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Journal of Environmental Quality 31:759-768 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Ecological Risk Assessment

Soil Temperature, Nitrogen Concentration, and Residence Time Affect Nitrogen Uptake Efficiency in Citrus

J. M. S. Scholberg*,a, L. R. Parsonsb, T. A. Wheatonb, B. L. McNealc and K. T. Morganb

a Agronomy Dep., 304 Newell Hall, Univ. of Florida, Gainesville, FL 32611
b Univ. of Florida, Citrus Res. and Educ. Center, 700 Experiment Station Road, Lake Alfred, FL 33850
c Soil and Water Science Dep., Univ. of Florida, IFAS, Gainesville, FL 32611

* Corresponding author (jmscholberg{at}mail.ifas.ufl.edu)

Received for publication October 10, 2000.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
We try to elucidate which environmental and soil factors control nitrogen uptake efficiency in citrus. Effects of residence time and nitrogen (N) concentration (three 500-mL applications of 7 mg N L-1, representative of reclaimed water used for citrus irrigation in central Florida, or one 150-mL application of 70 mg N L-1) on nitrogen uptake efficiency (NUE) of young citrus seedlings were studied. Increasing residence times from 2 to 8 h increased NUE from 36 to 82% and from 17 to 34% for high and low application frequencies, respectively. We developed a model to predict N uptake based on root density, N concentration, and soil temperature (Ts). Assuming a base temperature (Tb) of 10°C, N uptake temperature sum (UTS) = {sum}(Ts - Tb)/24 (°CdN, degree day units of N uptake). To eliminate the risk of N leaching for young seedlings, minimum uptake periods of 5 and 16°CdN were required at initial soil N concentrations of 0.9 and 2.5 mg N L-1, respectively. After correcting for differences in root length, this information was then used to predict the effect of irrigation practices on N uptake from reclaimed water for mature trees. Applying 2500 mm yr-1 vs. 400 mm yr-1 reclaimed water reduced the NUE of N in this water from 100 to 63% during the summer and from 100 to 28% during the winter. Reductions in NUE at higher irrigation rates appeared to be related to N displacement below the root zone prior to complete N uptake.

Abbreviations: ET, evapotranspiration • N7, 7 mg N L-1 treatment • N70, 70 mg N L-1 treatment • NS, nitrogen supply • NU, nitrogen uptake • NUE, nitrogen uptake efficiency • Tb, base temperature for nitrogen uptake • tR, residence time • Ts, soil temperature • UTS, uptake temperature sum


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
TRADITIONALLY, SOIL FERTILITY RESEARCH has focused on yield response to fertilizer application rates. Its main objective has been to maximize production with respect to yield and/or net returns. Research strategies shifted with increased concerns about the contribution of agriculture to ground water contamination in the early 1970s (Parr, 1973), and currently include the development of environmentally sound production techniques (Alva and Paramasivam, 1998).

The Water Conserv II research project is part of the City of Orlando's and Orange County's initiative to use municipal waste (reclaimed) water to irrigate agricultural crops. Use of reclaimed water from urban communities for citrus irrigation may provide a partial solution to the apparent conflict between increased urbanization and sustainable agricultural production (Parsons and Wheaton, 1996; Parsons et al., 2001). The use of reclaimed water to irrigate grapefruit (Citrus x paradisi Macf.) trees resulted in increased fruit yield and reduced N fertilizer requirements (Maurer et al., 1995). At high irrigation rates, appreciable amounts of N can be supplied to the crop. However, crop utilization of this dilute N source appeared to be low, and additional fertilization was required to sustain high growth and fruit yield (Maurer and Davies, 1993; Wheaton et al., 1997). High application rates of reclaimed water increased canopy growth and fruit yield but, in some cases, reduced leaf N concentrations due to increased N leaching (Wheaton et al., 1997). More detailed studies are required to elucidate processes that hamper efficient N utilization by citrus irrigated with reclaimed water.

Although plant growth response to N supply has been studied extensively, many aspects of the physiological basis for improving nitrogen uptake efficiency (NUE) in crops remain poorly understood (Rufty et al., 1990). Root growth has been shown to be affected by changes in rhizosphere environment (Paollilo et al., 1999). However, root adaptation may be too slow to make efficient use of greatly fluctuating N concentrations associated with high application rates of dry fertilizer. Seasonal variations in N uptake also occur, with uptake appearing to be highest during periods of active shoot growth (Maust and Williamson, 1994; Weinbaum et al., 1978). Roots in natural ecosystems are typically exposed to relatively low (<1 mg N L-1) N concentrations, and roots have been shown to be capable of depleting nutrients well below this level (Bloom, 1996; Cerezo et al., 1997). Most plants typically acclimate to a low yet constant supply of nutrients via continued growth, thereby maintaining relatively constant internal N concentrations (Ingestad, 1982). Extremely high N rates, on the other hand, may cause perturbances of the internal growth equilibrium. This may result in a down-regulation of N uptake (Bloom, 1996), reduced growth rates (Ingestad, 1982), and smaller root to shoot ratio (Maust and Williamson, 1994). After an initial growth flush, deficiency symptoms may occur once soil N concentrations fall below levels required to sustain previously established growth rates (Ingestad, 1982). As a result, plants will not attain a growth equilibrium nor can they make efficient use of root and canopy structures. Based on this premise, supplying N in phase with crop demand should enhance tree growth and yield.

