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a Center for Marine, Environmental Sciences and Biotechnology Research, Drew Griffith Hall, Savannah State Univ., Savannah, GA 31404
b USDA-ARS-PWA, 24106 North Bunn Road, Prosser, WA 99350
c Sentek Pty Ltd, 77 Magill Road, Stepney, South Australia, 5069 Australia
d CMESBR, Savannah State University, Savannah, GA 31404
Corresponding author (siva{at}savstate.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: CRF, controlled-release fertilizer • DGF, dry granular fertilizer • FRT, fertigation • FLDEP, Florida Department of Environmental Protection • IBDU, isobutylidene diurea • NBMP, N best management practices • PRCU, polyolefin resin-coated Urea • SL, suction lysimeter
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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60% of annual rainfall received during June to September) (National Oceanic and Atmospheric Administration, 1996), and occasional shallow water tables, these soils are vulnerable to rapid leaching of soil-applied chemicals and nutrients. Historically, fruit bearing citrus trees have been fertilized predominantly using DGF, often broadcast over the entire grove. Irrigation was predominantly done with a sprinkler system that applied water over the top of the tree canopy. Recent technological advances in fertilizer formulations and irrigation designs have prompted renewed interest in improving the efficiency of fertilizer and water delivery in order to enhance N use efficiency. Freeze protection capability and increasing demand for water conservation has encouraged the citrus industry to choose microirrigation systems. This change in irrigation technique has also facilitated the application of liquid fertilizers through the irrigation system, i.e. FRT.
Current N fertilization recommendations (Tucker et al., 1995) are primarily based on tree response studies conducted in the 1950s and 1960s using trees with low fruit production potential (30–40 Mg ha-1), on low-density plantings (usually <247 trees ha-1, at 7.6 by 6.0 m spacing) with an overhead irrigation system and broadcast application of DGF. There has been almost no attempt to examine the fate of N applied either as DGF or FRT to bearing citrus trees. Controlled-release N fertilizers have also been developed that release N slowly over an extended period of time. The release of N can be better synchronized with crop uptake to minimize NO3-N leaching losses. Results of several laboratory leaching studies conducted in citrus-growing soils of Florida with various controlled-release N fertilizers revealed low leaching (11–32%) losses of applied N compared with readily soluble NH4NO3 (Alva, 1992; Wang and Alva, 1996; Paramasivam and Alva, 1997).
The random drinking water quality survey in Highlands County in central Florida conducted by the Florida Department of Environmental Protection (FLDEP) revealed that 32% of total homeowner wells sampled (mostly into the surficial aquifer with poor construction) contained NO3-N in excess of the USEPA maximum contaminant level of 10 mg NO3-N L-1 (FLDEP, 1990, personal communication). This area is predominantly under citrus production; therefore, there is a need to investigate the fate of N applied to bearing citrus trees under current production practices, specifically with careful irrigation management. This study was conducted in a commercial grove of highly productive (with mean yield of 70–80 Mg ha-1) citrus trees that received different N sources and rates with optimal irrigation scheduling.
This study was a part of a comprehensive project aimed to develop N best management practices (NBMP) for bearing citrus trees on vulnerable soils to attain optimum fruit yield and quality with minimal leaching of NO3-N below the root zone. The tree response evaluation, in terms of leaf mineral concentrations, fruit yield, fruit quality, and concentration of NO3 in soil solution collected within the rooting depth were reported earlier (Alva and Paramasivam, 1998). The major objectives of this study were to evaluate; (i) the effectiveness of irrigation and fertilizer NBMP to minimize leaching of NO3-N below the root zone, (ii) the fate of applied N in soil and soil solution samples below the rooting depth, and (iii) the potential NO3 leaching losses below the root zone.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Suction Lysimeter, Tensiometer, and Rain Gauge Installation
Suction lysimeters, constructed using 2.5-cm-diam. (1-bar high-flow) porous ceramic cylinders (Soil Moisture Equipment Co., Santa Barbara, CA) connected to a polyvinyl chloride solution retainer cup at the bottom, were installed with ceramic cups placed at 60-, 120-, and 240-cm depths under the canopy. The SLs were installed 120 cm away from tree trunks. Suction lysimeters at these depths were installed in four of five replicates of all the treatment plots for a total of 180 SLs. Five clusters of tensiometers (Soil Measurement Systems, Tucson, AZ) were installed, one each at 15-, 30-, 90-, and 150- cm depths under the canopy along the dripline, to monitor soil water matric potential used as a basis for scheduling irrigation. After 5-cm-diam. holes were cored to desired depths, tensiometers were inserted into the holes. A sufficient quantity of soil from the bottom of the hole was poured back and tamped down slightly to ensure good soil contact with the porous ceramic cup. The remaining area around the tensiometer was back-filled with bentonite to completely seal the hole and guard against channeling water down the side of the tensiometers. A similar procedure was followed to install SLs. The tensiometers were read every 2 d, and irrigation was scheduled when the matric potential at the 15-cm depth attained -10 kPa for Jan to June and -15 kPa for July to December, (Smajstrala et al., 1987; Parsons, 1989). The 90- and 150-cm depth tensiometers were used to locate the depth of the wetting front for each irrigation and/or rain. Irrigation duration was calculated to replenish the water content of the uppermost 90-cm soil (a typical rooting depth in these sandy Entisols) back to field capacity. Two rain gauges were installed to record rainfall.
