 30-, and 30- to 45-cm soil depths every 50, 100, and 150 cm from the tree trunk in the N-S (within row), and E-W (between rows) directions. Samples were taken to the laboratory in sealed plastic bags and separated with a 0.2-cm mesh sieve. Two classes of root size were separated using forceps: (i) <0.2 cm in diameter and (ii) 0.2 to 1.0 cm in diameter. The roots were cleaned and fresh mass was recorded. Root length was determined by counting the number of horizontal and vertical intersections of roots in a grid system of 1.0 x 1.0 cm (Tennant, 1975), which multiplied by 11/14 and divided by the volume of the PVC corer (295 mL) gives the root density in cm cm-3 soil. Mean root radius (r0) was calculated assuming that fresh roots have a density of 1 Mg m-3 by the equation r0 = (Fwr/
L)1/2, where Fwr is the fresh mass of roots (in grams), and L is the total root length (in centimeters) (Barber, 1995).
Data Analysis
Standard deviations were calculated for mean Ndff, dry matter distribution of tree components, and average root density for each soil depth and trunk distance. A simple analysis of variance was used to test the hypothesis that means from AG biomass distribution of fertilized trees, root distribution obtained for soil quadrants, N content, 15N enrichment, and 15N recovery of tree components were equal (P = 0.05) using the GLM procedure of the SAS system (SAS Institute, 1996).
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Leaves accounted for 13% of the AG biomass. The summer plus fall 1999 flush represented the major portion of total leaf dry mass (Tables 1 and 2). Stansly et al. (1996) reported the seasonal growth pattern of 4-yr-old grapefruit (C. x paradisi Macfad.) trees, in which 20% of leaf area at the end of a growing season was carryover from the previous year and only 25% came from spring flush compared with 33% in summer and 35% in fall. This can be expected due to a larger size of individual leaves (summer + fall 1999 flush) compared with mature or spring flush leaves.
Roots accounted for 27.7% of the total dry mass of the tree. Fibrous roots accounted for the greatest proportion in the BG portion (35.6%) after the taproot (38.4%) (Table 1). More than 70% of fibrous roots were found in the 0- to 15-cm depth, and at deeper depths the root distribution data showed substantial variation as evident from greater coefficients of variation (Table 1). Woody roots followed a similar pattern as described above, and represented <26% of the total root system (as in the BG portion) within the 0- to 45-cm soil depth layer. Dasberg (1987) reported that the AG portion of 9- to 20-yr-old citrus trees including leaves, fruit, trunk, and branches accounted for >65% of the tree biomass. The dry matter partitioning of 22-yr-old ‘Shamouti’ orange trees (Feigenbaum et al., 1987) was: branches and twigs, 30.1%; trunk plus small branches, 25.3%; roots, 24.0%; fruit, 13.3%; and leaves, 7.3%. In the case of nonbearing (32-mo-old) Hamlin trees roots, trunk, large branches, leaves, and small branches accounted for 28.1, 26.1, 21.2, 18.0, and 7.8%, respectively, of the tree biomass (Alva et al., 1999). Our data are comparable to the above values since >70% of the total tree biomass was found in the AG components. Proportions of the dry mass of trunk or leaves deviate from values reported by Feigenbaum et al. (1987) and Alva et al. (1999) since the former presented a total value for trunk and main branches, whereas the latter harvested trees with no fruit. Dry mass distribution in citrus trees varies with the whole tree N status and with fruit load, which determines a competitive allocation effect of biomass among tree components (Lea-Cox et al., 2001). Furthermore, Swingle citrumelo is a superior rootstock for sweet oranges for high fruit yields under irrigation (Wutscher and Bistline, 1988; Castle et al., 1993). Such high yields, when related to a resulted smaller canopy volume of trees, can explain why fruit are a greater proportion of tree biomass compared with the value reported by Feigenbaum et al. (1987).
Root Density Distribution
Trees had uniform fresh root length density (Lv, cm root cm-3 soil) in all four directions evaluated (P > 0.05). Root density was greater closer to the tree trunk on both horizontal and vertical planes (P < 0.05) (Fig. 1A and B)
. Root density decreased from 1.85 cm cm-3 at the 0- to 15-cm depth to 0.16 cm cm-3 at the 30 to 45-cm depth within 50 cm from the tree trunk. At 150 cm from the trunk, root density was 48% less than at the 50-cm distance from the trunk in the 15-cm soil depth (Fig. 1A). The same pattern was found for each soil layer evaluated. Our values of Lv approach those reported for apple trees in the top 1.0 m of soil, which ranged from zero to 1.0 cm cm-3 (Hughes and Gandar, 1993; De Silva et al., 1999).
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Nitrogen-15 Taken up by Leaves and Fruit
Early effects of 15N application were detected by evaluating percentages of Ndff in the leaves (Fig. 2A and 3A)
. Maximum values of Ndff observed were 40% (Fig. 2A), indicating the importance of other N sources (tree reserve and soil) for the growth of citrus trees. Such limited contribution of fertilizer-N was also observed by Sanchez et al. (1992) for established pear (Pyrus communis L.) trees. A larger proportion of N from either the UR or AN labeled fertilizers occurred in younger leaves (22 to 38%), especially for the spring 1999 flush, compared with mature leaves (7 to 12%) (Fig. 2A and 3A). The N remobilization process involves several biochemical steps of protein degradation and translocation into different tree components (Titus and Kang, 1982; Kato, 1986; Engels and Marschner, 1995). Such mobilized N may not be enough to support large N sinks, and then newly absorbed soil N appeared in new growing tissues. Kato et al. (1982) showed that in the coldest season, N uptake by ‘Satsuma’ mandarin (C. reticulata Blanco) trees was 10% of the amount taken up in summer. More than 90% of the applied N was found in the roots in the winter; on the other hand, in the summer, 55% of the absorbed N translocated upwards and most of it was found in the developing new shoots. This suggests that the N taken up by the roots was translocated to the AG portion of the tree due to the high sink demand for N in protein synthesis of new developing organs (Legaz and Primo-Millo, 1984; Kato, 1986; Lea-Cox et al., 2001). Maximum root absorption efficiency is also reported to occur in late spring and early summer for peach trees [Prunus persica (L.) Bastch] (Muñhoz et al., 1993). The %Ndff increased gradually until 15 March for UR and 5 May for AN fertilized trees.