Nitrogen recommendations for mature citrus range from 132 to 224 kg N ha-1 yr-1 (Tucker et al., 1995), but commercial growers may use rates up to 300 kg N ha-1 yr-1 (Scholberg et al., 2000). Nitrogen rates for the production of commercially grown citrus nursery seedlings are on the order of 560 to 1000 kg N ha-1 yr-1 (Maust and Williamson, 1994). Excessive N fertilizer rates typically increase N leaching and may also result in ground water contamination (Alva and Paramasivam, 1998; McNeal et al., 1994). Concentrations of 200 to 400 mg N L-1 are commonly used for fertigation of containerized citrus grown in a potting medium. However, optimal growth occurred at N concentrations of 15 to 20 mg N L-1 in an inert soil medium (Maust and Williamson, 1994) and 10 mg N L-1 in solution culture (Chapman and Liebig, 1937). Leaf N concentrations were highest at soil solution concentrations of 200 to 400 mg N L-1 (Maust and Williamson, 1994; Wright et al., 1990).

Nitrogen uptake efficiency (NUE) can be defined as the percentage of applied N taken up by plants (Maust and Williamson, 1994). It is intrinsically linked to the plant's ability to intercept N before it is leached below the rootzone. For the production of containerized nursery stock, roots are confined in a small soil volume. This will result in an intense leaching of the growth medium, high potential N losses, and reduced NUE values. In Florida, the majority of the roots of mature citrus trees occur within the upper 0.3 to 0.4 m of the soil profile on shallow "flatwood" sites, compared with the upper 0.9 to 1.2 m for trees planted on deep-draining "ridge" soils (Menocal-Barberena, 1999). Under Florida conditions, most N is rapidly converted to NO3–N, which can be readily leached due to the inherent low water holding capacities and high pore velocities of sandy soils (Bouwer, 1985). Studies in Israel showed that, by increasing N rates from 140 to 416 kg N ha-1 yr-1, NUE was reduced from 57 to 40% (Dasberg, 1987). Reported values of NUE for lysimeter-grown citrus trees in Florida were on the order of 61 to 68% (Syvertsen and Smith, 1996). Plants grown under N-limiting conditions may have a greater affinity and capacity for N uptake, resulting in higher NUE (Dasberg, 1987; Jiang and Hull, 1998; Rufty et al., 1990).

The objectives of our study were to: (i) determine the effect of N concentration and application frequency on N uptake, (ii) evaluate the effect of residence time on NUE, (iii) elucidate some of the processes and mechanisms that hamper more efficient N use from reclaimed water, and (iv) develop a conceptual model to assess NUE of reclaimed water under field conditions. Based on apparent limited N uptake from reclaimed water, we hypothesized that: (i) citrus trees cannot make efficient use of low-concentration N sources, such as municipal waste water; or that (ii) N did not reside in the rootzone long enough for the trees to make efficient use of the supplied N. In order to test these hypotheses, we either applied the same amount of N more frequently as dilute solutions or less frequently in more concentrated form. We allowed the N to reside in the rootzone for different time periods to mimic the effects of excessively high irrigation rates and/or of leaching rainfall on NUE.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Research was conducted at the Citrus Research and Education Center, UF/IFAS, Lake Alfred, FL. Experiments were conducted in a nonshaded glasshouse from August 1998 to May 1999. Average minimum and maximum glasshouse air temperatures during the experimental period were 23.5 and 34.8°C, respectively. Average monthly relative humidity ranged from 33 to 98%. Monthly averages for maximum photosynthetically active radiation (PAR) values ranged from 677 to 1405 µmol-2 s-1 between December and July. Soil temperatures were measured at 0.5-h intervals by thermocouples placed at soil depths of 10 and 20 cm. Evapotranspiration (ET) rates of representative reference plants were determined by weight losses both during uptake periods and at daily intervals. Average soil temperatures and relative humidity values during the uptake periods, along with average ET values for the reference (ETreference) plants, are shown in Fig. 1 .



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  		Fig. 1. (A) Average soil temperature at the 20-cm soil depth and relative humidity (RH) during uptake periods. (B) Average weekly evapotranspiration (ET) rates of reference columns planted with Swingle (SWL) and Volkamer (VLK) seedlings (error bars represent ± one SE from the mean, n = 2).