Sampling and Analysis of Soil and Soil Solution
To each SL, vacuum was applied at a constant rate (400 mm Hg) for 3 minutes with a vacuum pump and the hoses were sealed air-tight to facilitate collection of soil solution from the vadose zone. Soil solution was sampled every 2 wk using a vacuum pump. Between sampling each SL, the sampling line was rinsed in 1 M HCl and then in deionized water. The volume of soil solution collected from SLs varied depending on the wetness of soil. If the application of vacuum to soil solution samplers was incidently followed by an irrigation or rainfall event, that resulted in higher volume of soil solution in the samplers. On average, 50 mL of soil solution was collected from samplers installed at 60- and 120-cm depths, and as much as 200 mL at the 240-cm depth. About 20 mL of soil solution was stored in polyethylene vials, placed in an ice chest with dry ice, and transported to the laboratory for analysis. At each sampling, the SL was emptied, and vacuum was applied for collection of subsequent leachate. The remaining water in the SL was pumped out prior to applying vacuum for the next sample collection. There were some occasions when the application of vacuum to soil solution samplers coincided with dry soil (-10 KPa), and we collected less than 2 to 3 mL of soil solution from SLs at 60- and 120-cm depths. The concentration of NO3-N in the soil solution was measured within 24 h of sample collection using an ion chromatograph (Dionex 100, Dionex Corporation, Sunnyvale, CA) following the procedure outlined by the USEPA (1991).
Soil samples were collected under the canopy using a 2.5-cm-diam. bucket auger 90 cm inside the tree drip line. Sampling was done during early spring prior to application of fertilizer, in summer, and in late fall after the last application of fertilizer during each year. Soil samples were taken at 15-cm increments for the first 30 cm, and then at 30-cm increments to a depth of 150 cm. Samples were air-dried, then ground to pass through a 2-mm sieve, and extracted with 2 M KCl. Rapid flow analyzer methods A303-S020 (Alpkem Corporation, 1989) and A303-S170 (Alpkem Corporation, 1986) were used to measure the concentrations of NH4-N and NO3-N, respectively.
Estimation of Nitrate Leaching Losses Below the Root Zone
The amount of NO3 leached below the root zone was estimated using, in part, the concentration of NO3-N in the soil solution collected at various sampling events at the 240-cm depth in the vadose zone under the citrus canopy. The mass of N leached below the root zone, (AN), was calculated as the product of the mean NO3-N concentration in the leachate sampled at 240 cm (CSL240) multiplied by the volume of drainage water (Q) that passed through this depth between the successive sampling periods.
 | (1) |
Summation of these quantities over a period of 1 yr provided an estimate of total mass of N leached below the root zone.
Assuming steady state water flow, we calculated drainage (Q) as the product of Darcy's flux (q) and the time period (
t) for which drainage was being calculated.
 | (2) |
Using soil water potentials measured at 90 and 150 cm and saturated hydraulic conductivities, water flux below the root zone was calculated using the following Darcy's flux equation:
 | (3) |
H is the total head gradient between 90 and 150 cm; z is the depth increment across which the head gradient was measured. The total hydraulic head (H; cm) at any depth in the profile is defined as:
 | (4) |
The unsaturated hydraulic conductivity for different soil pressure heads was estimated using measured saturated hydraulic conductivity, the soil pressure head, and fitting parameters (
, n, and m) from van Genuchten (1980) (Eq. [5]). The ks (cm d-1) is the saturated hydraulic conductivity.
 | (5) |
The parameter has unit of cm-1, while n and m are unitless. Additional details of drainage calculations were given by Paramasivam et al. (2000).