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7.0) following application of fertilizers demonstrated that volatilization accounted for 14.9 and 32.3% of applied N for AN and UR, respectively (Mattos, 2000). Since we assumed that tree biomass distribution was similar for UR and AN fertilized trees, the differences in %Ndff between treatments were probably due to differences in the amount of remaining N absorbed after fertilization. A plateau for Ndff was reached after 4 May, and the further decrease was probably associated with the uptake of nonlabeled N applied on 8 June and 9 September (Fig. 2A and 3A).
The total N concentration in the leaves of the orange trees before fertilization was 21 g kg-1 (Fig. 2B and 3B), and increased for 4 wk after application of the labeled fertilizer when levels of 26 g kg-1 for the summer/fall 1998 flush for both treatments were observed (Fig. 2B and 3B). Then N concentration declined after 15 March, probably as a result of combined processes of N redistribution from mature tissue and leaf expansion of young tissue (Calot et al., 1984). Nitrogen redistribution was evident since the concentration in mature leaves of AN-treated trees (25.3 g kg-1) was higher than that of UR-treated ones (21.7 g kg-1), whereas no major difference appeared for the summer/fall 1998 leaf flush as presented above. By 15 March, the residual soil inorganic 15N was very low (data not shown) for significant uptake and maintenance of the Ndff proportions in the leaves.
Nitrogen-15 Recovery by Tree Components
Nitrogen concentration was lowest in the trunk and taproot (3.7 to 4.4 g kg-1). The N concentration of twigs (4.0 to 7.8 g kg-1) and roots (5.8 to 17.0 g kg-1) varied depending on the tissue age. Younger roots had greater N concentration compared with older roots. Nitrogen concentration in the fruit had the least variation with values 8.3 g kg-1, whereas that of leaves varied from 21.0 to 25.5 g kg-1.
Nitrogen recovery from the labeled N source was greater for AN (39.5%) than for UR (25.5%) (Table 3). Recovery may have been slightly underestimated since roots were not totally collected from soil and there was also probably considerable loss of N due to senescence and shedding of mature leaves, flowers, and young fruit. Feigenbaum et al. (1987) reported that 15N-uptake efficiency from labeled KNO3 for 22-yr-old Shamouti orange trees was 40%. In their study, labeled fertilizer was applied with irrigation water in five monthly applications from April to August. Boaretto et al. (1999) found 33% 15N recovery from UR applied to soil for 1-yr-old ‘Pêra’ orange trees cultivated in closed pots where leaching losses of NO3 were prevented.
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5 to 6% of the applied labeled fertilizer, even though total removal by the vine tree was 50% (Ledgard and Smith, 1992). Weinbaum and Van Kessel (1998) found that total almond tree recovery of applied 15N-depleted N fertilizer during a 6-yr experimental period was 29.4%. Fruits were the dominant sink, and accounted for 78% of the labeled fertilizer-N recovered by the trees across the period of study. Percentage recovery of 15N applied to 4-yr-old grapefruit trees decreased with increasing N rate, which varied from 63.1 to 23.5%, respectively, and demonstrated decreased N use efficiency (Lea-Cox et al., 2001). The fate of added 15N during the spring in different tree components is shown in Fig. 5 . The largest amount of 15N was found in fruit (5.8 and 10.5 g tree-1), followed by roots and leaves. The amounts found in woody tissues (i.e., trunk, twigs > 1.5-cm diam., and woody roots) were very low (<0.6 g tree-1). The distribution of labeled N related to the total N content of trees treated with AN was calculated with data presented in Fig. 5. Percentage distribution (g 15N 100 g-1 N) was 16.7 for fruit, <8.9 for leaves, <8.6 for twigs, 4.9 for trunk, and 5.9 for roots. Trees that received UR showed percentage distribution (g 15N 100 g-1 N) as follows: 9.3 for fruit, <5.5 for leaves, <4.8 for twigs, 2.6 for trunk, and 4.4 for roots.
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37% of N stored in the framework components (leaves, twigs, trunk, taproot, and roots) (Fig. 5) and an important source of reduced N for growth and fruit yield. |
CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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30%). Fibrous roots were concentrated in the 0- to 15-cm depth, and represented >70% of the total BG class within the 0- to 45-cm layer. Recoveries of 15N by citrus trees fertilized during the spring with AN and UR were 39.5 and 25.5%, respectively. This difference in total 15N recovery was attributed to losses of N by NH3 volatilization, since fertilizers were applied to the soil surface with an alkaline reaction (pH > 7). Fruit appeared to be a large sink for applied 15N (recovery of 10–18%) and redistributed N in the citrus tree. The average 15N content of the tree biomass was small (8% of total N). The maximum observed quantities were associated with fruit. About 10.5 and 5.8 g 15N tree-1 were observed for AN and UR treated trees, respectively. Since fruit represented a large sink of applied 15N as UR and AN, respectively, we confirmed the importance of the spring application of N during early development of fruit. |
ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication November 20, 2001.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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