 
Ten-week-old seedlings of citrumelo [Citrus paradisi Macf. x Poncirus trifoliata (L.) Raf. var. Swingle] and lemon (C. volkameriana Ten & Pasq. var. Volkamer) were obtained from a commercial nursery. A total of 200 seedlings were transplanted in a large 150-L PVC container filled with Miracle-gro (Scotts, Marysville, OH) potting mix (0.14–0.14–0.14 N–P–K) and grown for 60 d. To monitor N uptake, columns were built using a 40-cm length of PVC pipe with a 10.8-cm i.d. The bottom of each column was fitted with a PVC end cap in which a center hole was drilled that had been threaded to fit a 1.27-cm-o.d. adapter connecting to a drainage tube. A 5-cm2, triple-folded piece of nylon screen was positioned above the center hole to retain sand in the columns.

On 26 Aug. 1998, seedlings were carefully uprooted and root systems were washed to remove residual potting soil. A total of 30 uniform (fresh wt. = 16.0 ± 0.5 g) seedlings were selected for each rootstock, and 24 of these seedlings were planted in the PVC columns that had been filled with 3 L (approximately 4.2 kg) air-dried soil. Initial leaf number ranged from 25 to 28 and stem diameter was 6 mm. A more detailed overview of seedling characteristics is presented in Table 1. The soil was a Candler fine sand (hyperthermic, uncoated Typic Quartzipsamment). The soil was excavated from the top 15 cm of a site adjacent to a citrus grove and passed through a 2-mm sieve before being steam-pasteurized at 120°C for 8 h. Subsequently the soil was air-dried to facilitate uniform filling of the columns. Soil pH was 7.4 and initial NH4–N and NO3–N concentrations were 2.7 and 2.5 mg N kg-1, respectively. Soil organic matter and total (Kjeldahl-extractable) soil nitrogen contents were 6.5 and 3.1 g kg-1, respectively. An additional 24 nonplanted columns were included and used as controls (reference columns). Columns were placed on a reinforced wooden table (1.6 x 2.6 x 0.9 m) using a plant spacing of 0.18 by 0.18 m. Columns were randomized according to a complete block design with four replicates. Nitrogen uptake measurements were delayed for 90 d to ensure adequate root proliferation throughout the entire soil volume. Uptake measurements were initiated in December and continued until May. All columns received a total of 10.5 mg N wk-1 with a complete N-depleted nutrient solution being applied to the columns once each month (Maust and Williamson, 1994).


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  		Table 1. Plant height, leaf area, fine root length, root length density (RLD), and dry weight of roots, stems, and leaves at transplanting and after harvesting.

 
The main treatment factors were: (i) seedling variety (Swingle vs. Volkamer); (ii) N concentration and application frequency (three 500-mL applications of 7 mg N L-1 [N7, representing reclaimed water] or one 150-mL application of 70 mg N L-1 [N70], applied weekly); and (iii) residence time (tR = 2, 4, or 8 h). Nitrogen from stock solutions of purified grade KNO3 (Fisher Scientific, Pittsburgh, PA) was applied with a precision dispenser (100-mL manual bottle-top dispenser; Wheaton, Millville, NJ). Nitrogen was applied at 0800, 1000, or 1100 hours and extracted at 1600, 1400, or 1300 hours for the 8-, 4-, and 2-h residence time (tR) treatments, respectively. At the end of the uptake period, residual soil N was extracted with excess water (4600 mL) and leachate was collected in 5.5-L containers. Preliminary studies had shown that this extraction volume resulted in N recovery between 98 and 102%. Columns were irrigated with "tube weight" drip emitters (EW50-36''; Chapin, Watertown, NY). Irrigation rates were 11.1 L h-1, resulting in soil ponding within 3 to 5 min. An additional two seedlings of each rootstock were planted in extra columns. These columns were weighed during uptake periods and at daily intervals, and ET rates were based on weight losses of these columns over time. These columns were irrigated manually each day and their water use was measured during N uptake periods and also for 24-h periods throughout the entire experimental period. Remaining seedlings were used for initial growth analysis (Table 1).

The following methodology was used for high extraction efficiency of residual soil N. Soil inundation with 0.01 m of water could be attained by turning on the irrigation for 3 to 5 min without applying a vacuum to the bottom of the columns. This brought any N that might have accumulated at the soil surface back into solution. Subsequently, a partial (0.005 MPa) vacuum was applied to the bottom of the columns. This, combined with high irrigation rates, resulted in rapid and intensive leaching that continued for 22 min. This leaching cycle then was followed by a drying cycle (vacuum only) that was maintained for 15 min. During this cycle, residual volumetric soil water content was reduced to 12 to 15% Total extraction time was 40 min, with most of the residual soil N being removed within the first 10 min. To account for this short delay in near-complete N removal, N extraction was initiated 5 min prior to the end of the uptake period. A detailed description of the N uptake monitoring system is presented elsewhere (Scholberg et al., 2001).