Statistical Analysis
Since the mean tensiometer readings were used in the estimation of geometric mean of unsaturated hydraulic conductivity, and water flux in between two sampling periods, additional statistical procedures were not employed on estimated cumulative drainage water below the root zone. The estimated drainage was assumed to be same for all the treatments. Total quantities of 2 M KCl extractable NH4-N and NO3-N in the top 60 cm of the soil profile data and the estimated N leaching losses were analyzed for statistical significance using the Analysis of variance (ANOVA) procedure (SAS Institute, 1988), and mean separation was done using Duncan's multiple range test (DMRT) at a significance level of P < 0.05. Impact of N rates on estimated N leaching losses were evaluated by regression analysis for N sources separately using SAS (SAS Institute, 1988).
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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60 mm) during April through June of 1994 (Fig. 2a) was due to a combination of prolonged irrigation and high incidence of rainfall. However, estimated peak drainage (80–120 mm) during 1995 (Fig. 3a) was purely due to high rainfall received during the months of August through October of 1995 (Fig. 1). Cumulative drainage below the rooting depth was 416 mm (Fig. 2b) in 1994 (24% of the total water input) and 615 mm (Fig. 3b) in 1995 (30% of the total water input).
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A citrus tree's root system is a relatively shallow, well-branched framework of woody laterals and fine fibrous roots (Schneider, 1968; Castle, 1980a). The fibrous root density decreases substantially below the 60-cm depth, with very few roots at 120 cm (Castle, 1980b). Nitrate in solution at the 60- and 120-cm depths, thus, represents N available for root uptake. However, the 240-cm depth represents the vadose zone below the major rooting depth of citrus. Nitrate at this depth is not available to the trees, nor can it be readily transformed (denitrified or assimilated) because of the limited microbial population and available C at that depth (Paramasivam et al., 1999), and thus, can be subject to leaching into groundwater.
Concentrations of NO3-N in soil solution sampled at 240-cm depth during the 1994 and 1995 cropping seasons are shown in Fig. 2 and 3. Few occasional rapid increases in concentration (peaks) of NO3-N in the soil solution were evident for DGF source in early 1994 (Fig. 2a) but not in 1995. Similar peaks in concentration of NO3-N in soil solution were evident for FRT source in both 1994 and 1995 (Fig. 2c and 3c). For CRF, NO3-N concentration in soil solution did not exceed 10 mg L-1 in 1994 (Fig. 2e) or in 1995 (Fig. 3e). In a very few instances, increases in concentrations of NO3-N were observed with DGF and FRT N sources in both years and were in the range of 17 to 33 mg L-1 at the 240-cm depth. Overall, this resulted in collection of twofold higher NO3-N concentrations in soil solutions from SLs installed in fertigated plots compared with DGF plots at the 240-cm depth (Fig. 2c and 3c).
Nitrate-N concentrations were the lowest at all depths and across all rates for CRF compared with DGF and FRT, probably due to the controlled release property of this source. The CRF material used in this study was polyolefin resin-coated urea. A 29-d leaching study by Wang and Alva (1996) with readily soluble NH4NO3 and slow-release fertilizers [isobutylidene diurea (IBDU) and a polyolefin resin-coated Urea (PRCU)] in Wabasso (sandy, siliceous, hyperthermic Alfic Haplaquod) and Candler (hyperthermic, uncoated, Typic Quartzipsamments) soils revealed that leaching of N represented 27 to 32% for IBDU, 11 to 12% for PRCU, and 88 to 100% for NH4NO3. Results of another 120-d column leaching study in Candler soil with urea-based CRF [such as Poly-S (Scotts Sierra Company, Marysville, OH), Osmocote (Scotts Sierra Company, Marysville, OH), and Meister (Helena Chemical Company, Tampa, FL)] revealed leaching losses of 30 to 59% of total N applied (Paramasivam and Alva, 1997). However, under careful irrigation and N management conditions employed in the current study, NO3-N concentrations were normally below 10 mg L-1 at a 240-cm depth during the 2-yr monitoring period.