After completion of the drying cycle, containers were disconnected from the drainage and vacuum ports, and shaken for 2 min before determining leachate volume gravimetrically with a waterproof bench scale (I20W; Ohaus, Florham Park, NJ). Representative subsamples were filtered (#5; Whatman, Maidstone, UK) and stored in 20-mL scintillation vials at -20°C until analysis. Samples were analyzed using an air-segmented automated spectrophotometer (Flow Solution IV; OI Analytical, College Station, TX) coupled with a Cd reduction approach (modified USEPA Method 353.2). Nitrogen uptake for a specific treatment (Utreatment) was calculated as: Utreatment = Vreference x [Nreference] - Vtreatment x [Ntreatment] where V is the leachate volume and [Ntreatment] and [Nreference] are the N concentrations of leachate from the treatment and reference columns, respectively. Nitrogen recovery percentages for reference columns were not affected by residence time treatments.

Leaf area of plants prior to the first N application was determined nondestructively by measuring leaf length. Leaf area could be predicted using previously established regression equations (r2 = 0.97). Plants were harvested on 31 May 1999 by cutting stems at soil level. After removal of all leaves, stems were cut into 3-cm segments. Root systems were separated into taproots, lateral roots (>1 mm), and fine roots (<1 mm). Leaves, stems, and root material were dried at 70°C for 72 h for dry weight determinations. Root length of fine roots was determined using a line intercept method (Tennant, 1975). Leaf area of each plant was measured at harvest using a LICOR (Lincoln, NE) 3000 leaf area meter. Leaf, stem, and root material were dried at 70°C for 72 h.

Experimental data were fitted using linear and quadratic regression models using SAS statistical software (SAS Institute, 1997). Coefficients of two regression equations for variables i and j were compared using the following test statistic: 0.5 with C being the numeric value of the regression coefficient and SE the standard error associated with that coefficient. Nitrogen uptake efficiency (NUE) was defined as N uptake divided by the amount of N supplied from either fertilizer or reclaimed water. Linear regression equations expressed nitrogen uptake (NU) as follows: NU = a + b x NS, with NS being nitrogen supply, and a and b being respective regression coefficients for the intercept and slope of the regression line. Initial evaluation of the results of the regression analysis showed that, in most cases, the values of a became more negative as residence time (tR) increased (Table 2). Negative values for a would suggest nonlinear N uptake with time. For this reason a quadratic regression equation to describe NUE with time was also included. Using this approach: NU = a + b x NS + c x NS2. In this case the value of b could be interpreted as the initial NUE value, and c as a time-dependent component of overall NUE. Alternatively, mean treatment values were plotted with corresponding standard error values (n = 4). Measured N uptake values were integrated over the entire growth season and end-of-the-season NUE values were analyzed using SAS statistical software (SAS Institute, 1997). In all figures, results of linear regression analysis are shown separately for each variety if the slopes of the regression lines were significantly different (<0.05); alternatively, data were combined.


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  		Table 2. Overview of regression equations, r2 values, and fertilizer N uptake efficiency for the 7 and 70 mg N L-1 treatments for Swingle (SWL) and Volkamer (VLK) seedlings as affected by residence time (tR). Regression equations express nitrogen uptake efficiency (NUE) as a function of nitrogen supply (NS, the amount of N fertilizer that was applied) as follows: NUE = a + b x NS + c x NS2. Comparison of overall NUE values expressed as a percentage of the total N supply are presented in the last column. n = 100.

 
A new concept referred to as the uptake temperature sum (UTS) was introduced to facilitate comparison of N uptake and potential N leaching across different uptake environments. The UTS values were calculated as follows: UTS = {sum}(Ts - Tb)/24 h (°CdN, degree day units of N uptake). Previous work had shown that N uptake for citrus ceased around 10°C (Scholberg, unpublished data, 1999). The value of the base temperature for N uptake of citrus (Tb) was therefore set at 10°C and UTS values were calculated by subtracting 10°C from hourly soil temperature values. Values were summed up for the entire uptake period (2–8 h) and resulting values express the effective uptake temperature for a corresponding uptake period. Dividing integrated values by 24 h resulted in UTS values expressed in°CdN (degree day units of N uptake). This approach is similar to the growing degree days approach used for growth analysis. One °CdN unit corresponds to 2.4 h uptake at 20°C or 1.6 h uptake at 25°C. A potential nitrogen leaching (PNL) index was introduced to quantify treatment effects on N leaching and potential ground water contamination, with values defined as follows: PNL = 100 - NUE for NUE <= 100% and set at 0 if NUE > 100%.

Results from this analysis were then combined with Michaelis–Menten uptake parameters for soil-applied N (Scholberg, unpublished data, 2001) to assess the utilization of N from reclaimed water for irrigation treatments as outlined in Table 4 at the Water Conserv II site (Wheaton et al., 1997). Root length distribution values for the field site were obtained from previous root study (Menocal-Barberena, 1999).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Nitrogen Uptake of Seedlings under Greenhouse Conditions
Leaf formation for Swingle seedlings ceased during the winter months while Volkamer seedlings continued leaf growth throughout the entire experimental period. This resulted in higher final leaf area values for Volkamer compared with Swingle. Greater root weight with Volkamer appeared to be related to a larger size taproot (data not shown). However, actual root length was slightly greater with Swingle (Table 1). Root length density (RLD), which is the root length per unit soil volume, increased from 0.24 km m-3 at planting to 3.6 to 4.1 km m-3 at harvest. Calculated RLD values in the upper 30 cm of the soil profile for citrus at the Water Conserv II site were 1.0 to 2.0 km m-3 (Menocal-Barberena, 1999).

Average soil temperatures during uptake periods ranged from 18°C in December to 27°C in May. Relative humidity values during uptake periods were typically between 35 and 70% and values were lowest during the winter months (Fig. 1A). Except for a few temporary decreases in ET during exceptionally cold or cloudy periods, overall daily water use of reference trees (ETreference) increased with time (Fig. 1B). The increase in ETreference was probably related to an increase in both plant size and air temperature during the course of the experiment. The increase in ET rates during the winter months was more pronounced for Volkamer seedlings compared with Swingle, due to continued leaf-area expansion by Volkamer during this period.

Average recovery values for the reference columns were 100.0 ± 1.2% and 106.8 ± 1.1% for the N7 and N70 treatments, respectively. Values greater than 100% were probably related to soil mineralization (Scholberg et al., 2001). Recovery values for planted columns ranged between 0 and 94%. These values decreased toward the end of the experiment as plants became larger and temperatures were higher. Overall N uptake during uptake periods increased with retention time and was greater with the N70 (10.5 mg N applied once each week) treatment compared with N7 (3.5 mg applied three times per week) treatment (Fig. 2 and 3) . However, this slight increase in N uptake with the N70 treatment was not proportional to the threefold increase in N dosage, and it appeared that an uptake period of 8 h was not adequate to ensure complete utilization of the 10.5 mg N applied to the seedlings. The smaller application volume with the N70 treatment could also have reduced the infiltration depth of the N solution and thereby reduced the root volume exposed to N. However, other research also showed a relatively small (35%) increase with a threefold increase in N rate (Lea-Cox and Syvertsen, 1996). Overall N uptake appeared to show a gradual increase with time for both the 4- and 8-h treatments. This trend was more pronounced with the N7 treatments compared with the N70 treatments. Weekly uptake rates for the 2-h treatments, on the other hand, remained remarkably constant throughout most of the growing season (Fig. 2). These trends may have been related to a 16 to 40% greater plant size for the 8-h treatment compared with the 2-h treatment (data not shown). Seasonal trends in N uptake have been shown to occur for citrus, and uptake during summer months was two to three times greater compared with that during winter months (Chapman and Parker, 1942; Roy and Gardner, 1945).



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  		Fig. 2. Nitrogen uptake (mg N per plant) as affected by residence time (t = 2, 4, or 8 h) for the 7 mg N L-1 treatment for Swingle (A) and Volkamer (B) seedlings (error bars represent ± one SE from the mean, n = 4).

 


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  		Fig. 3. Nitrogen uptake (mg N per plant) as affected by residence time (t = 2, 4, or 8 h) for the 70 mg N L-1 treatment with Swingle (A) and Volkamer (B) seedlings (error bars represent ± one SE from the mean, n = 4).

 
In addition to long-term trends, weekly fluctuations in N uptake rates occurred that were typically consistent across treatments and could have been caused by weekly temperature variation. Although the N uptake rates for the 2- and 4-h tR treatments occasionally overlapped, uptake rates toward the end of the season were significantly higher for the 4-h tR treatment compared with the 2-h tR treatment. Initial plant growth for the 4-h tR treatments was somewhat poor compared with other treatments, which may have masked initial treatment effects. Differences between the 2- and 4-h tR treatments became more pronounced with time, especially with the N7 treatment.

Results of regression analysis for cumulative N uptake as a function of the cumulative amount of N applied, along with end-of-the season NUE values, are shown in Table 2. Using this approach, some of the "environmentally induced" weekly variations in N uptake could be filtered out, which facilitated an assessment of treatment effects on changes in N uptake during the growing season. It could be argued that for highly leached sandy soils very low in organic matter nutrient, uptake in the absence of external N supply will be approximately zero. Based on this conceptual approach the value of a should be approximately zero, and the above equation can be simplified to: NU = b x NS. Rewriting this equation will result in the following: b = NU/NS = NUE. In this case the slope of the linear equation would approximate the average NUE value over the entire experimental period.

However, closer inspection of the regression coefficients showed that in most cases the values of a became more negative (p < 0.05) as tR increased (Tables 2 and 3). Negative values for a may be indicative of nonlinear N uptake with time. This trend appeared to be most pronounced at higher tR values and differences in a between 2- and 8-h tR treatments were significant for all treatments (Table 3). The use of quadratic regression equations to describe NUE with time was therefore also included. Using this approach: NU = a + b x NS + c x NS2. In this case the value of b could be interpreted as the initial NUE value, and c as a time-dependent component of overall NUE. Although the use of a quadratic equation only slightly improved r2 values, values for c (the quadratic component of the regression equation) were always significant (Table 2). After including a quadratic regression component, values of a were similar across all treatments and no longer affected by tR treatments (Table 3). Greater values of c could have been related to increased plant growth and/or more favorable N uptake conditions toward the end of the growing season. In all cases, values of c for the 8-h tR treatment were significantly greater compared with the 2-h tR treatment (Table 3). This may be related to better plant growth for this treatment. This is consistent with time-dependent increments in NUE showing a close relation with corresponding leaf area index (LAI) increases (Fig. 4) . This increase in NUE with plant size could be related to increased water use and/or increased N uptake capacity (Jiang and Hull, 1998).