Cumulative Nitrate-Nitrogen Leaching Losses
Estimated cumulative NO3-N leaching losses for various N sources and rates are presented in Table 2 for both cropping years. Since N leaching losses can be affected by both the availability of N and the amount of drainage water for leaching, the relationship between N rates and leaching losses were further explored by regression analysis and presented separately for various N sources by year (Table 2). In all cases, the relationship between N rates and N leaching losses was significant at the 0.05 probability level. The estimated cumulative amounts of NO3-N leached below the rooting depth from the DGF source accounted for 6.1 to 8.0% of applied N at 224 and 280 kg ha-1 yr-1, respectively. The corresponding values for the FRT source were 10.4 to 14.0%. For the CRF source, cumulative leaching losses of NO3-N accounted for 1.0 to 4.7% of the applied N at 112 and 168 kg ha-1 yr-1. Across all N rates, estimates of N leached below the rooting depth of mature citrus trees as percentage of N applied varied from 5 to 12% for DGF, 10 to 16% for FRT, and 1 to 5% for the CRF source.
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A study in a large southern California watershed planted with citrus reported 67 kg N ha-1 leaching losses (Bingham et al., 1971) that accounted for 45% of the annual applied N. Avnimelech and Raveh (1976) reported average leaching losses of 50 and 130 kg NO3-N ha-1 yr-1 (i.e., 21 and 47% of applied N) with mature ‘Shamouti’ orange grown in a clay loam and sandy loam soil, respectively, in Israel. Another N FRT study (Dasberg et al., 1984) over a 4-yr period, with mature Shamouti orange in the coastal plain of Israel, showed 8 to 48% N leaching loss with varying irrigation (580–880 mm) and N rates (87–393 kg N ha-1 yr -1). In the above studies, N leaching was estimated using the mean NO3-N concentration in the soil solution in the subsoil and annual volume of water that passed through the soil profile. The estimate of N leached in our study was lower than that reported in the above studies. The lower quantities of NO3-N leaching estimates in this study, compared with those reported in the other studies, may have been a consequence of improved management of N and irrigation.
Soil solution samples obtained using ceramic-cup SLs only provide information on soil solution chemical characteristics at the time of sampling, and not the flux passed the sampling zone. Therefore, the estimates of N leached using NO3-N concentrations in the SL samples only approximate the amount of N leached. Frequent sampling of soil solution is necessary to adequately characterize the changes in leachate concentrations with time.
Nitrogen in the Soil Profile
Nitrogen source or rate did not significantly affect 2 M KCl extractable NO3-N in 1994 (data not shown). The mean residual amount of NO3-N varied from 8 to 39 kg ha-1 in 1994. The mean residual amount of NH4-N in the top 60 cm of the soil profile varied from 35 to 93, and 48 to 119 kg ha-1 for 1994 and 1995, respectively. The concentration of NH4-N was much greater than that of NO3-N in the soil profile whenever samples were taken.
Since N rates were similar for the DGF and FRT sources only, mean comparisons of mean residual NO3-N was evaluated between these two sources (Table 3). Significant differences were evident in March and October only (Table 3). During all three sampling periods of 1995, residual NO3-N in the top 60 cm of the soil profile of the DGF treatment plot was numerically greater than in FRT plots (Table 3). The lower values for FRT treatment were expected because the annual amount of FRT N was applied in 15 split applications, while DGF was split into four applications.
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The nutrition program for bearing citrus trees should be aimed primarily at supporting the current crop of fruit and new growth. New vegetative growth is important for fruit yield during the subsequent year. The nutrient requirement is maximum from early spring through early summer (Tucker et al., 1995). Thus, citrus trees will take up most N during this period, while N may accumulate in the profile during the rest of the year excluding the losses due to leaching, and other associated N transformation processes.
In a mature Shamouti Orange tree study in a Hamra sandy loam with sandy subsoil on the coastal plain of Israel, with applications of 100 to 310 kg N ha-1 yr-1 as liquid NH4NO3, the NO3-N soil profile increased from 14 to 69, 8 to 27, and 6 to 22 mg kg-1 at 0- to 30-, 30- to 120-, and 120- to 300- cm depths, respectively, with increasing rate of N application (Dasberg et al., 1983). These results represented only a one-time sampling during July of 1982. Even though we sampled three times a year (March, June, and August of 1994, and March, June, and October of 1995), NO3-N soil profile distribution was neither significantly influenced by N source nor by rate, with the exception in 1995.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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1 Mention of product name does not suggest an endorsement of the product. 
Received for publication January 22, 2000.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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