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  		Table 3. Statistical comparison of linear and quadratic regression coefficients for specific residence time (tR) treatments for the 7 and 70 mg N L-1 treatments for Swingle (SWL) and Volkamer (VLK) seedlings. Coefficients of two regression equations for variables i and j were compared using the following test statistics: (Ci - Cj)/(SEi2 + SEj2)0.5 with C being the numeric value of the regression coefficient and SE the standard error associated with that coefficient.

 


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  		Fig. 4. Incremental nitrogen uptake efficiency (NUE) (quadratic component of the regression equations listed in Table 2) as related to leaf area expansion between 2 Dec. and 31 May for Swingle (SWL) and Volkamer (VLK) seedlings with the 7 mg N L-1 (N7) and 70 mg N L-1 (N70) treatments.

 
Statistical comparison of NUE between treatments was based on the total cumulative uptake at the end of the growing season divided by the total N supply. Means were separated using Duncan's Multiple Range Test (p < 0.05). More frequent application of dilute N solutions (N7) doubled NUE compared with less frequent application of more concentrated N solutions (N70). This provides support for the claim that frequent fertigation can enhance NUE. Increasing the residence time from 2 to 8 h resulted in an increase in NUE of 125 and 95% for the N7 and N70 treatments, respectively. This underscores the important role of irrigation management practices in improving NUE. Use of sufficient irrigation will move the N inside the active root zone, and thereby allow roots to take up the N prior to a potential leaching rainfall event. It will also reduce volatilization losses of surface-applied dry fertilizer N. Excessive irrigation, on the other hand, will reduce NUE because of N displacement below the root zone prior to complete nitrogen uptake.

Maximum overall NUE values were approximately 83% compared with 71% for reported for another greenhouse study with young seedlings (Maust and Williamson, 1994). The NUE value for lysimeter-grown trees was on the order of 68% (Syvertsen and Smith, 1996). High maximum NUE observed during this study may be related to the relatively low N supply. Toward the end of the experiment plants showed N deficiency symptoms, and leaf N concentrations were only 1.3 to 1.7%, which is low for citrus leaves (Tucker et al., 1995). The NUE typically decreases with N supply (Parr, 1973). Research in Israel showed that NUE values were reduced from 57 to 40% when N rates increased from 140 to 416 kg N ha-1 (Dasberg, 1987). Under N-limiting conditions, root to shoot ratios and root uptake capacity are increased (Jiang and Hull, 1998; Maust and Williamson, 1994) and plants may be more effective in mining the soil for N, resulting in an increase in NUE. Other researchers reported that NUE was greatest during periods of active growth. However, N uptake expressed on a unit weight basis remained fairly constant.

Overall trends and NUE values were similar for both varieties in the present studies. However, N limiting conditions affected the growth of the varieties in different ways. Swingle seedlings showed more pronounced leaf senescence under N-limiting conditions, resulting in prolonged winter dormancy. Both N uptake and stem thickening continued during the dormancy period, until increased internal N concentrations combined with warmer temperatures triggered a new leaf flush. It is of interest that N uptake for Swingle continued during winter dormancy. Deciduous species such as prune (Prunus domesticus L.), on the other hand, had greatly reduced N uptake during dormancy (Weinbaum et al., 1978). Volkamer seedlings did not show pronounced leaf senescence and, combined with continued leaf growth, this resulted in more pronounced leaf yellowing. This is in agreement with the high N use efficiency reported for Volkamer (Lea-Cox and Syvertsen, 1996).

One of the objectives of this study was to elucidate processes that control and/or limit NUE. Ideally, a generic index should be developed to assess N uptake and NUE for a range of uptake conditions. Greater N uptake rates and NUE values could result from an increase in either (i) plant size, (ii) root length density, (iii) evapotranspiration rate, (iv) soil temperature, (v) uptake duration, and/or (vi) a combination of these factors. Nitrogen uptake is typically proportional to plant size and, although incremental changes in NUE were closely related to incremental growth, plant size as such could not completely account for either seasonal or weekly fluctuations in N uptake. Alternatively, it could be argued that the increase in leaf area was the result of increased N uptake rather than the (primary) cause for differential N uptake rates and NUE. Plant size, therefore, may not be the best predictor for either N uptake or NUE.

Past research showed a fairly close relationship between water use and N uptake across different seedling varieties (Lea-Cox and Syvertsen, 1993). Results from regression analysis showed that overall N uptake increased linearly with the ET of reference plants (Fig. 5) . However, overall regression slopes were significantly greater (p < 0.001) with Swingle compared with Volkamer, which was probably related to the 50 to 100% higher ET rates of Volkamer seedlings (Fig. 1). Assuming a residual soil moisture content of 375 mL, averaged soil solution N concentrations for the N7 and N70 treatments would be 4.0 and 20 mg N L-1, respectively. Leaf area with the N70 treatment was approximately 30% lower than that with the N7 treatment. If ET would have been the main controlling factor for N uptake, then uptake by Volkamer should have been twice that by Swingle. Alternatively, uptake for the N70 treatment should have been 20/4 x (100 - 30%) = 350% of that for the N7 treatment (with 20/4 and -30% representing the increase in soil N concentration and reduction in leaf area, respectively). Since this was clearly not the case, it could be concluded that ET may not be the main factor that controlled N uptake. Reports in the literature show that the ratio between N uptake and water uptake could vary almost by an order of magnitude (Chapman and Parker, 1942). Active uptake, which is heavily temperature dependent, may thus be a key uptake process.



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  		Fig. 5. Nitrogen uptake (mg N per plant) as related to evapotranspiration rates of reference plants for Swingle (SWL) and Volkamer (VLK) seedlings with the 7 mg N L-1 (A) and 70 mg N L-1 (B) treatments. Regression equations express nitrogen uptake for Swingle (NUS) and Volkamer (NUV) as a function of the evapotranspiration (ET) rates for reference plants.

 
Effects of residence times on overall NUE were similar for both varieties (Table 2). Weekly N fluctuations, on the other hand, appeared to follow soil temperatures fairly closely (Fig. 1–3). It was concluded that a combination of residence time and soil temperature may be a good predictor of N uptake. However, this required the introduction of a function to account for temperature effects on uptake. This was done by introducing a new concept referred to as uptake temperature sum (UTS). Using this approach it was possible to compare N uptake across different soil temperature regimes. Nitrogen uptake was closely related to UTS values and trends were consistent (not significantly different at p < 0.05) for both varieties (Fig. 6) yet significantly different (p < 0.001) between N concentration treatments. Use of the UTS approach allowed us to account for a larger portion of the overall variability in N uptake compared with any other approach. It is therefore concluded that the UTS concept appears to be useful and may provide a sound basis for the development of expert systems for N management. However, this will require that the UTS approach will be extended to account for differences in soil N concentrations and root length densities.



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  		Fig. 6. Nitrogen uptake (mg N per plant) as related to the uptake temperature sum (UTS) during uptake for Swingle (SWL) and Volkamer (VLK) seedlings with the 7 mg N L-1 (A) and 70 mg N L-1 (B) treatments. Regression equations express N uptake as a function of UTS values expressed in °CdN (UTS values assume a base temperature for citrus of 10°C, with one °CdN unit being accrued by 2.4 h uptake at 20°C or 1.6 h uptake at 25°C).

 
The UTS approach was also used to assess N leaching under greenhouse conditions by calculating a potential nitrogen leaching (PNL) index. With the application of 7 mg N L-1, UTS values of 5.8°CdN were required to completely eliminate the risk of N leaching (Fig. 7) . This would translate to an uptake period of 17 h at 18°C or 9 h at 25°C. The corresponding value for the 70 mg N L-1 treatment was 16.5°CdN, which would translate to uptake periods of about 50 h at 18°C or 26 h at 25°C. This is in agreement with reports in the literature that leaching losses are greater at higher application rates and decrease linearly with an increase in uptake period (Lea-Cox and Syvertsen, 1996). It can be concluded that, with an increase in N rates, N should reside in the root zone much longer. This can promote N losses due to leaching, especially when combined with excessive rainfall and/or irrigation and ultimately will increase the risk of ground water contamination. Based on these results, use of the UTS approach may provide researchers with a simple yet useful tool for risk assessment of N leaching.



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  		Fig. 7. Potential nitrogen leaching (PNL) index as a function of the uptake temperature sum (°CdN) for Swingle (SWL) and Volkamer (VLK) seedlings with the 7 mg N L-1 (A) and 70 mg N L-1 (B) treatments. Regression equations expresses PNL index as a function of uptake temperature sum (UTS) values.

 
Estimation of Nitrogen Uptake Efficiency of Reclaimed Water under Field Conditions
The conceptual model to quantify NUE as a function of UTS developed for greenhouse conditions was extended to assess the NUE of reclaimed water at the Water Conserv II site during an uptake period of 2 to 3 d between irrigation events. Data from Fig. 6A were used to generate an uptake function for an N concentration of 4 mg N L-1 based on a root length density of 4 km m-3. Since N uptake is proportional to root length density (Clarkson, 1985), uptake data were corrected using root length densities determined at the Conserv II site (Menocal-Barberena, 1999). Effects of N concentration on N uptake were calculated using Michaelis–Menten parameters previously determined for soil N uptake of citrus (Scholberg, unpublished data, 2001). An overview of the calculated uptake times for complete N uptake as affected by N concentration, soil N concentrations, and root densities is presented in Table 4.


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  		Table 4. Estimated minimum uptake time required for complete uptake of NO3–N by citrus trees as affected by soil solution N concentration and selected root density and soil temperature (°C). The 16°C and 26°C temperatures are the mean soil temperatures during the winter and summer, respectively. The root length densities of 4.00, 1.33, and 0.25 represent those for seedlings, the upper 30-cm layer, or the 30- to 90-cm layer of the soil profile for field-grown trees (Menocal-Barberena, 1999).

 
Based on the predictive results from this conceptual model, it is concluded that more frequent application of small irrigation volumes would result in reduced N displacement and higher NUE values for reclaimed water (Table 5). However, with increasing irrigation rates, NUE of the reclaimed water decreased sharply, and application under cold conditions resulted in NUE values as low as 28%. The appreciable increase in required uptake times at greater soil depth was related to root densities of 1.33, 0.25, and 0.12 for the 15-, 60-, and 150-cm soil depths (Menocal-Barberena, 1999). With intermediate irrigation rates, N was displaced to a soil depth of 30 to 60 cm and some additional root uptake may still occur during the next irrigation cycle. At the highest irrigation rate, theoretically 50 to 60 kg N should be taken up by the crop. However, the intensive leaching associated with high irrigation rates may reduce NUE for soil N mineralization and/or additional fertilizer applications greatly, so overall NUE would be reduced as well.


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  		Table 5. Assessment of uptake efficiency for reclaimed water as affected by irrigation treatment and application month, based on information in Table 4. Average daily rainfall in January and July was 2.4 and 6.0 mm; corresponding daily evapotranspiration rates were 1.5 and 4.5 mm, respectively. Nitrate concentration in the reclaimed water was 7 mg N L-1. Actual N concentrations ([N]) were based on the ratio between irrigation rates and average rainfall. The effective soil storage capacity was 5%, and irrigation coverage was 70% of the total land area.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
It is concluded that N uptake cannot be uniquely predicted based on either ET values or plant size. Alternatively, the use of an uptake temperature sum allowed as to account for most of the overall variation in nitrogen uptake and results were similar for both varieties. Use of the uptake temperature sum should allow researchers to calculate potential N uptake and/or assess potential N leaching based on information on soil temperature, root distribution, and average soil N concentrations. This approach could also form a sound basis for development of computer-based systems for assessment of potential N leaching in agricultural production systems. Irrigation management should be geared towards maintaining N within the active root zone by more frequent application of smaller irrigation volumes based on crop water requirements. Based on results of the conceptual model it is concluded that the upper root zone, which has four times greater root concentrations (Barberena and Menocal, 1999), is probably the most effective site for N interception. Root concentrations at greater soil depth are minimal and displacement of water and/or N below the 60- to 90-cm soil depth will increase the potential risk of ground water contamination greatly. Excessively high N application rates will invariably increase required uptake periods and thereby increase the risk of N leaching below the active root zone prior to uptake. More frequent application of N should therefore improve NUE. It is of interest that highest N efficiencies may actually be observed under N-limiting conditions. However, with good irrigation and fertilizer management practices, NUE values of 60 to 70% for commercial citrus production appears to be realistic (Syvertsen and Smith, 1996) to sustain high yields (Wheaton et al., 1997).

Based on our original hypothesis, we conclude that the low N concentrations of reclaimed water are not the main cause for the low NUE associated with its use. Rather, N displacement below the rootzone associated with excessive irrigation appears to hamper efficient N utilization. Additional research is required to validate the UTS approach under field conditions. In our current conceptual model, the uptake sum was assumed to increase linearly with increasing temperature. This assumption has been shown to be valid over the 16 to 26°C temperature range. However, at more extreme temperatures deviation from this linearity may occur (Scholberg, unpublished data, 1999). We therefore intend to extend our current conceptual model to more universally applicable expert systems. This will require a more detailed description of the effects of soil temperature, soil N concentration, and root density on N uptake. The use of such systems should provide producers with a management tool that could enhance efficient N use and minimize the potential risk of ground water contamination.


   ACKNOWLEDGMENTS
 
Research conducted with the financial support of the Water Conserv II program, Orlando, FL. We gratefully acknowledge Dr. J.P. Syvertsen (Citrus Res. and Educ. Center, Univ. of Florida) for the use of his greenhouse facilities and for his helpful suggestions; John A. Cornell (Univ. of Florida Dep. of Statistics, Gainesville, FL) for his advice and assistance with statistical analysis; Jim Bartos (Analytical Research Laboratories, Univ. of Florida, Gainesville, FL) for water sample analysis; Deborah Van Clief (CREC, Lake Alfred, FL) for tissue analysis; Aija Paolillo (Florida Southern College, Lakeland, FL) for root length analysis; and Jacob Butler (Auburndale [FL] High School) for his assistance during installation and operation of the greenhouse system.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Florida Exp. Stn. Journal Ser. no. R-07763.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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